Gray Whale Sex, Reproductive Behavior, and Social Strategies

Abstract

Gray whale sexual behavior and copulation are observed throughout their range. The most prominent period for reproductive behavior is during the southward migration from summer feeding areas to wintering areas where some breeding occurs and calves of the year are reared. The seasonal migrations of gray whales are believed to function, in part, to bring together individuals that are otherwise widely distributed during the period of estrus to facilitate mating and reproduction. Sexual behaviors and sexual strategies for this species appear to align closely with those of balaenid (not rorqual) whales, although such comparisons need further investigation. Gray whales are polygynandrous (multi-mate) breeders. There does not appear to be female choice of mates, as groups of numerous females and males aggregate, and multiple copulations occur. Female estrus begins in mid-November and continues to early December; females may undergo a second estrus, extending into February, if they fail to conceive during their first cycle. Male gray whales have large testes and concomitantly produce large volumes of sperm, so they are believed to be sperm competitors; that is, they rely on multiple copulations (and sperm volume) to produce offspring. Multiple copulations with different males during the female estrus period may increase the likelihood that the timing of conception results in the birth of a calf approximately 13 months later near or in the wintering area(s). Mating bouts can last for minutes to hours, interspersed with surface-active-social-sexual behavior. Some all-male groups have been observed with erect penises engaged in social-sexual behavior in the absence of any females. Instances of male aggression toward postpartum females with calves of the year, sometimes resulting in injury or death, have been reported. As a result of dedicated long-term research in the past several decades, the state of knowledge on gray whale reproduction has greatly expanded and updated information on this topic is summarized in this chapter.

Full text

https://doi.org/10.1007/978-3-031-35651-3_21
Chapter 21
Gray Whale Sex, Reproductive Behavior,
and Social Strategies
Steven L. Swartz, Aimee Lang, Alexander Burdin, John Calambokidis,
Héloïse Frouin-Mouy, Sergio Martínez-Aguilar,
Fabian Rodríguez-González, Ludovic Tenorio-Hallé, Aaron Thode,
Jorge Urbán-Ramírez, and David W. Weller
Abstract Gray whale sexual behavior and copulation are observ ed throughout their
range. The most prominent period for reproductive behavior is during the southward
migration from summer feeding areas to wintering areas where some breeding
occurs and calves of the year are reared. The seasonal migrations of gray whales
are believed to function, in part, to bring together individuals that are otherwise
widely distributed during the period of estrus to facilitate mating and reproduction.
S. L. Swartz (✉)
Laguna San Ignacio Ecosystem Science Program (LSIESP), Darnestown, MD, USA
A. Lang · D. W. Weller
NOAA Fisheries, Southwest Fisheries Science Center, La Jolla, CA, USA
A. Burdin
Kamchatka Branch of Pacific Institute of Geography, Far East Branch, Russian Academy of
Sciences, Petropavlovsk, Kamchatka, Russia
J. Calambokidis
Cascadia Research Collective, Olympia, WA, USA
H. Frouin-Mouy
Cooperative Programs for the Advancement of Earth Science, University Corporation for
Atmospheric Research, Boulder, CO, USA
S. Martínez-Aguilar · F. Rodríguez-González · J. Urbán-Ramírez
Laguna San Ignacio Ecosystem Science Program (LSIESP), Darnestown, MD, USA
Departamento de Ciencias Marinas y Costeras, Universidad Autónoma de Baja California Sur,
La Paz, Baja California Sur, Mexico
L. Tenorio-Hallé
Cooperative Institute for Marine and Atmospheric Studies, Rosenstiel School for Marine and
Atmospheric Science, University of Miami, Miami, FL, USA
A. Thode
Laguna San Ignacio Ecosystem Science Program (LSIESP), Darnestown, MD, USA
Marine Physical Laboratory, Scripps Institution of Oceanography, University of California San
Diego, La Jolla, USA
© The Author(s) 2023
B. Würsig, D. N. Orbach (eds.), Sex in Cetaceans,
499

