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Population viability of sea turtles in the context of global warming


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Sea turtles present a model for the potential impacts of climate change on imperiled species, with projected warming generating concern about their persistence. Various sea turtle life-history traits are affected by temperature; most strikingly, warmer egg incubation temperatures cause female-biased sex ratios and higher embryo mortality. Predictions of sea turtle resilience to climate change are often focused on how resulting male limitation or reduced offspring production may affect populations. In the present article, by reviewing research on sea turtles, we provide an overview of how temperature impacts on incubating eggs may cascade through life history to ultimately affect population viability. We explore how sex-specific patterns in survival and breeding periodicity determine the differences among offspring, adult, and operational sex ratios. We then discuss the implications of skewed sex ratios for male-limited reproduction, consider the negative correlation between sex ratio skew and genetic diversity, and examine consequences for adaptive potential. Our synthesis underscores the importance of considering the effects of climate throughout the life history of any species. Lethal effects (e.g., embryo mortality) are relatively direct impacts, but sublethal effects at immature life-history stages may not alter population growth rates until cohorts reach reproductive maturity. This leaves a lag during which some species transition through several stages subject to distinct biological circumstances and climate impacts. These perspectives will help managers conceptualize the drivers of emergent population dynamics and identify existing knowledge gaps under different scenarios of predicted environmental change.
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790 BioScience August 2021 / Vol. 71 No. 8
Population Viability of Sea Turtles
in the Context of Global Warming
Sea turtles present a model for the potential impacts of climate change on imperiled species, with projected warming generating concern about
their persistence. Various sea turtle life-history traits are affected by temperature; most strikingly, warmer egg incubation temperatures cause
female-biased sex ratios and higher embryo mortality. Predictions of sea turtle resilience to climate change are often focused on how resulting male
limitation or reduced offspring production may affect populations. In the present article, by reviewing research on sea turtles, we provide an overview
of how temperature impacts on incubating eggs may cascade through life history to ultimately affect population viability. We explore how sex-
specific patterns in survival and breeding periodicity determine the differences among offspring, adult, and operational sex ratios. We then discuss
the implications of skewed sex ratios for male-limited reproduction, consider the negative correlation between sex ratio skew and genetic diversity,
and examine consequences for adaptive potential. Our synthesis underscores the importance of considering the effects of climate throughout the life
history of any species. Lethal effects (e.g., embryo mortality) are relatively direct impacts, but sublethal effects at immature life-history stages may
not alter population growth rates until cohorts reach reproductive maturity. This leaves a lag during which some species transition through several
stages subject to distinct biological circumstances and climate impacts. These perspectives will help managers conceptualize the drivers of emergent
population dynamics and identify existing knowledge gaps under different scenarios of predicted environmental change.
Keywords: climate change, temperature-dependent sex determination, thermal tolerance, operational sex ratio, effective population size
Climate change is driving population extirpations 
and species extinction at an accelerating rate (Urban
2015). Conservationists increasingly seek to assess what
populations and species will persist under future climate
scenarios—and why. This trend is at the forefront of sea tur-
tle conservation; the seven extant species are of conservation
concern and are susceptible to myriad aspects of environ-
mental change. A principal issue (and focus in the present
article) is that sea turtle demography is sensitive to tem-
perature. Warmer incubation temperatures produce female-
biased primary sex ratios (Standora and Spotila 1985), and
the mortality of developing embryos increases past thermal
thresholds (Bustard and Greenham 1968). In a warming
world, the threats of extreme sex ratio skew and declining
egg viability threaten population persistence. Will there be
enough males to maintain populations? Does this even mat-
ter if projected temperature increases result in progressively
higher embryo mortality? There is a growing concern that
sea turtle populations may have limited capacity to persist in
the warming world (Hays etal. 2017, Monsinjon etal. 2019).
Despite a dire contemporary outlook for sea turtle
persistence, they represent an ancient taxon whose lineage
has stood the test of time. Ancestors of the Testudines
order survived temperatures that were warmer than today
(Fastovsky and Weishampel 2005). A key difference now is
that the rise of Homo sapiens has contributed to a collapse in
turtle populations (Lovich etal. 2018, Stanford etal. 2020).
Current abundances have declined from historical levels
because of human pressures such as harvest and habitat
destruction, leaving today’s depleted stocks more vulnerable
than their ancestors. Despite this, the fact that sea turtles
have endured periodic fluctuations in atmospheric carbon
concentrations and temperatures suggests that adaptation
may enable their persistence. However, even if sea turtles
persist, it is likely that some populations will face extirpation.
Biological traits, conservation statuses, and climatic contexts
differ greatly among global populations (Fuentes et al.
2013, Mazaris etal. 2015). Therefore, estimating individual
population viability against a backdrop of ongoing climate
change is an important step to advance conservation.
Inferences of population viability depend on an under-
standing of demographic composition and the processes
driving changes therein (i.e., demographic dynamics), but
quantifying these elements in sea turtles is challenging.
Lengthy, complex, and cryptic life histories make accurate
estimates elusive. With time to sexual maturity ranging from
one to three decades, the lifespan of a sea turtle exceeds
the duration of most research programs. Rates of offspring
survival are low, and if sea turtles reach the juvenile stage,
they may inhabit multiple different foraging habitats as
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they mature. Adults then generally spend their lives in
one home foraging area but undertake sometimes distant
migrations to reproduce. Therefore, individuals are difficult
to locate and track in open marine habitats throughout their
lives. As a result, key demographic parameters (e.g., age
structure, sex ratio, and survivorship) remain unresolved
for most populations (Rees et al. 2016). Research on the
population viability of sea turtles is notably concentrated
around demographic effects on offspring, compared with
older life-history stages, because of conspicuous effects from
temperature (on sex ratios and survival) and the relative ease
of conducting research on nesting beaches (versus in marine
In the present article, we synthesize concepts that frame
our current understanding of how warming temperatures
may affect the viability of sea turtle populations. Our review
highlights that, from a population modeling perspective,
the effects of climate change on the population growth rates
for any organism will be realized via changes in survival or
reproductive output. To comprehend how sublethal climate
impacts will affect viability, we must first have a detailed
understanding of life history to link these impacts to
eventual changes in survival and reproduction. This reality
applies across diverse organisms and ecosystems and makes
it challenging to predict viability for species whose life
history is difficult to study.
Our review is oriented toward impacts on offspring
demography and how these outcomes ultimately affect
population growth rates (and viability). We first provide an
overview of the temperature-sensitive nature of offspring
demography. The process of incorporating direct impacts
on vital rates (embryo survival) in inferences of viability is
relatively immediate and simple when compared with more
drawn out and complex predictions for the effects of skewed
sex ratios (eventually realized in reproductive output).
We examine this reality, exploring how skewed primary
sex ratios (PSRs; defined in table 1) may translate to male
limitation by reviewing the links among PSR, adult sex ratio
(ASR), operational sex ratio (OSR), and mating (figure 1).
We transition to considering the implications of skewed sex
ratios for genetic diversity—an essential concern for viability
but one that is often overlooked for sea turtles. Finally, we
examine the importance of possible adaptations to climate
change and offer a concluding synthesis.
Effects of temperature on offspring demography
All sea turtle species exhibit temperature-dependent
sex determination (TSD), in which the sex ratio among
embryos developing in an egg clutch becomes female-biased
at warmer temperatures (Yntema and Mrosovsky 1980,
Standora and Spotila 1985). The exact mechanism trigger-
ing gonad differentiation remains unresolved. However, we
know that TSD plays out through temperature-linked gene
expression pathways that, in sea turtles, drive increased
expression of aromatase and therefore estrogen at high
temperatures, compared with testosterone at lower tempera-
tures, consequently leading to the respective development of
ovaries or testes (Singh etal. 2020, Weber etal. 2020). Sex
determination occurs during the middle third of embryonic
development, or the thermosensitive period (Girondot etal.
2018). Two key parameters are typically used to charac-
terize the relationship between incubation temperature
and sex ratio, or thermal reaction norm (i.e., a pattern in
phenotype across a range of temperatures; figure 2). The
first is the pivotal temperature, the constant temperature
resulting in a balanced sex ratio. It is typically presented as
constant because seminal TSD studies took place in labora-
tories employing constant temperatures (box 1). The second
key component is the transitional range of temperatures,
between which ratios transition from approximately 95%
male to 95% female (Girondot etal. 2018). The temperature
values parameterizing thermal reaction norms have been
shown to vary among species and populations of the same
species (Hulin etal. 2009, Bentley etal. 2020a) and possibly
among individuals from the same population (Carter etal.
2017, 2019).
Offspring survival. Determining what factors affect the embry-
onic survival of sea turtles (i.e., egg hatching success) has
been an aim of biologists for decades (box 2; Bustard and
Greenham 1968). A diverse suite of factors has been tied to
hatching success, both endogenous (e.g., parental genetics;
Table 1. Glossary of key terms related to sea turtle sex ratios.
Biological parameter Definition
Population (of sea turtles) A genetically related (i.e., partitioned) group of individuals originating from a nesting area. Natal homing
results in highly differentiated maternal lineages and unique genetic markers, and population mixing
(gene flow) is most frequently male mediated.
Primary sex ratio (PSR) Sex ratio of offspring.
Adult sex ratio (ASR) Sex ratio of reproductively mature adults.
Operational sex ratio (OSR) Sex ratio of adults seeking to reproduce. In sea turtles, OSRs are typically quantified annually—that is,
per breeding season.
Realized sex ratio (RSR) Used in the present article to refer to the sex ratio of adults contributing (parentage) to offspring in a
breeding season. RSRs can vary from OSRs because of behavioral mechanisms such as competition. In
many cases referred to as breeding sex ratio.
Breeding periodicity The frequency with which a sea turtle participates in reproduction—for example, annually, biennially,
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Phillips etal. 2017) and exogenous (e.g., proximity to beach
vegetation; Ditmer and Stapleton 2012). Temperature stands
out as a critical exogenous variable; thermal reaction norms
described for hatching success (figure 2) consistently show
declines at threshold incubation temperatures (Howard etal.
2014). This association between temperature maxima and
hatching success poses a clear threat to sea turtle offspring
production and is another example of how climate change
may affect population persistence (Laloë etal. 2017).
