Prey productivity and predictability drive different axes of life-history variation in carnivorous marsupials

Abstract

Variation in life-history strategies has usually been characterized as a single fast-slow continuum of life-history variation, in which mean lifespan increases with age at maturity as reproductive output at each breeding event declines. Analyses of plants and animals suggest that strategies of reproductive timing can vary on an independent axis, with iteroparous species at one extreme and semelparous species at the other. Insectivorous marsupials in the Family Dasyuridae have an unusually wide range of life-history strategies on both purported axes. We test and confirm that reproductive output and degree of iteroparity are independent in females across species. Variation in reproductive output per episode is associated with mean annual rainfall, which predicts food availability. Position on the iteroparity-semelparity axis is not associated with annual rainfall, but species in regions of unpredictable rainfall have longer maximum lifespans, more potential reproductive events per year, and longer breeding seasons. We suggest that these two axes of life-history variation arise because reproductive output is limited by overall food availability, and selection for high offspring survival favours concentrated breeding in seasonal environments. Longer lifespans are favoured when reproductive opportunities are dispersed over longer periods in environments with less predictable food schedules.

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Cite this article: Collett RA, Baker AM, Fisher
DO. 2018 Prey productivity and predictability
drive different axes of life-history variation in
carnivorous marsupials. Proc. R. Soc. B 285:
20181291.
http://dx.doi.org/10.1098/rspb.2018.1291
Received: 10 June 2018
Accepted: 10 October 2018
Subject Category:
Evolution
Subject Areas:
ecology, evolution, theoretical biology
Keywords:
Dasyuridae, life history, seasonality, fast– slow
continuum, iteroparity, semelparity
Author for correspondence:
Rachael A. Collett
e-mail: rachael.collett@uqconnect.edu.au
Electronic supplementary material is available
online at https://dx.doi.org/10.6084/m9.
figshare.c.4267451.
Prey productivity and predictability drive
different axes of life-history variation in
carnivorous marsupials
Rachael A. Collett1, Andrew M. Baker2and Diana O. Fisher1
1
School of Biological Sciences, University of Queensland, Brisbane, Queensland 4072, Australia
2
School of Earth, Environmental and Biological Sciences, Queensland University of Technology, Brisbane,
Queensland 4000, Australia
RAC, 0000-0002-9727-6622
Variation in life-history strategies has usually been characterized as a single
fast– slow continuum of life-history variation, in which mean lifespan
increases with age at maturity as reproductive output at each breeding
event declines. Analyses of plants and animals suggest that strategies of
reproductive timing can vary on an independent axis, with iteroparous
species at one extreme and semelparous species at the other. Insectivorous
marsupials in the Family Dasyuridae have an unusually wide range of
life-history strategies on both purported axes. We test and confirm that
reproductive output and degree of iteroparity are independent in females
across species. Variation in reproductive output per episode is associated
with mean annual rainfall, which predicts food availability. Position on
the iteroparity-semelparity axis is not associated with annual rainfall, but
species in regions of unpredictable rainfall have longer maximum life-
spans, more potential reproductive events per year, and longer
breeding seasons. We suggest that these two axes of life-history variation
arise because reproductive output is limited by overall food availability,
and selection for high offspring survival favours concentrated breeding
in seasonal environments. Longer lifespans are favoured when reproduc-
tive opportunities are dispersed over longer periods in environments
with less predictable food schedules.
1. Introduction
Variation between species in schedules of survival, growth, and reproduction is
usually considered on one axis of life-history variation from fast to slow [1– 3],
assuming that trade-offs between age at maturity, fertility, and lifespan con-
strain life-history strategies, so that species invest most in either reproduction
(faster species) or survival (slower species) [4,5]. However, several analyses
have suggested that the degree of iteroparity (i.e. breeding repeatedly in a
dispersed time period at one end of the spectrum and breeding once in a con-
centrated time period at the other) is independent of the fast–slow continuum.