Sexual behaviors and sexual strategies for this species appear to align closely with
those of balaenid (not rorqual) whales, although such comparisons need further
investigation. Gray whales are polygynandrous (multi-mate) breeders. There does
not appear to be female choice of mates, as groups of numerous females and males
aggregate, and multiple copulations occur. Female estrus begins in mid-November
and continues to early December; females may undergo a second estrus, extending
into February , if they fail to conceive during their first cycle. Male gray whales have
large testes and concomitantly produce large volumes of sperm, so they are believed
to be sperm competitors; that is, they rely on multiple copulations (and sperm
volume) to produce offspring. Multiple copulations with different males during the
female estrus period may increase the likelihood that the timing of conception
results in the birth of a calf approximately 13 months later near or in the wintering
area(s). Mating bouts can last for minutes to hours, interspersed with surface-active-
social-sexual behavior. Some all-male groups have been observed with erect
penises engaged in social-sexual behavior in the absence of any females. Instances
of male aggre ssion toward postpartum females with calves of the year, sometimes
resulting in injury or death, have been reported. As a result of dedicated long-term
research in the past several decades, the state of knowledge on gray whale repro-
duction has greatly expanded and updated information on this topic is summarized
in this chapter.
500 S. L. Swartz et al.
Keywords Acoustics · Eastern North Pacific population · Gray whales · Male
aggression · Migration · Polygynandrous (multi-mate) breeding · Scramble
competition · Sexual strategies · Social-sexual behavior · Sperm competition ·
Western North Pacific population
21.1 Migration and the Reproductive Cycle
Gray whales occur along the margins of the eastern North Pacific (ENP) and western
North Pacific (WNP), migrating annually from northern feeding areas to southern
wintering areas that represent a round trip journey of 15,000–20,000 km that spans
up to 55° latitude (Swartz 2018). Seasonally predictable sources of food have helped
shape the life history of gray whales into two general periods that are linked by the
annual migration: (a) summer feeding in the higher latitude waters wher e food is
abundant and whales are widely dispersed across a large territory and
(b) overwintering in lower latitude waters, where whales are more closely aggre-
gated. The seasonal occurrence in southern wintering areas serves several hypothe-
sized functions, including (a) escape from inhospitable environmental conditions
(e.g., sea ice, water temperature) in northern areas, (b) reduction of risk from killer
whale predation during the period when calves are born (Corkeron and Connor
1999; Sumich 2014; Weller et al. 2018; Black et al. 2023), and (c) the successful
rearing and survival of calves (Jones and Swartz 2009). The gray whale reproductive
cycle is tightly synchronized with this migration and this relation is described below.

21 Gray Whale Sex, Reproductive Behavior, and Social Strategies 501
21.1.1 Migration and Reproductive Timing
The southward migration helps to concentrate whales spatially and temporally,
thereby facilitating mating. By late November, most gray whales are moving south
from summer feeding areas to wintering areas. This southern migration is segregated
by age, sex, and reproductive condition. The first pulse of migrants is led by (a) near-
term pregnant females, followed by (b) estrus females and mature males and then
(c) immature animals of both sexes (Rice and Wolman 1971). The northward
migration begins about mid-February and is also segregated according to age, sex,
and reproductive condition. The first phase of this northward migration includes
(a) newly pregnant females followed later by (b) adult males and anestrus females
and then (c) immature whales of both sexes. The second phase of the northward
migration consists mostly of mothers with calves. These pairs are observed on the
migration route between March and May and they generally arrive to the summer
feeding grounds between May and June (Jones and Swartz 1984).
During summer and fall, most whales in the ENP feed in the Chukchi, Beaufort,
and northwestern Bering Seas. An exception to this generality is a relatively small
number (roughly 200) of “Pacific Coast Feeding Group” (PCFG) whales that
routinely return each summer and feed along the Pacific coast between southeast
Alaska and northern California (Darling 1984; Calambokidis et al. 2002; Moore
et al. 2007). Three primary wintering aggregation areas and lagoons in Baja Cali-
fornia Sur, Mexico, are utilized, and some females are known to make repeated
returns to specific lagoons (Jones 1990; Martínez-Aguilar et al. 2023). In the WNP,
gray whales feed off Far East Russia during summer and fall in the western Okhotsk
Sea and southeastern Bering Sea (Weller et al. 1999, 2002; Tyurneva et al. 2010).
Historical accounts (Andrews 1914; Nishiwaki and Kasuya 1970; Wang 1984) and
contemporary data (Weller et al. 2008, 2016; Nakamura et al. 2021) indicate that
coastal waters off Asia were, and continue to be, part of a WNP migratory route and
portions of the South China Sea are reputed wintering areas (Weller et al. 2002,
2013). Until about the mid-2000s, whales in the WNP were thought to be geograph-
ically isolated from whales in the ENP; however, satellite-tagging, photo-identifica-
tion, and genetic research has documented some whales identified during the
summer in the WNP migrating to the ENP in the winter, including coastal waters
off Canada, the USA, and Mexico (Weller et al. 2012, 2016; Mate et al. 2015;
Martínez-Aguilar et al. 2022). These findings suggest that some whales summering
in the WNP migrate to the ENP west coast of North America in winter, while others
migrate south to waters off Japan and China (Weller et al. 2008, 2013, 2016).
Studies of gray whales summering in areas off the Pacific Northwest between
southeastern Alaska and Northern California and off Sakhalin Island in Far East
Russia have found intra- and inter-annual fidelity for many of the whales observed
(Darling 1984; Weller et al. 1999; Calambokidis et al. 2002, 2017; Bröker et al.
2020). While this same pattern of seasonal site fidelity may exist in various parts
of the Arctic feeding grounds, data from those areas do not exist at the level
necessary to determine such.