Temperature-sensitive offspring demography and future warming. The
associations among temperature, primary sex ratios, and off-
spring mortality have concerning implications for sea turtle
populations in the context of current and projected warm-
ing (Santidrián Tomillo et al. 2014). For example, recent
evidence from a green turtle (Chelonia mydas) in-water
aggregation in Australia suggests that nearly female-only
populations are possible; juveniles and subadults originat-
ing from northern Great Barrier Reef nesting beaches were
more than 99% female (Jensen etal. 2018). Such a bias in sex
ratio is undoubtedly concerning, but at a broader scale, some
postulate that, under the projected warming scenarios, sea
turtle population viability may be more sensitive to embryo
mortality than to increasing sex ratio skew (Hays etal. 2017).
Predicting the impacts of these two factors is a difficult
challenge for population modeling. First, thermal reaction
norms appear to be largely context specific (but see Monsinjon
et al. 2017), and a suite of environmental parameters may
be locally important. Second, whereas hatching success is
comparatively easy to accommodate in population models,
predicting the effects of skewed PSRs is more complex.
This distinction can be illustrated by considering seminal
matrix population models (Crouse et al. 1987), although
these models center on female fecundity and approaches
Figure 1. Warming temperatures affect sea turtle embryos via increases in both mortality and female sex ratio bias.
In population models, these impacts will affect population growth rates in distinct ways. Changes to embryo survival
represent a direct impact on vital rates (or transition probability from stage one in a demographic matrix model). By
contrast, impacts on primary sex ratios will eventually affect adult reproductive output; before this impact is realized,
offspring cohorts must survive to reproductive maturity and be subject to a suite of factors, including hypothetical changes
to sex ratios among life-history stages, reproductive biology, and mating behavior. Longstanding knowledge gaps (denoted
with question marks) make it challenging to predict the impacts of temperature-sensitive offspring demography and
research continues to address these gaps. Figure created by Kate Maurer.
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accounting for sex ratios would be appropriate. Impacts
on offspring survival would be factored directly into the
transition probability from the first life stage, but the
impacts from PSR skew are difficult to derive and will only
be realized in eventual reproductive output. To predict how
PSRs will translate to actual measures of adult reproduction,
we must account for many factors reflecting changes in
sex ratios through life history, adult behavior, and mating
How many males are enough? Conceptualizing a
male-limited sex ratio
Below, we review research suggesting that female-biased
sex ratios are common and adaptive in sea turtles. But
how many males are too few? Determining a PSR at which
males will become limiting to population growth depends
on estimates of several types of sex ratios throughout sea
turtle life histories and how these ratios are linked (see
table 1). We first need to estimate a future ASR from a
PSR, or, more accurately, from the blend of cohort-specific
PSRs that contribute to a given ASR. Then, from an ASR,
we must derive a seasonal OSR, which differs from the
ASR because of sex-specific breeding periodicity. Behavior
at mating areas may result in differences between the OSR
and the sex ratio of realized parentage; that is, the sex ratio
among parents whose genes are passed on to an offspring
cohort. This ratio has been referred to as a breeding sex
ratio in many instances, but in other cases, it is conflated
with an OSR. In the present article, we refer to it as the
realized sex ratio (RSR).
From primary to adult sex ratios: Sex-specific survival. The con-
version factor for equating a PSR to an ASR depends on
male and female survival between the hatchling stage and
sexual maturity. Sometimes, a 1:1 conversion is used (e.g.,
Hays etal. 2017). However, evolutionary theory provides
support for the idea of sex-specific differences in survival
and fitness for species with TSD (Schwanz et al. 2016).
This theory may play out in sea turtles via hatchling phe-
notypic plasticity (table 2). Temperature simultaneously
affects offspring morphology and sex (Booth 2018), which
may result in a generalized trend of reduced fitness at
female-producing temperatures below thermal maxima
(Kobayashi etal. 2018). This would have a balancing effect
on sex ratios as offspring cohorts develop toward maturity.
However, the alignment between the thermal reaction
norms for TSD and hatchling morphology warrants further
Hypotheses about sex-specific survival in immature sea
turtles remain rooted in theory and lack empirical testing.
Information on preadult survival has eluded researchers for
decades, especially at the early pelagic stage dubbed the “lost
year.” Tracking offspring cohorts presents major logistical
challenges such that we lack empirical information regarding
the distribution of younger individuals in general (but see
Mansfield et al. 2014, Putman et al. 2020). Furthermore,
ex-situ experiments have limited capacity to simulate real
sources of mortality—namely, predation. Information about
early life survival is increasing via long-term mark–recapture
efforts using hatchling genetic fingerprinting (see Dutton
and Stewart 2013). After genetically marking hatchlings
and waiting until reproductive maturity, females may be
recaptured via genetic sampling on nesting beaches, or
both parents may be recaptured using parentage analysis of
offspring DNA (e.g., Wright etal. 2012a).
Characterizing survival via hatchling genotyping entails
immense sampling efforts (for low returns on decadal scales)
and is dependent on natal homing back to the study site.
Another approach to estimating patterns in sex-specific
survival is to compare sex ratios at different life stages.
Rees and colleagues (2016) noted a trend in sex ratios for
Mediterranean loggerheads (Caretta caretta), from strongly
female-biased PSRs to balanced or male-biased ASRs. In a
similar gradient, Wibbels and colleagues (1991) suggested
a juvenile loggerhead sex ratio of 2.1 females per male near
Hutchinson Island, Florida, which is notably less biased than
the more than 93% female PSRs found at nearby nesting
beaches in Cape Canaveral (Mrosovsky and Provancha
1989). Allen and colleagues (2015) likewise documented
more female bias in immature green turtles compared
with adults in San Diego Bay. Many confounding factors
limit inference into causation, such as the mixing of stocks
with distinct demographic rates and possible influences
from sex-specific behaviors. However, modeling efforts
may begin to resolve confounding factors in some systems.
For example, Vandeperre and colleagues (2019) used nest
numbers at Florida source rookeries with a 3-year lag to
predict juvenile loggerhead abundance in Azorean waters,
and sex ratio estimates could foreseeably be integrated into
such approaches.
Figure 2. Thermal reaction norms for primary sex ratios
(PSRs) and hatching success (i.e., embryonic survival)
provide the basis for many predictions regarding the
persistence of sea turtles under projected warming
scenarios. Two arbitrary examples illustrate the shape of
such norms, adapted from Hays and colleagues (2017;
not shown, at even lower temperatures, further left on the
x-axes, the curve trends would be reversed as PSR bias
rises and hatching success declines at cold thresholds).
Figure created by Kate Maurer.
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We are now more prepared to advance our understanding
of sex- and stage-specific survival in sea turtles. The
knowledge gap surrounding survival hinders estimation
of how PSRs translate to ASRs—a key consideration when
projecting the effects of warming on populations. Although
the first (difficult) step is to simply derive robust baseline
estimates of survival, it is important to note that this
parameter, among others, is dynamic and may vary with
environmental conditions. For instance, Kobayashi and
colleagues (2018) experimentally demonstrated that water
temperature affects hatchling swimming performance such
that early survival rates may change with spatiotemporal
variation in water temperature. Given an accurate PSR
estimate, improved estimates of survival (and transition
probabilities) will help to refine ASR estimation. However,
this step still falls short of estimating reproductive output, a
parameter ultimately mediated by OSR and mating biology.
From adult to operational sex ratios: Breeding periodicity. The
next consideration for unraveling how warming may affect
reproductive output via skewed sex ratios is the relationship
between ASR and OSR; the conversion factor between the
two is determined by breeding periodicity (table 1, figure
1). Whereas female periodicity has been comparatively
well documented by nesting beach-tagging programs (e.g.,
Kendall etal. 2019), there is a knowledge gap regarding male
breeding periodicity because of the difficulty of observing
males in marine habitats. We know that male sea turtles can
mate with multiple females in a single breeding season (Gaos
et al. 2018), and there is compelling evidence that males
may participate in breeding seasons more frequently than
females (Hays etal. 2014). This greater breeding frequency
makes sense energetically, because reproductive energy costs
should be lower for males and require shorter foraging peri-
ods to replenish (Hays etal. 2014). On the basis of these two
criteria alone (polygyny and breeding periodicity), an ASR
that maximizes reproductive output would theoretically be
female biased. But how many males are too few? Projecting
how changes to ASRs will affect OSRs, and therefore mating,
depends on detailed knowledge of male breeding periodic-
ity to complement existing data sets documenting female
periodicity. We highlight three methodologies advancing the
field in this regard: satellite tracking (e.g., Hays etal. 2014),
in-water surveys at breeding areas (e.g., Hays et al. 2010),
and genetic paternal reconstruction using hatchling DNA
(e.g., Wright etal. 2012a).
Box 1. TSD science: Progress and considerations.
Research advances have pushed TSD science beyond a paradigm developed through controlled laboratory studies that typically used
constant temperatures and did not account for a suite of other environmental factors that vary in situ (Bowden and Paitz 2018). For
example, whereas the thermosensitive period was originally defined as the middle third of incubation duration under constant tem-
perature, it has since been reconceptualized to take into account fluctuations in temperature and associated physiological develop-
ment (Girondot and Kaska 2014). Reanalyses using this new concept revealed that previous studies may have inflated estimates of
the production of male offspring in situ (Girondot etal. 2018). Some work has also suggested that when incubation temperatures are
inferred from proxies (e.g., air or sand temperatures) instead of directly sampling in egg clutches, resultant estimates of sex ratios can
be flawed (Fuentes etal. 2017) and may miss important mediating factors such as sand moisture (Lolavar and Wyneken 2020). We
note that most studies have focused on characterizing pivotal temperature, at times at the expense of defining the transitional range of
temperatures, which may be equally or more important for understanding links among incubation temperatures, PSRs, and viability
(Hulin etal. 2009).