That is, the degree of iteroparity (number and spacing of reproductive events) is
not necessarily traded off with life-history speed (investment in reproduction
versus longevity). Stearns [6] found a secondary precociality–altriciality conti-
nuum in mammals after accounting for the slow– fast continuum, and Gaillard
et al. [7] identified this as part of a semelparity-iteroparity axis that accounts for
up to 15% of variation in birds and mammals. In a more recent factor analysis of
mammals, Bielby et al. [5] identified a factor that explained up to half of the
variance between species, and included maturity, weaning time, and time
between reproductive bouts. A second factor explained a further quarter of
the variance and described output per reproductive episode. Species at one
end produced large litters of small young and species at the other end invested
more in large but few young. Bielby et al. [5] interpreted species position on
this output axis in terms of the well-known offspring number versus quality
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trade-off, which is grounded in physiological constraints
[8,9]. Salguero-Gomez et al. [10] have recently also demon-
strated that the fast–slow continuum in plants includes
traits of growth rate and lifespan on one axis and degree of
iteroparity on another. Iteroparity is a risk-spreading repro-
ductive tactic and is likely to be an advantage in variable
environments. When conditions favouring offspring survival
are unpredictable, bet-hedging by repeated breeding
increases fitness, given a trade-off between reproduction
and survival [11–13]. Bielby et al. [5] called for research to test
how the iteroparity and reproductive output axes in mammal
life-history variation are associated with environmental
variability.
Reproductive output, which includes number and size of
offspring, is expected to be constrained by food availability
and a female’s ability to use energy and nutrients for repro-
duction [1,14,15]. In marsupial taxa, adoption of higher
energy diets has been associated with evolutionary switches
to higher reproductive rates. Corresponding trade-offs
between reproductive output and other life-history traits
have led to the evolution of fast life-history strategies in car-
nivorous species [16]. Rainfall and temperature seasonality
might also influence age-specific survival [17], which deter-
mines how species should trade off reproduction and
survival [18]. We therefore predict that in carnivorous marsu-
pials from the Family Dasyuridae, which are distributed
widely across climate zones, features of climate that increase
overall arthropod availability will be correlated with higher
reproductive output in terms of litter size per reproductive
bout and a faster life history.
Adaptive reproductive timing coincides with events that
maximize offspring survival, such as seasonal rainfall and
peaks in prey abundance [11,19,20]. For example, the desert
chameleon (Furcifer labordi) from Madagascar has evolved
semelparity and extended incubation time in an arid, seaso-
nal environment [21]. Similarly, Australian dasyurids with
late maturity, monoestry and semelparity occur where there
are predictable annual peaks in arthropod abundance,
because only one favourable time to wean young per year
is possible, given that the marsupial trait of long lactation
precludes breeding in the season of an individual’s birth
[22]. We therefore predict that features of climate that increase
food seasonality and the predictability of peaks in prey abun-
dance will be correlated with semelparity in female
dasyurids.
Using databases of life-history traits (see the electronic
supplementary material for source references), location
records in the Australian marsupial Family Dasyuridae, and
long-term climate data, we test how female reproductive
output, degree of iteroparity, and lifespan covary with food
abundance and seasonal predictability of food.
(a) Specific predictions
We hypothesise that reproductive output and degree of iter-
oparity in females are independent: output will depend on
food availability and not food predictability, whereas
degree of iteroparity will depend on food predictability and
not food availability. We therefore predict that litter size
and the number of neonates at birth will covary with the
amount of rainfall, and lifespan, length of the annual repro-
ductive season, and the number of reproductive attempts
will covary with rainfall predictability.
2. Methods
(a) Study taxa
Dasyurids are predominantly insectivorous, range in size from
less than 5 g to 9 kg, and have a maximum lifespan of 1 –6
years [23]. The maximum number of young that can be reared
is determined by the number of teats, which vary from two to
14. Some groups such as antechinus produce supernumerary
young: they give birth to more young than the number of teats,
so some inevitably die at birth. Other species produce fewer
young than the number of teats [24]. In seasonal breeders, the
reproductive season lasts for two weeks to six months, depend-
ing on the species. Uniquely in mammals, dasyurid males
include the entire spectrum from obligate semelparity to itero-
parity. Females can breed multiple times and vary from
virtually semelparous to continuous breeding [16,18,25]. Com-
plete male die-off occurs in 20% of dasyurid species (Fisher
et al. [22]), including: all in the genera Antechinus,Phascogale,
and Dasykaluta [26]; Dasyurus and Parantechinus each contain a
single species with facultatively semelparous males [27,28].
Females from some species are monoestrous (i.e. breed once a
year), while others are polyoestrous (i.e. can produce multiple
litters per season) [29].