502 S. L. Swartz et al.
21.1.2 Implications of Summer Feeding Location on Mating
With the above in mind, the location(s) where whales feed in the summer has
possible implications for mating. For instance, some information on the timing of
migration has been gleaned from three whales satellite-tagged during 2010 and 2011
on the Sakhalin Island summer feeding area in the WNP. All three whales migrated
east, with one (a 13-year-old male) remaining off Sakhalin until 10 December while
the other two (both females, ages six and nine) remained there until 24 November
before migrating to areas occupied by ENP gray whales (Mate et al. 2015). These
data indicate that at least some, and perhaps all, animals making the winter migration
from the WNP to the ENP are still far to the west, and therefore only in proximity to
other whales migrating from the WNP, during the primary conception period that
occurs between late November and early December (Rice and Wolman 1971). The
plausibility of this possibility is consistent with the results of genetic analyses, in
which significant nuclear genetic differences have been found between the whales
that feed off Sakhalin Island in the WNP and gray whales in the ENP (Brüniche-
Olsen et al. 2018; Lang et al. 2022). These differences indicate that whales feeding
off Sakhalin Island are not mating at random with ENP gray whales, a finding that
suggests assortative mating may be occurring as a result of location (i.e., summer
area) and migratory timing (Lang et al. 2022).
Similarly, the above scenario for whales summering in the WNP may also apply
to PCFG gray whales and other similar seasonal aggregations that may exist in the
ENP. Over 50% of PCFG individuals have been sighted in areas off the Pacific
Northwest in Nove mber and December despite a limited amount of effort. Satellite-
tagging research on PCFG gray whales revealed that 18 whales started their south-
ward migrations between 3 December to 13 February (Lagerquist et al. 2019), a
period of time that is later than the estimated period of conception between
mid-November to early December (Rice and Wolman 1971). Although these factors
could result in a higher likelihood of assortative mating among PCFG whales,
genetic analyses do not support such a hypothesis; research has yet to detect
significant nuclear DNA differences when PCFG whales are compared with ENP
whales that feed in Arctic waters or those using the wintering aggregation areas and
lagoons in the Baja California peninsula, Mexico (D’Intino et al. 2013; Lang et al.
2014).
21.2 Reproductive Life History
21.2.1 Estrus and Conception
Most gray whale conceptions occur each year during a 3-week period, overlapping
the early portion of the southward migration, from mid-November to early
December (Rice and Wolman 1971). In the ENP, studies have shown that sexual

activity and copulatory behavior occur during migration (Norris et al. 1983; Swartz
1986) and continue once the whales have reached their wintering areas off Mexico.
These observations seemingly support the suggestion that females that failed to
conceive after their first ovulation may experience a second estrus cycle that is
later, possibly extending into February, in the same breeding season (Rice and
Wolman 1971). Newly pregnant females are the first to leave the Baja California
peninsula wintering areas in the spring, migrating to summer feeding areas to begin
replenishing their metabolic “energy stores” that had been greatly depleted during
the previous southward migration in combination with the nutritional demands
related to the growth of their fetuses (Sumich 2014).
21 Gray Whale Sex, Reproductive Behavior, and Social Strategies 503
21.2.2 Age, Sexual Maturity, and Sex Ratio
The oldest female gray whale killed during scientific whaling operations between
1959 and 1969 in the ENP was estimated to be 75 years old and was pregnant (Rice
and Wolman 1971). Minimum age estimates for individual living gray whales photo-
identified in the Mexican wintering areas range from 45 to 53 years (Martínez-
Aguilar et al. 2023). Males and females attain sexual maturity between 5 and
11 years (average is 8 years for both sexes; Rice and Wolman 1971; Bradford
et al. 2010). Females generally produce one calf every 2 years, although longer
inter-birth intervals of 3 or more years may occur (Rice and Wolman, 1971; Jones,
1990; Weller et al. 2008; Martínez-Aguilar et al. 2023). Gray whale calves are born
singly and the sex ratio has been reported to be 1:1 in the ENP (Rice and Wolman
1971). A recent study using genetic analysis found that 66.1 % of whales first
identified as calves on the WNP feeding area in the Okhotsk Sea were males
(Weller et al. 2009; Lang 2010). It is not known what factors influence the proportion
of male calves born per year, and the male bias found in the aforementioned study
may simply reflect stochastic variation (Cooke et al. 2016).
21.2.3 Gestation, Size at Birth, Weaning
The gestation period for gray whales is estimated to be 11–13 months with births
occurring from late December to early March (Jones and Swartz 1984; Sumich
2014). The median birth date is 27 January when near-term pregnant females are
migrating southward and in or near their wintering areas. At birth, calves are
4.5–5.0 m in length and weigh approximately 1000 kg. Females have a close and
protective bond with their calves and fight fiercely to defend them when threatened
(Scammon 1874). Weaning occurs at 6–8 months, primarily in summer feeding
areas, when calves are 7.6–8.5 m in length (Rice and Wolman 1971; Weller et al.
1999; Sychenko 2011). Post-weaning survival (survival from their first feeding