Historically, empirical PSR characterization required researchers to euthanize hatchlings to assess gonadal histology. The permit-
ting obstacles and ethical considerations associated with sacrificing hatchlings have motivated the development of novel, nonlethal
approaches. For instance, Tezak and colleagues (2020) developed an immunoassay that, from a small sample of hatchling blood, can
identify a marker for antimüllerian hormone that appears to be male specific. There is also promise in other methods that may be
extended to hatchlings in the future, such as measuring eggshell steroids (Kobayashi etal. 2015), blood hormone assay techniques for
juveniles (Allen etal. 2015), and near-infrared spectroscopy (used to classify sex of amphibians; Vance etal. 2014). All these techniques
will facilitate more empirical PSR validation to accompany increasingly sophisticated modeling approaches for in situ temperature data
(Abreu-Grobois etal. 2020), bolstering our understanding of TSD in sea turtles.
Maternal effects on offspring phenotype (Mousseau and Fox 1998) may be an important area of research for understanding TSD,
because some effects may be attributed to and therefore conflated with the underlying thermal reaction norm (figure 2). A substantial
body of literature documents how maternal behaviors affect offspring phenotype, such as nest site selection (e.g., Reneker and Kamel
2016), in which the ultimate mechanism is the modification of the incubation environment. By contrast, maternal effects can act
through different pathways, such as varying sex steroid hormones within eggs (Bowden and Paitz 2018). Carter and colleagues (2017)
documented changes in maternally sourced egg estrogen concentrations throughout a nesting season in red-eared sliders (Trachemys
scripta) and, after controlling for temperature, associated higher concentrations with an increased likelihood of female sex ratio bias.
Hormone-mediated maternal effects may act independently from temperature and, when left unaccounted for, are conflated with the
underlying sex ratio thermal reaction norm. Therefore, growth in this research area will strengthen the understanding of PSR deter-
mination in general (Bowden and Paitz 2018).
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Satellite tracking can facilitate inferences of male breeding
from the periodicity of movements to breeding areas. We
found six published subsets of data for males that were
tracked long enough to infer periodicity over multiple
seasons (typically more than 365 days). These data included
four species and 35 individuals (supplemental table S1;
James et al. 2005, van Dam et al. 2008, Casale et al. 2013,
Varo-Cruz et al. 2013, Hays et al. 2014, Naro-Maciel et al.
2018). A yearly remigration pattern was exhibited by 19 of
these turtles. Unfortunately, this approach is constrained by
the duration of transmitter retention and will typically only
show whether a male exhibits consecutive annual breeding
migrations. Either an annual or biennial pattern is often
assumed, although in many cases a triennial (or longer)
pattern cannot be ruled out. Moreover, intraindividual
variability has not been estimated for males but is likely
considering the complexities that dictate reproductive
energetics and periodicity. Another caveat for inferring
male breeding periodicity on the basis of migrations is that
it is unclear what to infer from males that reside in breeding
areas (e.g., Varo-Cruz et al. 2013). Although migration is
assumed to culminate in reproductive activity, a lack of
migration may not be indicative of reproductive status
because males may simply remain resident in breeding
areas (furthermore, reproductively active males may not
successfully mate). For example, Blanvillain and colleagues
(2008) found that as high as 15% of males present at a
loggerhead breeding area in Florida were not reproductively
active. Although satellite tracking males to infer periodicity
has clear limitations, these can be alleviated to some degree
by complementary in-water work at breeding areas, such
as surveys using photo ID or unmanned aerial vehicles
(UAVs; Schofield etal. 2017). Hays and colleagues (2010)
complemented satellite tracking with photo-ID surveys and
female nesting data to suggest that loggerhead males visited
a Zakynthos Island breeding area 2.6 times more frequently
than females; such a disparity would lead to a greater male
component in the OSR relative to ASR.
A newer approach to estimating periodicity is the use of
hatchling genetics to reconstruct paternal genotypes (e.g.,
Wright et al. 2012a). Hatchling DNA contains a definitive
record of paternity, and therefore sampling over multiple
seasons can reveal periodicity. As molecular techniques
advance and continue to decline in cost, this approach
holds great promise for answering questions surrounding
periodicity and OSRs. We note two key considerations for
using this method to estimate male periodicity. First, the
strategy is only as good as the detection probability for
fathers. This probability is dependent on a priori knowledge
of the population of interest and associated sampling design.
For example, if sired clutches are distributed among several
beaches (e.g., Wright etal. 2012b), then sampling hatchlings
at one beach will likely not be enough to detect all fathers.
In addition, clutch sampling should take into account the
potential for multiple paternity. Second, a full record of
offspring parentage represents an RSR, not an OSR, unless
all males attempting to mate successfully sire offspring. This
distinction neutralizes concerns about males that reside at
Box 2. Sea turtle offspring survival.
Contemporary understanding of reaction norms for embryo thermal tolerance (hatching success) has advanced through the descrip-
tion of taxonomic and geographic variation, as well as through more rigorous evaluations of impacts from other environmental vari-
ables. Although temperature poses a clear threat, its relative importance (when below lethal extremes) compared with other factors
is unclear across species and populations. This is in part because of inter- and intraspecific differences in the temperature at which
hatching success declines (Howard etal. 2014). These differences are accompanied by variation in apparent susceptibility to other
regional environmental parameters such as aridity and rainfall (Santidrián Tomillo etal. 2015a, Rafferty etal. 2017, Rivas etal. 2018).
Varying findings suggest context-specific associations between regional climate variables and hatching success and key trade-offs
between temperature and moisture.
In contrast to TSD, there has been less investigation into when temperatures ultimately factor into embryonic survival and how ther-
mal tolerance (i.e., how a given temperature affects embryonic development) may vary through incubation, especially under natural
conditions. Whereas lethal upper thresholds exist, certain durations of exposure to lower temperatures may also affect hatching success
(Howard etal. 2014, Bladow and Milton 2019).
Warming incubation temperatures also may affect the survival of sea turtle offspring after hatchlings exit nests, although there are
fewer empirical data for this stage compared with eggs. Temperature-linked phenotypic variability in reptile offspring affects more
than just sex (Singh etal. 2020), and sea turtle hatchling morphology appears to follow a thermal reaction norm with middle tempera-
tures resulting in the best morphological outcomes (Fisher etal. 2014, Mueller etal. 2019). Morphological effects are likely driven by
physiological factors, such as the increased conversion of egg yolk to tissue at lower temperatures, as well as effects on muscle fiber
development (Booth 2018). The overall result is that warmer temperatures are associated with the development of smaller and slower
hatchlings (although perhaps with higher energy reserves; Booth 2018). High variance in incubation temperature may also negatively
affect phenotype (Horne etal. 2014). If the average hatchling becomes smaller and slower as temperatures warm, and if these mor-
phological effects are conserved as the hatchling develops (as has been suggested by Noble etal. [2018]), then survival during early
life history stages may decline.
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breeding areas and may be important when conceptualizing
male limitation, because it is the OSR that should ultimately
dictate mate availability. That is, the male component of
an OSR represents all males available to mate and as male
numbers decline, competition should decline, and therefore,
OSRs may ultimately converge on RSRs. Future research is
warranted to evaluate how similar a seasonal RSR is to an
associated OSR by quantifying competitive exclusion.
Parentage analyses for a single season can produce an RSR
snapshot. For example, Gaos and colleagues (2018) sampled
eastern Pacific hawksbill (Eretmochelys imbricata) hatchlings
and found a single season RSR of 1.41 females per male.
Sampling over several nesting seasons can identify trends
in RSRs (that may reflect trends in OSRs) and patterns in
male breeding periodicity. Wright and colleagues (2012a)
used paternal reconstruction to suggest 3 of 99 genotyped
male green turtles participated in more than one breeding
season over 3 years in Cyprus, despite an estimated RSR
of 1.3 males per female. Phillips and colleagues (2014)
found that 4 out of 91 Seychelles hawksbill fathers sired
clutches in multiple seasons over four nesting seasons.
Lasala and colleagues (2013) and (2018) sampled loggerhead
hatchlings in Georgia and western Florida over three nesting
seasons and did not find any repeat fathers despite male-
biased RSRs. Together, these genetic studies may suggest a
more infrequent male periodicity pattern when compared
with satellite tracking work. However, the more pertinent
and broader takeaway may be that the knowledge gap
surrounding male periodicity and OSRs is far from being
resolved, limiting understanding of how increasingly skewed
sex ratios affect mating outcomes.
From operational to male-limited sex ratios: Mating biology and
behavior. A primary conclusion thus far is that, although we
know strikingly little about the conversion factors between
the different sex ratios, our ability to arrive at an OSR or
RSR is improving. Genetic parentage analyses may have the
most promising future as a single methodology, but in the
end, the confluence of different methodological approaches
may be necessary to fill knowledge gaps surrounding male
biology. For instance, we could learn much from a study that
integrates hatchling genetics with UAV surveys, in-water
photo ID, and satellite tracking to estimate OSRs or RSRs,
residence time at breeding areas, and the levels of male
fidelity to a single breeding area. A second conclusion is
that given more frequent male mating and polygyny, highly
female-biased PSRs may promote viability (Hays etal. 2017,
Santidrián Tomillo and Spotila 2020), although this idea
contrasts with Fisherian sex ratio theory (see Girondot etal.
1998). But what proportion or number of males represents
a tipping point? This question represents an impasse in this
discussion because even if breeding periodicity is known
and an OSR can be estimated, the point at which that OSR
will feature insufficient numbers of males to maintain
population stability depends on mating behavior. The real-
ity is that we know little about behavioral determinants in
sea turtle mating systems in general. For instance, a key
question is how many females a single male can mate with.
Documenting declines in egg fertility rates represents one
approach to inferring when male limitation occurs and is a
promising first step if combined with estimates of sex ratios
as detailed above (Phillott and Godfrey 2020). However, in
a theoretical scenario of warming-driven male limitation, in
which a decreasing proportion of males accounts for avail-
able paternal DNA, the consequences of skewed sex ratios
for population genetics become problematic.
The genetics of skewed sex ratios
Genetic diversity is a central component of population
viability, but it is often omitted in considerations of the
resilience of sea turtles to climate change (but see Fuentes
et al. 2013). Because extremely skewed sex ratios should
theoretically lead to negative effects on diversity and fitness
(Allendorf etal. 2013), omitting genetic diversity may lead
to overly optimistic conclusions. Advances in conservation
genetic research and molecular techniques will be critical
Table 2. Glossary of key terms related to sea turtle genetics and microevolution.