(b) Data
We collated published female life-history data on 34 Australian
dasyurids taken from 82 studies (table 1; electronic supplemen-
tary material). We only included species that are
predominantly insectivorous (arthropods are greater than 75%
of their diet), because associations between rainfall and arthro-
pod availability have been quantified [22], allowing us to use
rainfall as a proxy for food availability [30]. Traits analysed
included: body mass at adulthood, maximum lifespan, polyestry
versus monoestry, duration of reproductive season (indicating
number of possible reproductive attempts), litter size, and
number of supernumerary young. Where possible, we used pub-
lished field studies. Because potential within-breeding episode
trade-offs with short-term food supply are likely to be important
in dasyurids [30], we used offspring number per reproductive
bout rather than a ratio of long-term output over time such as
reproductive rate. We used litter sizes recorded within a week
of birth, because mothers may progressively lose pouch young
during lactation. We calculated mean values for traits when
there were multiple studies of the same species. We defined the
duration of the reproductive season as the number of weeks
with births [31].
We used rainfall as a proxy for arthropod availability (abun-
dance and activity) [32]. For each species, we used mean annual
rainfall at the centroid of geographical range based on all
recorded locations [22]. We calculated seasonal predictability
of rainfall by collating monthly rainfall from the Bureau of
Meteorology [33] at the study sites where life-history infor-
mation was collected. We gathered these data for the 10 years
preceding the end of the study, as Fisher et al.[22]foundthat
3–8 years of insect and climate data gave repeatable results
and clear outcomes in tests of hypotheses at these sites. For
each site, we categorized monthly rainfall as ‘high’ if it was in
the top 25% of abundances, or ‘low’ if it was in the lower
75% of abundances. We used these categorical data to calculate
a Colwell index for rainfall at each site where marsupial life-his-
tory data were collected [22]. Colwell’s index (P) uses
categorical data to measure how tightly an event is linked to a
season. P is composed of C (constancy) and M (contingency).
Constancy describes how uniform the event is across seasons.
Contingency measures the repeatability of seasonal patterns
between years. P is maximized when the event occurs con-
stantly throughout the year or if the pattern of occurrence is
repeated across years [34].
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(c) Statistical analyses
We log-transformed body mass, lifespan, and length of the repro-
ductive season and arcsin-transformed rainfall predictability (P)
to normalize the distributions [35]. We used phylogenetic gener-
alized least-squares (PGLS) models in R, using the packages ape
[36] and nlme [37] to test the relationships between predictor
variables: annual rainfall, rainfall predictability (P), lifespan,
length of the reproductive season, and polyestry, and response
variables: lifespan, length of the reproductive season, litter size,
and supernumerary young, incorporating phylogenetic infor-
mation and body mass into models. We used a recent
marsupial phylogeny [38] to account for interspecific autocorre-
lation due to phylogeny [39]. We used a multivariate normal
prior for the phylogenetic random effects, with unit variances
and correlation structure derived from the phylogenetic tree
using Grafen’s branch lengths [40]. We calculated a pseudo
r-squared for each PGLS model [41].
3. Results
(a) Trade-offs and climate predictors of reproductive
output
In agreement with our hypothesis that food availability limits
reproductive output, species in more arid climates produced
fewer young per reproductive bout (litter size versus mean
annual rainfall: t¼2.72, p¼0.01, d.f. ¼34, slope ¼0.001,
s.e. ¼0.0005; figure 1). Litter size was negatively associated
with mass (t¼22.89, p¼0.007, d.f. ¼34, slope ¼20.68,
s.e. ¼0.24). Species with larger litters were more likely to
have supernumerary young (t¼3.47, p¼0.002, d.f. ¼34,
slope ¼1.64, s.e. ¼0.47), and the number of supernumerary
young was correlated with annual rainfall (t¼2.14, p¼
0.04, d.f. ¼34, slope ¼358, s.e. ¼167), further supporting
our prediction that there would be a positive relationship
between food availability and reproductive output. Species
occurring in Australia’s arid and semi-arid zones (less than
350 mm annual rainfall) never had more than seven young,
and only one desert species, the kowari (Dasyuroides byrnei),
produced any supernumerary young. In agreement with
our prediction that reproductive output would not vary
with food predictability, litter size was not significantly
related to P (rainfall predictability) (t¼20.4, p¼0.69,
d.f. ¼34, slope ¼20.79, s.e. ¼2) (pseudo r-squared for
model one ¼0.32). Litter size was also not associated with
traits that indicate the degree of iteroparity in females
(litter size versus length of reproductive season: t¼21.85,
10
0
500 1 5001 000
Partial residuals of annual rainfall (mm)
Litter size (no. pouch young)
2
4
6
8
Figure 1. The association between litter size and partial residuals of mean
annual rainfall for Australian insectivorous dasyurid species. The line indicates
the fitted regression from model one, including 95% confidence intervals.