season as calves to the following season) is estimated at 0.65–0.70 and varies
annually (Bradford et al. 2006; Cooke et al. 2019).
504 S. L. Swartz et al.
21.2.4 Lactation and Female Resting Period
To nurse, a female gray whale swims slowly or rests motionless at the surface while
her calf approaches from below and nudges the abdominal area of its mother as a
cue, whereupon milk is pumped from the female’s teat into the mouth of the calf.
Calves consume about 189 l of milk daily, which is 53% fat and 6% protein (Sumich
2014). Mothers and calves remain in the wintering areas off Mexico until April or
May, allowing calves sufficient time to strengthen and rapidly increase in size before
their first northward migration. After weaning their calves in July and August on the
summer grounds, females then have a 3–4 month resting period to focus on feeding
to fatten and replenish their “energy stores” in advance of their next estrus cycle that
begins anew around November to December, thus completing the 2-year reproduc-
tive cycle (Rice and Wolman 1971; Swartz 1986; Jones 1990).
Studies in the WNP show that lactating females are commonly in poorer body
condition than other whales (Bradford et al. 2012). Although their body condition
typically improves somewhat through the feeding season, the probability that lac-
tating females would have complete within-season recovery is generally low, indi-
cating that post-parturient females are typically not able to fully replenish their
energy stores before their next opportunity to breed. Calves are in consistently
good body condition, resulting from high maternal investment during lactation,
even after weaning.
21.3 Gray Whale Reproductive Strategy
Female gray whales enter their first seasonal estrus at the onset of the southward
migration and along the migratory route. The compressed breeding season of gray
whales combined with the segregated and phased nature of the migration appears to
have helped shape their mating system by precluding the monopolization of avail-
able females by males.
21.3.1 Polygynandrous Mating System
The mating system of gray whales is polygynandrous (multi-male and multi-female).
Male defense of mates in “harems” or other similar aggregations that give dominant
males exclusive access to females, as seen in other polygynous mammals with
compressed breeding seasons, does not appear to be the case with gray whales.

When in estrus, females copulate with multiple mates to enhance the likelihood of
conception. There does not appear to be female-based pre-copulatory choice of
mates, as mating groups vary in size, often with numerous females and males
aggregated together. In these groups, multi-mate and multiple copulations occur.
Social-sexual behavior is not lim ited solely to the wintering areas. Video capable
suction cup tags have revealed that gray whales feeding off the Pacific Northwest
coast in the spring engage in close physical contact both underwater and at the
surface, rubbing against each other with their bodies and flippers, which is some-
times accompanied by males with erect penises (JC, pers. comm.). Sexually active
all-male groups are also seen during migration as well as winter and summer areas
(Darling 1977; Jones and Swartz 1984, 2009; Youngson and Darling 2016;
Fig. 21.1).
21 Gray Whale Sex, Reproductive Behavior, and Social Strategies 505
Fig. 21.1 A group of three sexually aroused male gray whales. No female was identified in this
group. While this “all male” behavior is observed throughout the gray whales’ range, it’s utility or
significance remains unclear. Photo credit: Fabian Missael Rodríguez-González
21.3.2 Operational Sex Ratio and Scramble Competition
The 2-year breeding cycle reported for gray whales (Rice and Wolman 1971) means
that at the start of each breeding season, 50% of the mature females are available
(or unavailable) for mating. This proportion of available females skews the opera-
tional sex ratio to a 2:1 male bias. Thus, competition between males for mating
opportunities would be expected (Emlen and Oring 1977). Evidence for overt
aggressive male–male competition for available females, however, has not been
reported. Instead, gray whales appear to have evolved a more cooperative “scramble