Genetic term Definition
Effective population size NeA theoretical number of individuals for a genetically idealized population that would have the same loss
of heterozygosity because of drift as the true population in question (i.e., census population). Ne can
be reduced by nonrandom mating, overlapping generations, high interfamily variance in contribution to
offspring, historical population bottlenecks, and sex ratio skew.
Genetic drift Chance variation in the frequency of different genotypes (i.e., not due to natural selection). Drift is
stronger in small populations and can lead to loss or fixation of alleles.
Inbreeding or outbreeding According to genetic theory, breeding by closely related parents will result in deleterious fitness
outcomes for offspring (inbreeding depression), but breeding by distantly related parents can also lead
to negative fitness outcomes (outbreeding depression) by—for example, introducing maladapted genes.
Phenotypic plasticity The ability for a single genotype to encode multiple phenotypes as a function of environmental variation.
Directional selection Natural selection for genotypes in a directional manner in response to some selection pressure—for
example, unidirectional change to the genotype encoding flipper length when swim speed is consistently
Selective sweep Rapid directional selection of a region of the genome due to strong natural selection. Alleles not
influenced by selection can be swept up to high frequency or fixation because of physical proximity to an
allele under selection.
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for evaluating how global warming will affect the viability of
sea turtle populations via skewed sex ratios (see Komoroske
etal. 2017 for relevant technical detail on genetic markers
and methodological advances).
Genetic diversity, effective population size, and fitness. Monitoring
genetic diversity through time can illuminate the effects of
TSD on effective population size (Ne; defined in table 2 along
with other terms). Although other measures of genetic diver-
sity are important to monitor, such as expected heterozygos-
ity and allelic richness (Allendorf etal. 2013), we focus on
Ne in the present article because it integrates genetic effects
with life history and is therefore highly relevant to viability
inferences (Hare etal. 2011). Notably, for a TSD context, Ne
for a mating population is maximized at a balanced sex ratio
and declines precipitously at highly skewed sex ratios. For
instance, at increasingly female-skewed sex ratios a closed
mating populations offspring are fathered by a decreasing
proportion of males. As such, all offspring must receive
paternal genes from a dwindling genetic pool. As this process
iterates through generations amid ongoing warming, genetic
diversity becomes a major concern. Changes in Ne over time
are therefore important to monitor as they may reveal when
skewed sex ratios start to limit genetic diversity. This infor-
mation is fundamental because genetic variation dictates
adaptive potential (Lande and Barrowclough 1987, Hare etal.
2011), a concern in the context of ongoing climate change.
Estimating Ne is challenging for sea turtle populations that
exhibit complex genetic structure with highly differentiated
maternal lineages at rookeries, male-mediated gene flow,
and overlapping generations (Bowen and Karl 2007, Hare
etal. 2011). Most studies characterize a static Ne value or
back-cast changes to assess bottlenecks (e.g., LeRoux etal.
2012), and few investigate contemporary change—a reality
that may be related to the difficulty of observing change
given long generation times. Phillips and colleagues (2014)
estimated Ne for a hawksbill population in Seychelles
and concluded that mating behavior and population
connectivity maintained elevated Ne, which may confer
adaptive resilience. González-Garza and colleagues (2015)
did not derive estimates of Ne, but provided evidence that
hawksbill neophytes (first-time reproducers) nesting in the
Yucatán Peninsula exhibited decreased individual genetic
diversity compared with older remigrants. This result
may support the idea that genetic diversity is declining
through generations, although mechanisms may exist
in other contexts for the maintenance of diversity even
amid population decline. Frandsen and colleagues (2020)
reported stable population-level genetic diversity in Kemp’s
ridley turtles (Lepidochelys kempii) despite a marked
population decline (and perhaps the most severe historical
bottleneck experienced by extant sea turtle populations).
As more studies track genetic diversity through time,
we will be better able to assess how population genetics
vary with demographic composition (e.g., abundance,
generation time, and sex ratio).
The concept that reduced genetic diversity can have
negative effects on fitness is well established (Allendorf etal.
2013); however, this idea lacks empirical testing in sea turtles.
Fitness is difficult to measure for sea turtles, and proxies are
typically used such as hatching success, emergence success,
or clutch size. Two innovative studies involving hawksbill
sea turtles attempted to relate measures of genetic diversity
to such proxies. In the study of Yucatán hawksbills, there was
no association between genetic diversity and selected fitness
proxies such as clutch size (González-Garza et al. 2015).
Phillips and colleagues (2017) provided more nuanced
results, using inbreeding and outbreeding hypotheses
(table 2) to explain that when parental relatedness was
high, hawksbill reproductive success was reduced, but when
parents were unrelated, lower paternal diversity increased
success. However, although Phillips and colleagues (2017)
did control for maternal body size and incubation duration
in analyses of clutch size and hatching success, respectively,
there are many important variables they did not account
for, such as incubation conditions (though a relatively large
data set of 142 clutches may help to overcome this). Studies
such as these pave the way for more research investigating
the consequences that genetic diversity has on sea turtle
population dynamics, a crucial research area for sea turtle
The genetic future. A better understanding of the ties among
sex ratio skew, genetic diversity, and fitness will be key to
forecasting sea turtle resilience under projected climate
change scenarios. As climate change unfolds in the near
term, many sea turtle populations may increase because of
greater female production and population-level fecundity
(Santidrián Tomillo etal. 2015b, Laloë etal. 2017), although
a suite of climate change impacts across sea turtle habitats
may negate this scenario. If populations do increase, it
will be important to monitor the genetic diversity patterns
underlying this trend. Mechanisms such as male-mediated
gene flow may help to maintain Ne but, in general, as sex
ratios change, proportional decreases would be expected
in Ne relative to the census population size. If Ne reaches
a critical threshold, populations will decline. Indeed, if
genetic theory holds, many populations could already be at
risk. This is especially true for small populations because of
heightened loss of genetic diversity owing to genetic drift
and associated increases in inbreeding depression, leading to
the loss of adaptive capacity (Hare etal. 2011). This reflec-
tion on genetic diversity logically leads to a discussion of
adaptive responses.
Adaptive responses
Adaptive responses represent a source of uncertainty (and
perhaps optimism) for inferences into population viability.
Above, we highlighted that genetic diversity is key to long-
term population viability, but a more mechanistic under-
standing of potential adaptive responses and their likelihood
(i.e., by determining if certain traits are under directional
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selection) would be useful for management and conserva-
tion. There is concern that adaptation via microevolution
in sea turtles may be too slow to cope with the current pace
of climate change; we discuss this idea later in the section.
Adaptive potential is inherently linked to sex ratios, a rela-
tionship mediated via Ne and genetic diversity (although
we note that other factors affecting phenotypic expression,
beyond the scope of our discussion, are also important to
adaptive potential; Allendorf etal. 2013). The implications
of skewed sex ratios for population genetics mean that when
projecting impacts on viability from skewed PSRs, account-
ing for effects on possible adaptive responses represents yet
another layer of complexity.
To persist in the context of warming climates, populations
will have to respond via genetic adaptation (microevolution)
or phenotypic plasticity. As temperatures increase, there
should be a corresponding increase in selection pressure
for those traits that confer fitness benefits. For sea turtles,
responses to warming could include changes in geographic
distributions (explored in depth in supplemental box S1),
thermal reaction norms, reproductive phenology, and
maternal effects on offspring phenotype. Trait variation
has been demonstrated for philopatry (i.e., fidelity to a
geographic region; e.g., Levasseur et al. 2019), thermal
reaction norms (e.g., Carter etal. 2019), and nesting behaviors
that may confer maternal effects (e.g., Reneker and Kamel
2016). Physiological maternal effects, such as maternally
sourced egg hormone concentrations, may also be relevant
(Bowden and Paitz 2018). Given this variation, these traits
should be subject to natural selection to the extent that
genotypic variation underlies phenotypic variation. Shifts
to reproductive phenology are unique in that they may not
represent a change in traits but, rather, the maintenance
of a trait (cueing phenology to sea surface temperatures)
while climates change (e.g., Patel et al. 2016). In the end,
when evaluating all the possible ways in which sea turtles
may respond to climate change, combinations of multiple
responses should be considered. For instance, Monsinjon
and colleagues (2019) suggested that phenological shifts
alone may be insufficient for many loggerhead populations
to persist under projected warming scenarios.
Quantitatively evaluating the suite of responses that
sea turtles may exhibit to cope with climate change is a
complex undertaking. Shifts in distributions may be the
most proximate to consider, because unless the thermal traits
that dictate sea turtle geographic ranges change through
selection or plasticity, warming temperatures will drive
ranges poleward. Mechanistic species distribution models
that take into account biophysical attributes (e.g., thermal
niches) can be especially useful to predict and understand
shifts (Dudley et al. 2016). Furthermore, the mechanistic
nature of such approaches leaves them open to incorporate
various environmental forces, aspects of species biology, and
responses beyond distributional shifts (Mitchell etal. 2008,
Wang et al. 2018, Bentley et al. 2020b, Stubbs etal. 2020).
Considering range shifts, in particular, exposes a possible
interplay of trade-offs: Sea turtles have high philopatry for
both foraging and nesting areas that presumably evolved
to ensure access to suitable habitats (Levasseur et al. 2019,
Shimada etal. 2020), but philopatry may impede range shifts
(box S1).
For any phenotypic response, it will be important to
distinguish between (directional) selection and plasticity,
because these have distinct implications for population-
level responses to climate change (Fox etal. 2019). Plasticity
may be more advantageous for coping with short-term
environmental fluctuations (e.g., annual temperature
extremes), whereas directional selection may be more
advantageous in the long run for dealing with environmental
trends such as long-term warming. Genomic approaches
will be instrumental in distinguishing between the two—
for example, through identifying genomic regions under
selective sweeps due to directional selection (table 2).
Can evolution keep pace with climate change? Although
long generation times slow the negative effects of genetic
drift, they can also decelerate the process of evolution.