Table 1. Dasyurid species included in this study and the number of
published studies data was collated from. PTR ( personal trapping records)
and PC (personal correspondence).
Species No. of studies
Antechinomys laniger 2
Antechinus agilis 1 and PTR
Antechinus bellus 2
Antechinus flavipes 3 and PTR
Antechinus godmani 1 and PTR
Antechinus leo 2
Antechinus minimus 3
Antechinus stuartii 1 and PTR
Antechinus subtropicus PTR
Antechinus mimetes 2
Dasycercus cristicauda PC
Dasykaluta rosamondae 3
Dasyuroides byrnei 2
Dasyurus hallucatus 3
Dasyurus viverrinus 2
Ningaui ridei 3
Parantechinus apicalis 3
Parantechinus bilarni 3
Phascogale calura 2
Phascogale tapoatafa 4
Planigale gilesi 3
Planigale ingrami 3
Planigale maculata 2
Planigale tenuirostris 3
Pseudantechinus macdonnellensis 2
Pseudantechinus ningbing 1
Sminthopsis crassicaudata 4
Sminthopsis douglasi 1
Sminthopsis griseoventer 1 and PC
Sminthopsis leucopus 3
Sminthopsis macroura 4
Sminthopsis murina 2
Sminthopsis ooldea 2
Sminthopsis virginiae 2
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p¼0.07, d.f. ¼34, slope ¼20.08, s.e. ¼0.04; litter size
versus polyestry: t¼1.19, p¼0.24, d.f. ¼34, slope ¼1.1,
s.e. ¼0.92) and dasyurids do not trade off litter size against
lifespan (litter size versus lifespan: t¼20.04, p¼0.97,
d.f. ¼34, slope ¼20.04, s.e. ¼1.03) (pseudo r-squared for
model two ¼0.25). Litter size of species in our study
ranged from four to 10.
(b) Trade-offs and climate predictors of the degree of
iteroparity
Female maximum lifespan of species in our study ranged
from one to six years and were positively associated with
mass (t¼2.27, p¼0.03, slope ¼0.11, s.e. ¼0.05). As pre-
dicted, lifespan was longer in areas with more
unpredictable food supplies (lifespan versus rainfall predict-
ability index P: t¼23.23, p¼0.003, d.f. ¼34, slope ¼1.24,
s.e. ¼0.38, figure 2) (pseudo r-squared for model three ¼
0.32). Species with long lifespans are more likely to have
long reproductive seasons (lifespan versus reproductive
season length: t¼4.29, p¼0.0002, d.f. ¼34, slope ¼0.02,
s.e. ¼0.005) ( pseudo r-squared for model four ¼0.38) and
to have multiple litters per season (lifespan versus polyestry:
t¼2.51, p¼0.02, d.f. ¼34, slope ¼0.25, s.e. ¼0.1) (pseudo
r-squared for model five ¼0.19). Reproductive season
length alone was also strongly associated with rainfall pre-
dictability (reproductive season length versus rainfall
predictability index P: t¼24.73, p¼0.0001, d.f. ¼34,
slope ¼20.89, s.e. ¼0.26, figure 3) (pseudo r-squared for
model six ¼0.14). This supports our hypothesis that adap-
tation to a seasonal climate, and therefore predictability of
food schedules, favours a short reproductive period in
seasonal environments, and a long lifespan with repeat
breeding over a long period is more likely to evolve where
there is less predictable rainfall. Annual rainfall did not sig-
nificantly predict rainfall predictability (t¼1.14, p¼0.26,
d.f. ¼32, slope ¼731, s.e. ¼643.1), as some regions of arid
Australia where dasyurids were sampled have highly predict-
able rainfall, and some more mesic areas have unpredictable
rainfall (figure 4). For example, Ningaui timealeyi (body
weight 5.8 g) has a maximum lifespan of one year, and
although its Western Australia Pilbara location is a dry
environment, summer cyclones are common and most
annual rainfalls predictably in February [43]. Planigale gilesi
(body weight 6.9 g) in arid western New South Wales is simi-
lar in size and ecology but lives for a maximum of five years
in a region where low, annual rainfall falls unpredictably
across the year [44].