competition” mating strategy where males jostle to be in position next to an estrus
female without being directly aggressive to each other (Clutton-Br ock 2016). Such
mating can occur in what are termed “explosive breeding assemblages,” where both
sexes converge (e.g., during migration) for a relatively short-lived, highly synchro-
nized mating period. Sexual activity occurs during their migration and while the
whales occupy the wintering aggregation areas and lagoons in the ENP (Gilmore
1960; Rice and Wolman 1971; Norris et al. 1983; Jones et al. 1984), suggesting that
rather than mating in one specific portion of their range or location, the timing of
reproduction is more important. That is to say, a “core time” rather than “core area”
is the factor that assures gray whales conceive within a limited period and, in turn,
best ensures that calves will be born 11–13 months later in an optimal wintering
location: one that increases their potential for growth, development, and survival
(Jones and Swartz 2009). This line of reasoning suggests that the long-time use of
“breeding area” to describe the lagoons of Mexico in the ENP, and similar regions in
the WNP, is not accurate and should be properly termed “wintering aggregation
areas,” where some mating occurs.
506 S. L. Swartz et al.
21.3.3 Testes-to-Body Weight Ratio and the Multi-mate
Breeding System
The lack of direct evidence for male–male competition in gray whales, such as
pronounced sexual dim orphism, male territorial defense, and communal displays,
suggests that competition and sexual selection may occur at some level other than
that of the individual. The testes-to-body weight ratio in gray whales predicts a
multi-male breeding system, which is consistent with the breeding behavior
observed in this species. Brownell and Ralls (1986) reviewed the literature on baleen
whale testes size, penis length, and mating system. They reasoned that in species
with sperm competition, large testes that produce large amounts of sperm per
ejaculation would serve to dilute and displace the sperm of rival males and that
longer penises would deliver the sperm closer to the ova. Gray, right (Eubalaena
glacialis), and bowhead (Balaena mysticetus) whales are species without obvious
male–male competitive behavior and possess both large testes-to-body weights and
penis-to-body lengths. Conversely, the humpback whale (Megaptera novaeangliae)
with smaller testes, a shorter penis, and conspicuous interactions between breedi ng
males has been selected to compete to a greater extent by preventing rival males from
mating rather than by sperm competition (Brownell and Ralls 1986; Eichenberger
et al. 2023, this book; Würsig et al. 2023, this book).

21 Gray Whale Sex, Reproductive Behavior, and Social Strategies 507
21.4 Reproductive Behavior in the Eastern North Pacific
Wintering Lagoons
The scientific understanding of gray whale reproductive behavior during the winter,
when calves of the year are nursing and breeding among single whales continues,
comes almost strictly from studies conducted in the coastal lagoons along the Pacific
coast of the Baja California peninsula, Mexico.
21.4.1 Surface-Active Behavior
Early reports of gray whale mating behavior that described “trios” consisting of a
mating pair and a third individual or “helper” were oversimplifications (Samaras
1974; Miller 1975; Gilmore 1976). More recent observations have confirmed that
mating groups range in size from pairs to several individuals of mixed age/sex
(except calves) and may stay engaged in mating bouts that last for two or more
Fig. 21.2 A typical gray whale courting/mating group sequence photographed by drone in Laguna
San Ignacio. (a) Three gray whales (presumed males) investigating and stimulating an estrus female
in advance of initiating a mating bout and copulation. (b) Courting/mating bouts typically begin
with male gray whales gathering and positioning themselves around the estrus female for attempts
at copulation. (c) Typical “hugging” position of male gray whales when attempting copulation with
an estrus female. (d) While one male copulates with the estrus female, other males remain with the
courting group and stand by for additional mating opportunities. Photo credit: Fabian Missael
Rodríguez-González