Moreover, microevolutionary rates are estimated to be
slower for the Testudines lineage relative to other reptilian
and vertebrate lineages (Avise etal. 1992). High throughput
genetic sequencing and genomic approaches, such as
genome-wide association studies (Korte and Farlow 2013),
have great potential to elucidate the relationship between
genotype and phenotype. Isolating critical genomic regions
and genes under selection will help to monitor responses,
determine rates of change, and assess adaptive capacity
(Pardo-Diaz etal. 2015). Chow and colleagues (2019) offered
some of the first findings in this area of research, presenting
loggerhead genomic regions that may be under selection.
The next step is understanding the function of these genes.
Genomic techniques may also aid in evaluating the long-
term efficacy of controversial management strategies,
in particular those that attempt to improve survival by
removing sea turtles from their natural environments at key
developmental stages. Functional genomics could help to
reveal to what extent such methods interfere with beneficial
natural selection (see supplemental box S2).
Forecasting viability: Synthesis and conclusions
As regional climates change, the persistence of populations
will ultimately depend on how local environmental changes
affect population-specific demographic parameters. For sea
turtles, research on climate impacts is frequently focused
on the embryo stage because of notable associations among
temperature, survival, and PSRs, and because so much
research occurs at nesting beaches. We reviewed concepts
and literature that frame the implications of temperature-
sensitive offspring demography for viability, and we focused
this narrative on discussing sex ratios because of the com-
plex nature of linking PSRs to (adult) population dynamics.
Impacts on populations from decreases in hatching success
are relatively straightforward to estimate given accurate tem-
perature projections and well-described thermal reaction
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norms. By contrast, PSR skew cascades through life history
to eventually affect population growth rate through changes
in reproductive output (figure 3). Scrutinizing this cascade
exposes a suite of research areas to prioritize as the field con-
tinues to better understand the implications of warming for
viability. We summarize these areas with six points below.
Variation in thermal reaction norms. The understanding of how
temperature affects embryonic survival and sex ratios has a
longstanding basis, but more empirical description of such
norms continues to capture variation among species, popu-
lations, and even in individuals (Howard etal. 2014, Carter
et al. 2019). Notably, sex ratio thermal reaction norms are
more difficult to quantify, and empirical data are lacking;
many studies have used egg incubation temperature prox-
ies or base predictions about one population on the norm
described for another. Cutting-edge techniques in endo-
crinology are making it easier to characterize and describe
variation in sex ratio reaction norms (e.g., Tezak etal. 2020).
Preadult survival. It remains difficult to quantify survival to
maturity in sea turtles. Theory regarding the evolution and
adaptive significance of TSD suggests the possibility of sex-
specific patterns in survival (Schwanz etal. 2016), but with
Figure 3. Sex ratios are core parameters dictating mating opportunities, reproductive output, and, therefore, population
growth rates. In scenarios (a) and (b), a starting 75% female primary sex ratio (PSR) is used to explore how different
possible rates of sex-specific survival to adulthood, sex-specific adult breeding periodicity, and levels of male competition for
mating opportunities may cascade to affect the sex ratio of sea turtles contributing parentage (realized sex ratio; RSR) to an
offspring cohort. We encourage readers to consider alternative starting PSRs. At some level of female bias, we expect male
limitation will affect reproductive output. In scenario a, we show how this process is sometimes represented, with primary
sex ratios directly equated to adult sex ratios (ASRs) and assuming single rates (for the shaded transitions) when converting
to the next sex ratio. Although this approach may work well for single, well-studied populations, for broader inferences with
less certain demographic rates and transitions, we suggest embracing uncertainty and considering a range of values. In
scenario b, we explore various arbitrarily chosen rates that reflect some of the myriad possibilities that might be considered.
We further note that warming impacts on embryo mortality factor into population growth rate r at the beginning of a sea
turtle life cycle (i.e., the far-left narrow bar in this figure), whereas impacts on sex ratios affect r through reproductive
output after the cascade of factors affecting sex ratios (i.e., the far-right bar). Figure created by Kate Maurer.
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any such patterns mostly unknown, it is unclear how PSRs
may translate to an eventual ASR. We highlighted produc-
tive approaches to understanding preadult survival, includ-
ing hatchling genetic fingerprinting (Dutton and Stewart
2013) and monitoring sex ratios among life-history stages
(Jensen etal. 2018).
Male breeding periodicity. Our ability to convert between an ASR
and OSR is improving as we develop more of an understand-
ing of male breeding intervals. Methods such as satellite track-
ing (Hays etal. 2014) and genetic parentage analysis (Lasala
etal. 2018) continue to make important progress in this realm.
Mating system and behavior. The behavioral ecology for sea
turtle mating systems represents a key frontier for future
research to understand population viability. Importantly,
levels and consequences of competition are difficult to
describe, such that we do not know how different an OSR
may be from a corresponding RSR. Parentage analysis as a
means of tracking breeding intervals does provide essential
RSR information (Wright et al. 2012a) but does not shed
light on what a given OSR:RSR ratio may be. Characterizing
mating systems will require innovative strategies that inte-
grate research methods—for example, combining in-water
monitoring and hatchling genetic sampling.
Reproductive biology and male limitation. Knowledge gaps sur-
rounding reproductive biology make it difficult to conceptual-
ize what proportion or number of males in a mating population
may be limiting. Given an understanding of OSRs and RSRs
for a population, monitoring egg fertility rates shows promise
toward addressing this gap (Phillott and Godfrey 2020).
Sex ratio skew, genetics, and adaptive potential. We are building
toward a future in which sea turtle genetics and genomics
will be integrated into population assessments more rou-
tinely. Genetic diversity should be included when consider-
ing male limitation. Even if a population is not male limited
in terms of females being able to find mates, it may be male
limited genetically. That is, at extreme sex ratio skew the
decreasing proportion of males in the population represents
a shrinking pool of genetic diversity, and as this pattern iter-
ates through generations of warming the risk of inbreeding
and deleterious fitness effects increases. Moreover, as genetic
diversity declines, adaptive potential declines with it and
may limit resilience to ongoing climate change. Broader,
more intensive sampling of genetic diversity and monitoring
of Ne will be crucial moving forward, as will genomic tech-
niques to evaluate avenues for adaptive response.
These research topics present data deficient areas of
need as sea turtle conservation moves into a future with
accelerating climate change. In the absence of empirical
data for key demographic parameters, predicting viability
across a range of biologically realistic values may be
prudent (e.g., exploring varying male breeding intervals
or rates of sex-specific survival). Such exercises can help
to quantify uncertainty and may aid in identifying what
demographic variables should be prioritized for future
research. Embracing variation will also be important for
elucidating the effects of warming on viability, because
characterizing traits such as thermal reaction norms at the
population level can gloss over key intrapopulation and
intraindividual variation (e.g., Carter et al. 2019). From
refining the understanding of thermal reaction norms to
documenting breeding periodicity and identifying genomic
regions of accelerated change, research continues to address
data deficiencies and push the field toward answers.
As the understanding of demographic dynamics
(especially male demography) expands, so too will the
ability to project the future impacts of warming. As such,
this review has focused primarily on sex ratios and male
biology. We note, however, that impacts on egg survival
may be more threatening to populations than changes to
sex ratios (Saba etal. 2012, Hays etal. 2017), and they are
easier to understand and project. Nonetheless, in a scenario
in which sea turtles can adapt and persist, we suggest
that working toward a comprehensive understanding of
warming’s impacts is warranted. We acknowledge that
climate-associated changes to demography across all life-
history stages, rather than just offspring, will be important
to accommodate in modeling and projections (Hamann
etal. 2013). Furthermore, it will be beneficial to advance the
capacity to incorporate diverse facets of global change into
viability inferences (see box 3). In the present article, we
focused on warming temperatures but recognize that other
factors may be equally important to evaluate. Hamann and
colleagues (2013) reviewed many environmental changes
relevant to sea turtles, including changes in sea level, sea
surface temperatures, and precipitation. Much work has
advanced our understanding of how these factors may affect
populations, and a promising trend is the integration of
multiple facets within a single analysis (e.g., Montero etal.
2018, Patrício et al. 2019). Mechanistic models represent
a cohesive approach to such integration. For instance,
Stubbs and colleagues (2020) used mechanistic modeling
to evaluate different pressures on an Australian green turtle
population and suggested that climate change impacts on
food availability could have more severe effects than direct
impacts on demographic parameters.
Projecting population persistence is an important exercise
to evaluate and inform conser vation action. However, tracking
the fate of sea turtle cohorts is logistically challenging and
resource intensive, leaving information gaps for fundamental
aspects of demography. Currently, predictions must rely on
assumptions lacking empirical support about important
aspects of viability (e.g., genetic diversity, male reproductive
ecology). However, we highlighted advances across diverse
disciplines that provide evidence for an accelerating wave of
research on how global climate change will affect sea turtle
population viability. As research builds and continues to
address data deficiencies, the field of sea turtle conservation
has a solid base to inform adaptive management and future
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studies under changing climates. The lessons we glean from
sea turtles also apply broadly to any population viability
context: To understand how sublethal impacts of climate
at one life-history stage will affect reproductive output or
survival at another life-history stage, we must consider the
full natural history of organisms.
We thank Kate Maurer for producing the figures for the arti-
cle. We appreciate helpful feedback from Kathryn Levasseur,
Erica Henry, Kristen Hart, Larisa Avens, and three anony-
mous reviewers. Andrew Maurer is supported by a National
Science Foundation Graduate Research Fellowship (award
no. 1746939).
Supplemental material
Supplemental data are available at BIOSCI online.
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Box 3. Global change and sea turtles: Emergent threats.
Beyond many relatively well-documented aspects of climate change, new threats continue to emerge and may interact with each other
or with longstanding threats to populations. We highlight five threats garnering research attention. First, broadscale ecological regime
shifts may unfold with changing climates. Bjorndal and colleagues (2017) implicated such a regime shift in association with somatic
growth rate declines in immature turtles across three species throughout the Western Atlantic. Second, plastics in the marine environ-
ment are increasingly linked to sea turtle mortality (Wilcox etal. 2018). Third, apparent increases in episodic proliferation of algae
may inundate nesting habitats (e.g., Sargassum spp.; Maurer etal. 2015) or cause the release of toxins in coastal environments (e.g.,
red tides; Foley etal. 2019). Fourth, the rising incidence of disease associated with anthropogenic impacts on the marine environment
threatens many populations (e.g., fibropapillomatosis; Jones etal. 2016). Fifth, and related to the fourth, environmental contaminants
from products such as pharmaceuticals and pesticides have been shown to affect immune, neurological, and endocrine function in
aquatic wildlife (e.g., Arnold etal. 2014, Desforges etal. 2016). The feminizing effects of certain plasticizers—even at low doses—are
particularly relevant to sea turtle biology given projected changes to sex ratios (Vandenberg etal. 2012). Epigenetic effects from certain
contaminants may be heritable and therefore influence microevolutionary pathways for adaptation (Anway etal. 2005).