4. Discussion
Our results agree with several previous analyses, which con-
cluded that aspects of the fast–slow continuum are
independent of the semelparity-iteroparity axis in mammals.
We focused on offspring number, because previous studies
revealing multiple axes of life-history variation in mammals
identified reproductive output as a key variable [5,7]. The
theory basis of within-bout trade-offs with litter size is well
established [15,45]. Experiments and descriptive tests in
small eutherian mammals have shown that the trade-off
between the number and prenatal growth rate of offspring
in a litter is strongly affected by physiological constraints of
energy, nutrients, temperature, and tissue capacity. Total
0.5
1.6
0.4
0.6
0.8
1.0
1.2
1.4
1.00.90.80.7
Partial residuals of sin Colwell’s seasonalit
y
index (P)
Log female maximum lifespan (years)
0.6
Figure 2. The association between log female maximum lifespan and partial
residuals of sin Colwell’s predictability index of rainfall for Australian insecti-
vorous dasyurid species. The line indicates the fitted regression from model
three, including 95% confidence intervals.
0.5
1.5
Log10 length of breeding season (weeks)
0
0.5
1.0
1.00.90.80.7
Partial residuals of sin Colwell’s seasonality index (P)
0.6
Figure 3. The association between log10 length of the breeding season and
partial residuals of sin Colwell’s predictability index of rainfall for Australian
insectivorous dasyurid species. The line indicates the fitted regression from
model six, including 95% confidence intervals.
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investment in reproduction is expected to reduce long-term
survival under the disposable soma theory, which states
that investment in reproduction reduces individual somatic
maintenance [46,47]. In an environment with high extrinsic
mortality in adults, organisms should invest in early and
high reproductive output rather than long-term maintenance
and survival. Position of a species on the fast– slow conti-
nuum is therefore expected to depend on aspects of its
ecology and environment that affect age-specific mortality
risk [48]. Litter size and growth is traded off within each
reproductive episode based on maternal investment capacity
at the time [9,15]. However, distributing this investment over
a longer breeding period does not necessarily change the
upper limit on the number of offspring per litter. For
example, rate of milk transfer and heat production are mech-
anisms limiting investment within a reproductive bout
[15,45]. Habitats and ecology that cause higher extrinsic mor-
tality risk do not necessarily have higher or lower seasonal
predictability of food. If they do, the direction of selection
can be reversed. For example, Reznick et al. [49] found that
guppies in high predation sites evolved faster reproduction
when high predation environments had scarcer food, perhaps
because predators indirectly reduced net mortality by redu-
cing density and thus competition for food. In variable
environments, organisms that hedge their bets by dispersing
reproductive effort over a longer breeding season and have a
longer reproductive lifespan have a lower risk of failure
[11,45]. Orzack & Tuljapurkar [50] showed that unpredictable
environments could favour either high or low reproductive
output through their effect on reproductive costs.
In our study, rainfall seasonality was unrelated to annual
rainfall. Therefore, aspects of the environment that affect
whether iteroparity or semelparity is likely to lead to greater
fitness in females are at least partly disconnected from
aspects of the environment that affect whether females can
invest in large litters and whether mortality risk and repro-
ductive costs are likely to lead to higher fitness in females
that increase reproductive effort.
As predicted, a climate variable related to the predictabil-
ity of peaks in prey abundance (rainfall predictability) was
correlated with species position on the semelparity-iteropar-
ity axis, and a variable that alters food availability and
reflects energy limitation (annual rainfall) was associated
with variation in reproductive output. These findings con-
cord with some previous predictions in mammals and
other vertebrates. For example, in the mammal family Lepor-
idae (rabbits and hares) temperature seasonality predicted
71% of the global variation in litter size and body size, and
the authors interpreted this in terms of food limitation
caused by seasonality. Unpredictable timing of stressful
environmental conditions was associated with increased iter-
oparity, whereas nest predation rate predicted 55% of
variation in the timing trait of gestation duration [19]. In
endemic mammal families in Madagascar, iteroparity invol-
ving short intervals between breeding episodes, a long
breeding season, and high adult survival is common, and
this has been attributed to the particularly unpredictable
timing of rainfall on this island [11]. Comparing desert popu-
lations of a ground squirrel on a gradient of increasing
seasonal predictability, Whorley [20] also found that more
unpredictable rainfall was associated with longer breeding
seasons, lower synchrony, and smaller litter size. In Rose’s
mountain toadlet (Capensibufo rosei), 94% of variation in
toad lifespan between years is explained by variation in
breeding season rainfall. In dry years, survival is increased
and reproductive output is low, and in wet years, toads
increase reproduction at the expense of survival [51].