hours with additional whales joining in the bouts as if stimulated by the sexual
activity of the core group (Jones and Swartz 1984; Fig. 21.2a–d). Mating bouts may
also include high-speed chases, termed “freight train races,” where group members
(presumably males) appear to pursue a lead animal (presumably a female in estrus),
with multiple individuals lunging through the water, creating spectacular
bow-waves, and sometimes cover 3–4 km before resuming a mating bout. As the
winter season progresses and the northward migration begins, the occurrence of
sexual activity becomes less frequent as the numbers of single adult whales in and
near the lagoons decline.
508 S. L. Swartz et al.
21.4.2 Acoustic Behavior
While gray whales are acoustically active throughout their range, including during
their migration (e.g., Guazzo et al. 2017; Burnham et al. 2018), and while feeding
(e.g., Moore and Ljungblad 1984), they appear to be most acoustically active in their
wintering areas (Dahlheim et al. 1984; Ollervides 2001), both in terms of the variety
Fig. 21.3 Spectrogram of a gray whale S1 call recorded on bottom mounted acoustic recorders in
Laguna San Ignacio, Baja California Sur, Mexico: February 10, 2008, at 00:19:49 local time. Note
the varying peak frequencies in each pulse, or “note.” Photo credit: Aaron Thode

of sounds produced and the frequency of vocalizations. Acoustic calls attributed to
gray whales in the wintering lagoons include “low-frequency rumbles,”“pulses,
moans, and whistles,”“chirps,”“clicks,”“bongs,”“knocks,”“pulses,”“snorts,”
“slamming,” and “bubble bursts.” There are over 14 distinct, mostly
low-frequency, sound types produced by gray whales reported in the scientific
literature.
21 Gray Whale Sex, Reproductive Behavior, and Social Strategies 509
While gray whales have been reported to produce a variety of different calls, the
“S1” call is common in the wintering lagoons (Dahlheim et al. 1984; Ollervides
2001; Charles 2011), but relatively rare during fall and spring migrations (Guazzo
et al. 2017). The S1 is characterized by a sequence of knocks with the tonal texture of
conga drums (Dahlheim, 1987; Fig. 21.3) and has a high source level, or volume,
which makes it detectable over greater distances than other types of calls (Dahlheim
1987). The S1 call is thought to serve a communication function, with animals
producing their own S1 call in response to hearing another animal’s S1 call (Ponce
et al. 2012). Field observations of gray whales found associations between the
occurrence of the S1 call and social-sexual reproductive behavior (Charles 2011).
The S1 call has also been reported during social-sexual interactions by feeding gray
whales off British Columbia, Canada (Youngson and Darling 2016). In summary,
the S1 call is a loud, common call that is used for two-way communication between
individual whales of all demographics but is particularly common among individuals
engaged in mating or social-sexual behavior.
21.4.3 Spatial Segregation and Behavioral Incompatibility
In January and February, females with calves and adult courting whales occupy the
lagoons, and although female–calf pairs routinely travel the entire length of a lagoon,
they actively avoid passing through areas with concentrations of courting adult
whales. In broad terms, gray whales in the wintering lagoons behave as two separate
segments: (a) social-sexual mating groups of single adult males and females in estrus
and (b) post-parturient females with their newborn calves. Single breeding adults of
both sexes engage primarily in social-sexual behavior, while females with new
calves generally remain solitary, devoting their energies to nursing and nurturing
their offspring (Jones and Swartz 1984).
In one of the wintering lagoons (Laguna San Ignacio) courting groups composed
of adult whales (males and females without calves) occur at highest densities in the
“lower” one-third of the lagoon nearest the lagoon entrance and their density
decreases with increasing distance from the entrance. In contrast, most females
with newborn calves are concentrated in the inner two-thirds of the lagoon furthest
from the entrance. These inner waters are often referred to as “nurseries,” where
mothers and their calves are mostly resting and nursing (Figs. 21.4 and 21.5). Few
single whales visit these nursery areas, providing relative solitude for females
following parturition and during early development of their calves (Jones and Swartz
1984, 2009).

510 S. L. Swartz et al.
Fig. 21.4 Female gray whale and her newborn calf in Laguna San Ignacio, Baja California Sur,
Mexico. Female gray whales will often support their newborn calves by swimming underneath the
calf and keeping it near to the water surface until the calf develops a regular swimming rhythm for
surfacing to breathe. Photo credit: Sergio Martínez-Aguilar
The average minimum length of stay for single adult whales (adult males and
females without calves) in the wintering lagoons is short (7.5 days) in comparison to
the average length of stay for females with calves (32.2 days). These residency times
reflect a high rate of turnover for single whales in the lagoons (Martínez-Aguilar et al.
2023). Further, photo-identification studies show that during the winter, single adult
whales circulate among the three primary wintering lagoons and the nearshore bay
waters, presumably looking for mating opportunities, while females with calves