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Andrew Maurer ( is a doctoral candidate in
the Department of Biological Sciences at North Carolina State University, in
Raleigh, North Carolina, in the United States; he is also a research associ-
ate with the Jumby Bay Hawksbill Project in Antigua, in the West Indies.
Jeffrey Seminoff leads the Marine Turtle Ecology and Assessment Program at
the National Oceanic and Atmospheric Administration’s Southwest Fisheries
Science Center, in La Jolla, California, in the United States. Craig Layman
is a senior fellow at the Center for Energy, Environment, and Sustainability
at Wake Forest University, in Winston-Salem, North Carolina, in the United
States. Seth Stapleton is the principal investigator of the Jumby Bay Hawksbill
Project and serves as the director of conservation and animal health sciences
at the Minnesota Zoo, in Apple Valley, Minnesota; he is also an adjunct faculty
member in the Department of Fisheries, Wildlife, and Conservation Biology at
the University of Minnesota, in Minneapolis, Minnesota, in the United States.
Matthew Godfrey is a biologist with the North Carolina Wildlife Resources
Commission, in Raleigh, North Carolina, in the United States. Martha Burford
Reiskind is an assistant professor in the Department of Biological Sciences and
the director of the Genetics and Genomics Scholars program at North Carolina
State University, in Raleigh, North Carolina, in the United States.
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... The consequences of skewed primary sex ratios at the population-level can be complex and depend on reproductive behavior and physiology of adults, as well as the age at sexual maturity (ASM), reproductive longevity of males vs. females (Maurer et al., 2021) and methods of assessment (e.g., Lasala et al., 2013;Patrício et al., 2019). While breeding frequency, nest number, and clutch size are commonly measured parameters for adult female sea turtles, adult male sea turtle behavior is far more difficult to study because they do not return to shore. ...
... The relative benefits of such activities can be evaluated with population models that simulate the effects of variable primary sex ratios through time. Conservation strategy evaluation with population models has been used for sea turtles since the 1980s (e.g., reviewed by Heppell et al., 2003;Patrício et al., 2019Patrício et al., , 2021Maurer et al., 2021) and can help managers identify actions that are more or less likely to contribute to population persistence (Heppell and Crowder, 1998;Rout et al., 2013). ...
... To explore the potential population-level effects of warming beach temperatures on the peninsular Florida loggerhead population under different assumptions about adult male behavior and reproductive success, we incorporated long-term primary sex ratio observations from nesting beach studies in southern Florida into a simple, age-structured simulation model with a variety of outputs, including nest and nesting female abundance, adult sex ratio, and effective population size (N e ) of adults and breeders. The last is an important response variable because highly female-skewed populations could continue to function while suffering inbreeding depression (Maurer et al., 2021). We then evaluated the benefits of increasing the frequency of strong male production years under various model assumptions to simulate mitigation strategies. ...
Full-text available
Because the sex of sea turtles is determined by temperature during embryonic development, many populations are vulnerable to increased bias in primary sex ratios as global temperatures rise. Higher temperatures produce more females, and some populations are already showing years with all-female offspring production. But because sea turtles take decades to mature and have long adult lifespans, these primary sex ratio biases can take years to impact adult sex ratios, and the males from cohorts that are produced during cooler years may compensate for the sex ratio bias if they can breed more frequently and with multiple females. To date, little is known about male sea turtle reproductive behavior, making predictions of sex ratio skew impacts highly speculative. We used data from southern Florida loggerhead sea turtle nests to parameterize a simple population model to explore the effects of an increase in the proportion of female hatchlings over time on population trends, effective population size (N e ), and quasi-extinction probability. We also tested the effects of increasing the frequency of relatively high male production years to simulate potential mitigation strategies. While heuristic rather than predictive, our results expectedly show a rise in nest counts due to the increase in females over time, followed by population decline as males become limiting. Population collapse due to increased female bias will take many decades to occur, but sex ratio skew can have large impacts on N e , and thus increase the potential for inbreeding. An increase in the frequency of male production years, even just one additional “good male year” per decade, can help mitigate these outcomes if the rate of feminization is not too rapid. Male breeding frequency and mating success are critical drivers of the results and must be prioritized for research.
... Because the percentage of female hatchlings in sea turtles increases at high temperatures and global air temperatures are rising, there is some concern about the potential over-feminization of populations (Jensen et al. 2018;Tanner et al. 2019). Highly skewed sex ratios from climate warming could reduce genetic diversity, the effective population size and increase the potential for inbreeding (Heppell et al. 2022;Maurer et al. 2021;Lockley and Eizaguirre 2021). On the other hand, high temperatures also increase egg failure and hatchling mortality, which are therefore threatened by climate change (Santidrián Tomillo et al. 2009;Valverde et al. 2010). ...
... Operational sex ratios are the sex ratios of sexually active females and males at a given time (Emlen and Oring 1977). In sea turtles, operational sex ratio has been considered as the ratio of adult turtles that are ready to mate in a season (Maurer et al. 2021). Based on the estimations of operational sex ratios obtained at different nesting beaches around the world, these tend to be relatively balanced in sea turtle populations (reviewed in Santidrián Tomillo and Spotila 2020). ...
Full-text available
Sea turtles have temperature-dependent sex determination. Because females are produced at high temperatures, increasing global temperature may lead to population feminization. Primary sex ratios (PSR) of sea turtle hatchlings are naturally female-biased, but this translates into a more balanced operational sex ratio because male turtles reproduce more often than females. As a consequence, a balanced PSR and the temperature that produces it (pivotal temperature) are of limited use to guide climate mitigation management because an equal PSR may be demographically suboptimal. Here, I define population-advantageous primary sex ratios (PA-PSR) as the PSR that will tend to be in equilibrium in a population and that will result in balanced operational sex ratios; I then estimate PA-PSR for different reproductive frequencies (years elapsed between reproductive seasons) of adult female and male turtles. I also define population equilibrium temperature (PET) as the temperature that would result in the equilibrium PSR of hatchlings (i.e., PA-PSR). These concepts may help assess the influence of rising temperatures on populations, as they can better indicate if PSRs depart from those at equilibrium. I compared PA-PSR and beach PSR for two populations of sea turtles for which male and female remigration intervals were known and found that a mild or no feminization over the PA-PSR may be occurring. Because PSR varies inter-annually, and hatchlings coming from beaches of different thermal conditions could recruit to the same population, it is critical to estimate beach PSR at the right temporal and spatial scales. Climate mitigation strategies based on these concepts could provide better management guidance for conservation practitioners. Similar approaches could be considered for other female-biased species with temperature-dependent sex determination.
... Journal-Agrarian and Natural Resource Economics In addition, the incubation temperature between 29°C and 29.5°C, suffered an increase of 0, 21°C in autumn Maurer et al., 2021a, Maurer et al., 2021b, and a decrease of 0.17°C in summer, over which the percentage of female offspring increased from 17% at 85%, and Andrew S. et al., 2022. In 2018, the Mexican Caribbean coast received a massive influx of pelagic Sargassum SPP. that accumulated and decomposed on the beaches turning the water brown in color. ...
The invasive presence of pelagic sargassum on the coasts has increased disproportionately in the last decade, causing great damage to the ecosystems of coastal and marine flora, and fauna, as well as the tourism sector, due to the fact that the sargassum when it enters into decomposition generates fetid odors, detachments of Ammonium concentrations and Hydrogen Sulfide H2S that together with hypoxic conditions were the mass death cause of species, therefore it is necessary to clean affected areas. The aim of this research was to analyze how to reduce erosion in beach dunes, through the technological implementation for the treatment of the mixture sand - dead pelagic sargassum. The methodology had a mixed approach to propose the application of centrifugation and precipitation technologies to significantly reduce beach dunes erosion. However, the machines that do not have this process present a sand-sargassum mixture as residue that, when removed, erodes the dunes. The results obtained were the proposal for the implementation of a new complementary process to those carried out by beach cleaning machines to reduce erosion, in addition to compacting the sargassum for its transfer optimization.
... Given the high nest temperatures experienced by eggs in the Gulf (Loughland 1999;SCENR 2006in Pilcher et al. 2015Al-Ghais 2009;Chatting et al. 2021), and predicted to occur in the Red Sea (Tanabe et al. 2020) and Arabian Sea (Willson et al. 2020), yolkless eggs in hawksbill turtle nests may fulfill roles proposed for leatherback turtles, including that of thermal buffering (Frazier and Salas 1984) and moisture reservoir (Hall 1990;Dutton and McDonald 1995;Wallace et al. 2006). This is a new area of consideration, because other studies of sea turtle responses to warming climates have focused on changes in geographic distribution, reproductive phenology, thermal reaction norms for sex determination, and maternal effects on offspring phenotype (see Maurer et al. 2021). The limited clutch count data available for other species in the northern Indian Ocean suggests that green sea turtles nesting in the Gulf may also lay large numbers of yolkless eggs Al-Mohanna et al. 2014;Table 5), so the functional role and occurrence of yolkless eggs in different species throughout the region also needs to be examined more closely as a potential adaptation to extreme nest environments. ...