In dasyurids, we found that species with large litters were
more likely to occur in high rainfall habitats and to have
supernumerary births. Arid zone species rarely had
high: 4 160mm
Annual rainfall
low: 130 mm
Figure 4. The centroid point of the geographical range of dasyurid species included in this study and mean annual rainfall throughout Australia. Species are marked
with a Dif Colwell’s P is less than 0.7 (less seasonally predictable) and a Wif Colwell’s P is equal to or more than 0.7 (more seasonally predictable). For full species
names see table 1. Rainfall raster data were taken from Reside et al. [42].
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supernumerary young and often failed to have all teats occu-
pied by neonates, suggesting they cannot reliably obtain
enough food to produce excess young. We conclude that
energy or nutrient availability constrains female reproductive
output, consistent with many studies of limitations to repro-
ductive output in small mammals (e.g. [16,52–58]. For
example, Sibly & Brown [58] found that mass of mammal
neonate tissue was associated with reliable and abundant
food. However, seasonality is often also associated with
reproductive output, because seasonal environments have a
reliable annual pulse of abundant food, especially at high lati-
tudes. For example, offspring number often increases with
environmental seasonality in birds [59,60] and mammals,
including European lagomorphs [19], boars (Sus scrofa) [61],
and ground squirrels (Ammospermophilus leucurus) [20]. Simi-
lar trends have not been obvious in carnivorous and Southern
Hemisphere mammals at lower latitudes [62,63], with the
exception of Antechinus agilis in the relatively low latitude
of southern Australia [64]. Unlike seasonal Australian
environments, Northern Hemisphere habitats with severe
winters have large seasonal peaks in food availability relative
to the scarcest season [59].
We found that degree of iteroparity in female dasyurids
across the continent was correlated with predictability of rain-
fall and thus schedules of reliable food availability. Species in
environments with seasonally predictable rainfall were more
likely to be monoestrous, have shorter lifespans, and shorter
reproductive seasons. These species time reproduction so that
late lactation, which is energetically costly [56], coincides
with the peak in arthropod availability [22]. Female dasyur-
ids in regions of unpredictable rainfall live longer, are more
likely to be polyestrous, and have longer reproductive
seasons. The opportunity for multiple breeding attempts
over several years is likely to be adaptive if survival of
young is highly variable [12,65], as a long reproductive
period enables bet-hedging, which increases the likelihood
of some births during times of high food availability [66].
Bet-hedging strategies occur in plants (desert annuals)
[13], bees (Perdita portalis) [67], tortoises (Gopherus agassizii)
[68], primates [66], and many other taxa, which spread
their reproductive effort over multiple episodes in
unpredictable environments [65,69].
Patterns of rainfall explained significant variation in pro-
duction of young, lifespan, and length of the reproductive
season. However, there was still a large proportion of var-
iance unexplained by our models. These effects might be
mediated by competition and population density [70], temp-
erature [45], rates of age-specific predation [49,71–73], and
torpor [74], which would be promising future avenues for
further understanding of the mechanisms.
Data accessibility. The datasets supporting this article have been
uploaded as part of the electronic supplementary material.
Authors’ contributions. R.A.C. and D.O.F. created the database. R.A.C.
performed the analyses. R.A.C., D.O.F., and A.M.B. contributed to
the manuscript. All the authors gave their final approval for
publication.
Competing interests. We declare we have no competing interests.
Funding. This research is supported by the Australian Government’s
National Environmental Science Program through the Threatened
Species Recovery Hub and an Australian Research Council fellow-
ship, Grant/Award no. FTll0100191.
Acknowledgements. We thank Simon Blomberg for assistance with R
scripting and April Reside for assistance with mapping.
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