spend longer periods within the lagoons (Martínez-Aguilar et al. 2023). Harassment
by courting whales is a major factor driving female–calf pair avoidance of courting
whales (Jones and Swartz 1984). Members of courting groups (presumably males)
pursue females with calves and attempt to mate with the females. Mating bouts are
highly disruptive events and involve groups of multiple individuals thrashing at the
surface, throwing their flukes and flippers, as they maneuver for mating position
(Norris et al. 1983; Jones and Swartz 1984; Swartz 1986; Figs. 21.6 and 21.7).
Calves may become separated from their mothers and/or can sustain injuries or even
be killed in these circumstances. In one instance, a well-known female with her calf
was relentlessly pursued and harassed by courting single adult whales, and the
mother was separated from the calf. A few days later, this female was found dead,
likely as a result of her injuries. The fate of her calf was never determined
(S. Martínez-Aguilar. Pers. Comm).
21 Gray Whale Sex, Reproductive Behavior, and Social Strategies 511
Fig. 21.5 When traveling the gray whale calf often positions itself close to and just behind the head
of its mother where the “slipstreaming” effect of water passing around the mother’s body assists the
calf’s swimming by pulling it along. Photo credit: Steven L. Swartz
21.4.4 Acoustic Crypsis
The levels of ambient biological noise in the inner lagoon areas, where females with
young calves segregate themselves, can be high and thereby provide acoustic
“crypsis,” or the ability of mother–calf pairs to avoid observation or detection by
other animals through the exploitation of sound. This ambient noise arises from

snapping shrimp sounds, tidal flow across the ocean bottom, wind-generated waves
on the surface, and to a lesser extent panga (small vessel) transits and fish chorusing
(Seger et al. 2015). Sounds recorded from mainly mother–calf pairs suggest a larger
repertoire than previously reported, possibly due to faint calls recorded only at close
range (Frouin-Mouy et al. 2020). Thus, in the wintering lagoons, mothers and calves
may take advantage of acoustic cryptic communication strategies (low-level sounds
in high-noise areas) for maintaining contact in a low-visibility environment while
simultaneously reducing a potential risk of eavesdropping by predators or courting
whales. Acoustic crypsis strategies have also been proposed in North Atlantic right
whales (Parks et al. 2019), southern right whales (Nielsen et al. 2019), and hump-
back whales (Videsen et al. 2017).
512 S. L. Swartz et al.
Fig. 21.6 A gray whale surface-active social-sexual courting/mating group of mixed sexes of
individual adult whales in Laguna San Ignacio, Baja California Sur, Mexico. Photo credit Steven
L. Swartz
21.5 Future Directions for Studying Gray Whale
Reproduction
21.5.1 Paternity Research
Although valuable information on female reproductive success can be obtained
through visual observations of mothers associated with their calves, evaluating

factors that influence male reproductive success is only feasible through genetic
analyses of paternity (Gerber and Krützen 2023, this book). Such analyses provide
the most information when a high proportion of animals in the population have been
sampled, and the genetic results can be integrated with long-term sighting histories
of individual whales. For example, in North Atlantic right whales, where ~63% of
identified individuals were sampled, the combination of genetic analysis of paternity
with 20 years of photo-identification records revealed that the age distribution of
assigned fathers was biased toward older males, with the youngest assigned father
being approximately twice the average age of first fertilization in females (Frasier
et al. 2007). These results suggested that mate competition may prevent younger
male right whales from reproducing. A similar approach in areas where a high
proportion of whales have been genetically sampled (e.g., WNP, PCFG) and those
samples are linked to long-term sighting histories of individuals could provide
insights into factors influencing male reproductive success in gray whales.
21 Gray Whale Sex, Reproductive Behavior, and Social Strategies 513
Fig. 21.7 Gray whales courting/mating. (a) A gray whale male approaching and positioning itself
at the side of a presumed female for a mating opportunity. (b) A gray whale male (right)
approaching and “hugging” a presumed female (left) with its flippers. (c) Gray whale males in a
courting/mating group approaching a presumed estrus female and positioning themselves for a
mating opportunity. (d) The erect penis of a male gray whale in Laguna San Ignacio, Baja Califor nia
Sur, Mexico. The gray whale penis with its bifurcated tip appears to be “prehensile” and this
capability may serve to assist the male in locating the entrance of the estrus female’s vagina for
intromission of semen. All photos taken in Laguna San Ignacio, Baja California Sur, Mexico. Photo
credit: Steven L. Swartz
Another method that may increase understanding of gray whale reproduction is
assessment of reproductive hormones. In other baleen whale species, reproductive