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This study presents the first published data on the morphometrics of nesting hawksbill turtles (Eretmochelys imbricata) and their eggs and counts of yolked and yolkless eggs per clutch from Iranian Islands in the northern Persian Gulf. We found significant variation in some variables among 4 main nesting islands (e.g., weight of yolked eggs, p = 0.000; number of yolkless eggs per clutch, p = 0.017), but effect size was moderate or less (r < 0.5 or d < 0.8). The diameter (p = 0.039), weight (p = 0.028), and number of yolked eggs (p = 0.000) per clutch increased significantly with curved carapace length (CCL), but the effect size was only large for the number of yolked eggs (f² = 0.152). The number of yolkless eggs did not vary with CCL (p = 0.453) or with the number of yolked eggs (p = 0.523). A meta-analysis of global hawksbill turtle morphometrics and reproductive output revealed significant variation in CCL (p = 0.000) and number of yolked eggs (p < 0.001), with nesting region having a very large effect size on both variables (g² = 0.880 and 0.616, respectively). Hawksbill turtles from the Gulf were smaller than populations from the Gulf of Oman (p < 0.001), Arabian Sea (p = 0.000), Caribbean (p = 0.000), West Atlantic (p = 0.000), and Southwest Pacific (p = 0.000) but not the Red Sea (p = 0.104), and laid fewer yolked eggs than populations in the Caribbean (p < 0.001) and West Atlantic (p = 0.001) but not the Red Sea (p = 0.636). This may be due to hawksbill turtles nesting in the Gulf remaining within its waters postnesting so adult body size is restricted by the relatively poor foraging habitat and/or success and/or the extreme environments, subsequently limiting clutch size. More information on home range and foraging habitat is required to draw similar conclusions about hawksbill turtles nesting in the Red Sea. Hawksbill turtles nesting in the Gulf, Red Sea, and Arabian Sea also appear to lay larger number of yolkless eggs per clutch than other populations worldwide (mean = 17.6 ± 10.8 SD [range = 0–59]). The functional role of yolkless eggs, potentially in the maintenance of thermal and hydric conditions within tolerable conditions in shallow nests laid in extreme environments, requires further investigation. Similarly, the potential for yolkless eggs to be an adaptation to extreme nest environments in other sea turtle species also nesting in the northern Indian Ocean also warrants examination.
... Whether males exhibit similar migratory patterns merits further study. Hawksbills are highly imperiled in this region (Mortimer and Donnelly 2008) and face threats from human activities and global environmental change (Hamann et al. 2013;Maurer et al. 2015Maurer et al. , 2021aMaurer et al. , 2021bMaurer et al. , 2022. Because the recovery of the Caribbean population will depend in part upon survival of adults, especially considering their high reproductive value (Crouse et al. 1987), identifying and protecting adult migratory and foraging habitats should be among our top priorities for conserving this species. ...
Adult female sea turtles are highly migratory, moving between foraging and nesting areas that can be thousands of kilometers apart. Conserving sea turtles and their habitats therefore depends on knowledge of space use across these migration-linked environments. Here, we describe migratory behavior of hawksbill sea turtles (Eretmochelys imbricata), a globally imperiled species. We used satellite telemetry to characterize the movements of females from nesting areas in Jamaica (n = 4) and Antigua (n = 4), West Indies, over 1998–2001. We mapped migrations and summarized space use during inter-nesting and foraging periods with kernel utilization distributions (UDs) and minimum convex polygons. Seven of eight turtles made post-nesting migrations, with paths ranging 56–1324 km in length, representing straight-line displacements of 68–1206 km. Two turtles sampled in southern Jamaica made short-range migrations within southern Jamaican waters, whereas two from northern Jamaica migrated further to foraging areas in the waters of Belize and Honduras. Three migrants sampled at Long Island, Antigua migrated to St. Eustatius, St. Kitts, and Redonda, respectively, with a fourth individual remaining resident in northeastern Antigua. Inter-nesting movements observed for three turtles produced 50% UDs ranging 12–44 km2, with centroid depths between 4–13 m. Foraging UDs for seven turtles spanned 8–111 km2 and 2–161 m in depth. Our results reveal variable migratory strategies, demonstrate international connectivity between hawksbill foraging and nesting habitats, and provide important information for Caribbean conservation efforts such as the design of protected areas or fisheries policies.
... Sea turtles have Type Ia ESD and might initially benefit from increasing nest temperatures that increase the proportion of females (Tomillo and Spotilla, 2020). However, nest temperatures predicted by current warming models would produce sex ratios that are excessively female-biased by 2050, potentially leading to population declines Maurer et al., 2021). Mitigation practices, such as shading nests or moistening nests with sea water to reduce temperatures have been attempted, but we lack the information needed to evaluate the outcomes (Patrício et al., 2021). ...
Most of the more than 11,000 extant species of nonavian reptiles are squamates (lizards and snakes); there are about 360 extant species of turtles, 26 crocodylians, and one rhynchocephalian. Although the diversity of reptiles is greatest in the tropics, many species occur in temperate regions and a few have geographic ranges that extend north of the Arctic Circle. Antarctica is the only continent with no extant reptiles. Oviparity is the ancestral mode of reproduction, but viviparity has evolved repeatedly among squamates. Both genetic sex determination (XX/XY and ZW/ZZ) and environmental sex determination are represented, and genetic, environmental, and non-genetic maternal factors interact in some species. Environmental sex-determination is universal in crocodylians, widespread among turtles, and present in some clades of squamates. Parental care is universal among crocodylians and is present in some species of squamates and turtles. Ectothermy, an ancestral character, is central to the biology of reptiles, and is responsible for their low metabolic rates and their high efficiency of secondary production. Lizards typically eat daily and consume many small prey items, whereas snakes eat less frequently and consume larger prey items relative to their body size. Low metabolic rates make small body sizes energetically feasible for ectotherms, and more than half of the extant species of lizards are smaller than nearly all mammals and birds. Among squamates, the mode of predation – from sit-and-wait to widely foraging – has a strong phylogenetic component and correlates with many elements of ecology, morphology, physiology, and behavior. Many species of snakes and a few lizards are venomous, and some snakes are poisonous because they sequester toxins from their prey. Although most species of reptiles have little economic value, they are important components of energy and nutrient flow in terrestrial ecosystems. Habitat loss, pollution, invasive species, disease, and global climate change affect many species. The life histories of most large species of turtles, lizards, snakes, and crocodylians depend on prolonged adult survival and reproduction, and these species are vulnerable to commercial exploitation.
1. Reproduction is generally more sensitive to high temperatures than survival and arguably a better predictor of the response of populations to climate change than survival estimates. Still, how temperature simultaneously impacts male and female reproductive success, the mating system and the operational sex ratio remains an open question. 2. Here, we addressed how a sublethal high temperature affects the reproductive system of the haplodiploid spider mite Tetranychus urticae. Males and females maintained at 25°C or 36°C during development were paired and the fertility of both sexes, their mating and remating eagerness, and the paternity of the offspring of females with different mating histories were measured. 3. Female and male fertility decreased at 36°C compared to 25°C, resulting in lower offspring production and a more male‐biased sex ratio, respectively, because of haplodiploidy. However, when either heat‐stressed females or females that mated with heat‐stressed males remated, there was a shift in paternity share, with more than one male contributing to the offspring. This was accompanied by reduced mating eagerness in pairs with partially sterile males and increased remating eagerness in pairs in which at least one sex was partially sterile in the first mating. 4. The observed temperature‐induced changes in female remating eagerness and sperm use allowed restoring the offspring sex ratio, by increasing the proportion of fertilized offspring, but did not lead to the recovery of offspring number. 5. The temperature‐induced changes in the mating behaviour and mating system should alter the interactions within and between the sexes, and with it the strength of sexual selection and sexual conflict in this species. Whether such changes are sufficient to prevent population extinction, despite the inability to recover offspring number, remains an open question.
The ranges of many species are shifting poleward as global warming intensifies, but this process might be less intense in philopatric species, i.e. those with the tendency to reproduce at their natal sites. Marine turtles are highly philopatric and their vulnerability to global warming is exacerbated by a life history combining delayed sexual maturity, temperature‐dependent sex determination and low embryo survival at high incubation temperature. Detection of nesting events of loggerhead turtles in the Western Mediterranean Sea has increased largely during the first two decades of the 21st century, which could be a response to global warming or the result of increased sampling effort. The deployment of temperature data loggers at 52 beaches scattered along the Mediterranean coast of Spain demonstrated that sand temperature is currently high enough to allow the incubation of loggerhead turtles clutches in most Spanish beaches. However, the reconstruction of sand temperature from 1950 to 2019 revealed that thermal conditions suitable for the regular nesting of loggerhead turtles have existed in Spanish beaches only since 2010, although sporadic nesting was possible previously in exceptionally warm years. The future establishment of a self‐sustained population would probably require further increases in temperature, to ensure a female‐biased offspring production, but the process might be jeopardized by increased erosion and beach flooding resulting from sea level rise. Muchas especies están desplazando su rango de distribución geográfica hacia los polos a medida que el calentamiento global se intensifica, pero este proceso podría ser menos intenso en las especies filopátricas, es decir, aquellas con tendencia a reproducirse allí donde nacieron. Las tortugas marinas son muy filopátricas y su vulnerabilidad al calentamiento global se ve incrementada por una historia de vida en la que se combinan una madurez sexual tardía, la determinación del sexo por la temperatura de incubación y una baja supervivencia embrionaria a temperaturas elevadas. La detección de eventos de nidificación de tortuga boba en el Mediterráneo Occidental se ha incrementado notablemente durante las dos primeras décadas del siglo XXI, lo que podría deberse el calentamiento global o a un mayor esfuerzo de muestreo. La colocación de registradores de temperatura en 52 playas repartidas por las costas mediterráneas de España demuestra que la temperatura de la arena es actualmente lo suficientemente elevada como para permitir la incubación de puestas de tortuga boba en la mayoría de las playas del mediterráneo español. Sin embargo, la reconstrucción de la temperatura de la arena entre 1950 y 2019 ha revelado que las condiciones térmicas adecuadas para la puesta regular de esta especie en España se han registrado únicamente desde 2010, si bien las puestas esporádicas eran posibles previamente en años excepcionalmente cálidos. El establecimiento de una población autosuficiente de la tortuga boba en España requeriría un incremento aun mayor de la temperatura, para asegurar la producción de hembras, pero el proceso se vería comprometido por la erosión de las playas resultante del incremento del nivel del mar. Average summer sand temperature (July 20th–August 10th) at 40 cm deep in Spanish Mediterranean beaches, as reconstructed from air temperature at El Prat beach, has increased from 1950 to 2019 (top figure). As a result, the duration of the viable nesting window for loggerhead turtles (bottom figure) has also increased and the thermal environment has become suitable for the development of their clutches since 2010. Arrows denote years when at least one hatchling emerged from one loggerhead turtle clutch laid less than 200 km from El Prat beach.
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Land-dependent marine species are a unique guild of species whose life histories rely on both land and sea. This group is exposed to climate change-related stressors 2-fold, as climate change impacts likely occur at different velocities across land and sea habitat, leading to a greater probability of evolutionary traps. Thus, it is difficult to assess vulnerability and subsequently manage these populations in response to climate change. Without consideration of the factors unique to land-dependent marine species, current vulnerability assessment frameworks may fall short when evaluating climate impacts on these species. We identified commonalities in climate-related threats across taxa and geographic regions, highlighting the specific life history strategies that may be better suited to adapt to the changing climate. Accordingly, we suggest 3 considerations for assessing the vulnerability of land-dependent marine species: (1) degree of specialization, (2) intraspecies population-level differences, and (3) non-climate stressors. Where possible, we suggest how the exclusion of this information in management and conservation planning may lead to less successful outcomes. Potential compounding impacts of multiple stressors puts this group at particular risk of population collapse when losing land and/or sea habitat and functionality. Each of these considerations should be included when assessing vulnerabilities to climate change, as well as in effective and proactive management responses.
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Temperature-dependent sex determination, present in most turtle species, is a mechanism that uses temperature to direct the sex of the embryo. The rapid increase of global temperatures highlights the need for a clear assessment of how sex ratios of organisms with TSD are affected. In turtles with TSD, quantifying primary sex ratios is challenging because they lack external dimorphism and heteromorphic sex chromosomes. Here we describe a new technique used to identify sex in neonate turtles of two TSD species, a freshwater turtle (Trachemys scripta) and a marine turtle (Caretta caretta) via analysis of small blood samples. We used an immunoassay approach to test samples for the presence of several proteins known to play an important role in sex differentiation. Our results show that Anti-Mullerian Hormone (AMH) can be reliably detected in blood samples from neonate male turtles but not females and can be used as a sex-specific marker. Verification of sex via histology or laparoscopy revealed that this method was 100% reliable for identifying sex in both T. scripta and C. caretta 1–2 day-old hatchlings and 90% reliable for identifying sex in 83–177 day-old (120–160 g) loggerhead juveniles. The method described here is minimally invasive, and for the first time, greatly enhances our ability to measure neonate turtle sex ratios at population levels across nesting sites worldwide, a crucial step in assessing the impact of climate change on imperiled turtle species.
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There is increasing concern about feminization of sea turtle populations resulting from female-biased production of hatchlings due to climate change and selective loss of males from other anthropogenic drivers. Extreme female-biased breeding populations would reduce the likelihood of successful mating and potentially result in high rates of infertile eggs. Infertile eggs are those in which none of the events between sperm penetration of the ovum and syngamy have occurred. Distinguishing between fertile and infertile eggs is challenging, especially in field conditions, and researchers often have relied on physical evidence gathered from unhatched eggs at the end of the incubation period, which likely have experienced tissue decomposition. We argue that infertility in sea turtle eggs can be demonstrated only by the absence of holes caused by sperm penetration of the inner perivitelline membrane; sperm bound between the inner and outer perivitelline membranes; nuclei in the blastodisc; embryonic tissue or membranes in egg contents; and/or the characteristic white spot on the egg exterior. Unhatched eggs can be examined at the end of the incubation period, but we recommend that studies specifically investigating infertility examine at least 20 oviposited eggs each from clutches laid by at least 20 different turtles at the peak of the nesting season.
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Reptiles are critically affected by temperature throughout their lifespan, but especially so during early development. Temperature-induced changes in phenotype are a specific example of a broader phenomenon called phenotypic plasticity in which a single individual is able to develop different phenotypes when exposed to different environments. With climate change occurring at an unprecedented rate, it is important to study temperature effects on reptiles. For example, the potential impact of global warming is especially pronounced in species with temperature-dependent sex determination (TSD) because temperature has a direct effect on a key phenotypic (sex) and demographic (population sex ratios) trait. Reptiles with TSD also serve as models for studying temperature effects on the development of other traits that display continuous variation. Temperature directly influences metabolic and developmental rate of embryos and can have permanent effects on phenotype that last beyond the embryonic period. For instance, incubation temperature programs post-hatching hormone production and growth physiology, which can profoundly influence fitness. Here, we review current knowledge of temperature effects on phenotypic and developmental plasticity in reptiles. First, we examine the direct effect of temperature on biophysical processes, the concept of thermal performance curves, and the process of thermal acclimation. After discussing these reversible temperature effects, we focus the bulk of the review on developmental programming of phenotype by temperature during embryogenesis (i.e., permanent developmental effects). We focus on oviparous species because eggs are especially susceptible to changes in ambient temperature. We then discuss recent work probing the role of epigenetic mechanisms in mediating temperature effects on phenotype. Based on phenotypic effects of temperature, we return to the potential impact of global warming on reptiles. Finally, we highlight key areas for future research, including the identification of temperature sensors and assessment of genetic variation for thermosensitivity.
What is the minimum viable population (MVP) of a particular species? Besides the obvious implications for conservation, especially of endangered species, this question raises important issues in population biology. MVP obviously varies with demographic, life history and environmental factors, but also depends upon genetic load and genetic variability. This book addresses the most recent research in the rapidly developing integration of conservation biology with population biology. Chapters consider the roles of demographic and environmental variability; the effects of latitude, body size, patchiness and metapopulation structure; the implications of catastrophes; and the relevance of effective population size on inbreeding and natural selection. Other topics addressed include the role of decision theory in clarifying management alternatives for endangered species, and the opportunities for improved co-operation between agencies responsible for management. The book concludes with a forward-looking and plain-speaking summary on future research and its application for conservation practice.
The adaptive significance of temperature-dependent sex determination (TSD) in reptiles remains unknown decades after TSD was first identified in this group. Concurrently, there is growing concern about the effect that rising temperatures may have on species with TSD, potentially producing extremely biased sex ratios or offspring of only one sex. The current state-of the-art in TSD research on sea turtles is reviewed here and, against current paradigm, it is proposed that TSD provides an advantage under warming climates. By means of coadaptation between early survival and sex ratios, sea turtles are able to maintain populations. When offspring survival declines at high temperatures, the sex that increases future fecundity (females) is produced, increasing resilience to climate warming. TSD could have helped reptiles to survive mass extinctions in the past via this model. Flaws in research on sex determination in sea turtles are also identified and it is suggested that the development of new techniques will revolutionize the field.
Both the development rate and sex of sea turtle embryos depend on incubation temperature, as all species of sea turtle are ectotherms and show temperature‐dependent sex determination (TSD). Theory predicts that selection should act on populations to optimise developmental times and primary sex ratios. In this study, we use a consistent methodology to measure development rates and model the reaction norm that defines TSD in three populations of flatback turtles (Natator depressus) and two populations of green turtles (Chelonia mydas) that nest in Western Australia. We show that development rates vary between and within species, likely reflecting adaptation to local beach microclimates. Similarly, the parameters that define the TSD reaction norm vary between the two species, and among N. depressus populations, with pivotal temperatures (TPIV) varying by 1.5 °C (29.6 – 31.1 °C), and the transitional ranges of temperatures (TRT) varying by 1.4 – 3.3 °C. In contrast, we found a similar TPIV for the two C. mydas populations, but a wider TRT at the northernmost tropical rookery. Our findings support the view that thermal parameters in geographically‐separated populations of sea turtles are broadly similar, but the variation we describe will be highly relevant for predicting how populations will respond to climate change.
Environmental conditions experienced by animals constrain their energy acquisition and its subsequent allocation to growth and reproduction, which ultimately contributes to population dynamics. Understanding how environmental conditions affect these physiological processes is therefore important for predicting how threatened species will respond to altered food and temperature conditions. Here we use a mechanistic modelling approach based on Dynamic Energy Budget (DEB) theory to demonstrate that changing food availability has a strong impact on growth and reproduction for a Western Australian population of green sea turtles (Chelonia mydas), particularly in scenarios with simulated marine heatwaves. Models predicted increasing time between nesting years in scenarios of decreasing food availability. Furthermore, increased frequency of marine heatwaves reduced reproductive output with the number of eggs a female produced in its lifetime predicted to be approximately 20% lower when heatwaves occurred every five years compared to every 20 years. Our predictions suggest that frequent marine heatwaves could have similar adverse effects to long-term decreases in food availability. In all scenarios, direct impacts of changes in temperature were less pronounced and suggest that the strongest impacts of the increasing temperatures of climate change will be mediated through food availability. The approach demonstrated here provides a strong foundation for understanding how the Ningaloo C. mydas population will respond to climate change, and can be refined as new physiological, behavioural, and environmental data become available.
Turtles and tortoises (chelonians) have been integral components of global ecosystems for about 220 million years and have played important roles in human culture for at least 400,000 years. The chelonian shell is a remarkable evolutionary adaptation, facilitating success in terrestrial, freshwater and marine ecosystems. Today, more than half of the 360 living species and 482 total taxa (species and subspecies combined) are threatened with extinction. This places chelonians among the groups with the highest extinction risk of any sizeable vertebrate group. Turtle populations are declining rapidly due to habitat loss, consumption by humans for food and traditional medicines and collection for the international pet trade. Many taxa could become extinct in this century. Here, we examine survival threats to turtles and tortoises and discuss the interventions that will be needed to prevent widespread extinction in this group in coming decades.
How egg temperature sets sex In many reptiles, sex is determined by nest temperature during egg incubation. Temperature regulates the expression of an epigenetic modifier gene called Kdm6b , which is responsible for testis development. However, the molecular connection between temperature and sex-specific expression of this factor was previously unknown. Weber et al. have identified a link between temperature and the activation of a key regulator of Kdm6b called signal transducer and activator of transcription 3 (STAT3). After an influx of Ca ²⁺ at the warmer, female-producing temperature, STAT3 is phosphorylated and silences Kdm6b transcription to repress testis development. Science , this issue p. 303