hormone assays of blubber biopsies and fecal samples have been used to determine
pregnancy rates (e.g., Atkinson et al. 2020), better understand the physiological
stress levels associated with different reproductive states (e.g., Valenzuela-Molina
et al. 2018), and infer patterns of male reproductive seasonality based on trends in
testosterone levels over time (e.g., Vu et al. 2015; Carone et al. 2019). Reproductive
hormones have also been measured in whale blow samples (e.g., Burgess et al.
2018). In addition, analysis of reproductive hormones in baleen, while only attain-
able from dead whales, provides the opportunity to track the recent reproductive
histories of individuals, increasing understanding of calving intervals, age of sexual
maturity, and timing of the breeding season (Hunt et al. 2016, 2022; Lowe et al.
2021). In gray whales, analysis of reproductive hormone concentrations in fecal
samples and blubber biopsies have focused on evaluating how reproductive hor-
mones vary with age, reproductive status, season, and life history stage (Lemos et al.
2020, Melica et al. 2021). Although analysis thus far has been limited to a single
sample, Hunt et al. (2017) demonstrated that reproductive hormones can be detected
in gray whale baleen. These studies highlight the potential utility of reproductive
hormone assessments to provide insight into gray whale reproduction.
514 S. L. Swartz et al.
21.5.2 Linking Reproductive and Acoustic Behavior
Bioacoustic tags, capable of collecting acoustic data as well as information on
acceleration and depth, and unoccupied aerial vehicles (UAVs or drones) equipped
with video, photographic, and other sensors (e.g., thermal) are examples of rapidly
developing technologies that can further expand the understanding of reproductive
behavior in gray whales (Ramos et al. 2023, this book). When paired together,
underwater calls of gray whales that are linked to their behavior at the surface can be
collected simultaneously, including data from breeding and other social-sexual
groups. Alternatively, a recent study using two drones, one capable of landing on
the water near whales to collect acoustic recordings and the other for obtaining aerial
imagery of whale behavior, demonstrated the feasibility and utility of this research
approach (Frouin-Mouy et al. 2020).
Acoustic communication plays a vital role in the social interactions of many
marine mammal species, but gray whale acoustic behavior and vocalizations are just
beginning to be evaluated. While the S1 call described in Sect. 21.4.2 above seems
associated with gray whale reproduction, its specific role has yet to be determined.
The statistical approach taken by Ponce et al. (2012) could be designed to count the
number of S1 calls detected, not only in relation to the total number of whales but to
the two demographic classes: single whales and female–calf pairs. If the S1 call is
indeed a social contact call used mostly by single whales that are actively trying to
breed, its presence should be correlated with the presence of single whales but
uncorrelated with the presence of female –calf pairs, when observed across multiple
seasons.

21 Gray Whale Sex, Reproductive Behavior, and Social Strategies 515
21.5.3 Role of Wintering Lagoon Entrance Aggregations
in Reproduction
Whalers in the nineteenth century first reported aggregations of gray whales at the
wintering lagoon entrances off the Pacific coast of the Baja California penin-
sula, Mexico, leading to the eventual discovery and exploitation of the high concen-
trations of whales inside of the lagoons proper. In recent decades, reports from
scientists and observations of local fishers and eco-tourism operators confirm that
gray whales actively enter and leave the winter lagoon interiors and form aggrega -
tions at the entrance points (Jones and Swartz 1984; Jones 1990). These entrance
point aggregations of whales are difficult to observe due to their distance from the
shore and high surf where they occur, precluding detailed study and evaluation of the
importance of these areas to gray whale reproductive behavior in the winter. A new
generation of affordable “fixed-wing” and “hovering” drones have good potential to
serve as effective and safe platforms to observe gray whale sexual and mating
behavior in these portions of their winter range.
Acknowledgments We express our thanks to Dara Orbach and Bernd Würsig for providing the
opportunity to contribute information on gray whale reproduction to this volume. We also thank Jim
Darling for his review and constructive comments that improved an earlier draft. We all have those
that inspired and encouraged us to pursue marine science, and on whose shoulders we will always
gratefully stand. Collectively we acknowledge and thank our teachers, mentors, and colleagues, past
and present, for fueling our desire to better understand the natural history and behavior of cetaceans,
and particularly of the gray whale. We are hopeful that our endeavors will inspire new generations
of marine scientists to follow and carry out novel lines of investigation and that they will be as
fortunate as we have been to experience the exhilaration of discovery and the satisfaction of sharing
their findings with their colleagues and with the world. Finally, we are grateful to the gray whales
for providing the opportunity to learn from these most remarkable cetaceans of their lives in the
ocean environment.
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