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Infanticide in a mammal-eating killer whale population


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Infanticide can be an extreme result of sexual conflict that drives selection in species in which it occurs. It is a rarely observed behaviour but some evidence for its occurrence in cetaceans exists in three species of dolphin. Here we describe observations of an adult male killer whale (Orcinus orca) and his postreproductive mother killing a neonate belonging to an unrelated female from the same population in the North Pacific. This is the first account of infanticide reported in killer whales and the only case committed jointly by an adult male and his mother outside of humans. Consistent with findings in other social mammals, we suggest that infanticide is a sexually selected behaviour in killer whales that could provide subsequent mating opportunities for the infanticidal male and thereby provide inclusive fitness benefits for his mother.
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Scientific RepoRTS | (2018) 8:4366 | DOI:10.1038/s41598-018-22714-x
Infanticide in a mammal-eating
killer whale population
Jared R. Towers1,2, Muriel J. Hallé2, Helena K. Symonds3, Gary J. Sutton2,
Alexandra B. Morton4, Paul Spong3, James P. Borrowman5 & John K. B. Ford1
Infanticide can be an extreme result of sexual conict that drives selection in species in which it occurs.
It is a rarely observed behaviour but some evidence for its occurrence in cetaceans exists in three species
of dolphin. Here we describe observations of an adult male killer whale (Orcinus orca) and his post-
reproductive mother killing a neonate belonging to an unrelated female from the same population
in the North Pacic. This is the rst account of infanticide reported in killer whales and the only case
committed jointly by an adult male and his mother outside of humans. Consistent with ndings in other
social mammals, we suggest that infanticide is a sexually selected behaviour in killer whales that could
provide subsequent mating opportunities for the infanticidal male and thereby provide inclusive tness
benets for his mother.
Conspecic infanticide occurs within many taxa. Among terrestrial mammals, it is mostly reported in primates,
carnivores and rodents1. However, compelling evidence for infanticide in cetaceans exists in three species of
dolphin. Patterson et al.2 and Dunn et al.3 found that dead neonate and juvenile common bottlenose dolphins
(Tursiops truncatus) in the eastern and western North Atlantic respectively had scarring that suggested they were
killed by conspecics. More recently, Kaplan et al.4, Robinson5, Perrtree et al.6 and Díaz López et al.7 witnessed
adult common bottlenose dolphins attack young in the North Atlantic. Furthermore, Zheng et al.8 documented
adult Indo-Pacic humpback dolphins (Sousa chinensis) killing neonates in the western North Pacic and in the
South Atlantic, an attack on a neonate Guiana dolphin (Sotalia guianensis) conducted by adults likely resulted in
its death9.
In many cases, it is dicult to put the causes and eects of infanticidal acts into context because the natural
occurrence of this behaviour is so rarely observed10,11. However, infanticide occurs in a myriad of dierent situ-
ations and several hypotheses have been proposed to explain it1113. Salient among these is the sexual selection
hypothesis which states that infanticide can benet the tness of males that commit it because loss of a dependent
ospring ends lactational amenorrhea and can quickly return the mother to a fertile condition. In these cases,
infanticide not only creates a mating opportunity but can also remove the progeny of a competing male from the
gene pool12. e predation hypothesis suggests that infanticide precedes cannibalism and is more likely to take
place in energy-stressed populations13,14. However, cannibalism does appear to be a secondary benet of some
infanticide events best explained by the sexual selection hypothesis15,16. e resource competition hypothesis
predicts that the removal of an infant may provide greater access to prey, mates or habitat for the perpetrator
or its descendants12,13. Lastly, non-adaptive explanations for infanticide purport that it is a socially pathological
behaviour that may be conducted accidentally or as a result of environmental stressors17,18.
e risk and occurrence of infanticide is likely to have inuenced social structures and mating systems in
some species11,19. Although much research has been conducted on the social societies of cetaceans20, very little
is known of the reproductive behaviour of most odontocetes21. However, male Indo-Pacic bottlenose dolphins
(Tursiops aduncus) have been documented sexually coercing females by chasing, ramming and biting them22.
ese aggressive behaviours and incidents of newly acquired scars on females caused by the teeth of conspecics
are positively correlated with the timing of their reproductive cycles23,24. Additionally, while scars on several other
odontocete species are thought to occur in mating competitions between males25,26, these intraspecic agonistic
interactions are rarely observed. For example, although killer whales (Orcinus orca) are one of the most stud-
ied and widespread species of cetacean, only a few observations of aggression between individuals of the same
1Pacic Biological Station, Fisheries and Oceans Canada, 3190 Hammond Bay Road, Nanaimo, BC, V9T 6N7, Canada.
2Bay Cetology, Box 554, Alert Bay, BC, V0N 1A0, Canada. 3OrcaLab, Pacic Orca Society, Box 510, Alert Bay, BC, V0N
1A0, Canada. 4Raincoast Research Society, Box 399, Sointula, BC, V0N 3E0, Canada. 5Whale Interpretive Centre, Box
2–3, Telegraph Cove, BC, V0N 3J0, Canada. Correspondence and requests for materials should be addressed to J.R.T.
Received: 8 December 2017
Accepted: 27 February 2018
Published: xx xx xxxx
There are amendments to this paper
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Scientific RepoRTS | (2018) 8:4366 | DOI:10.1038/s41598-018-22714-x
population have been reported (see27). Nevertheless, individuals of all sex and age classes from many killer whale
populations around the world have scars on their bodies from the teeth of conspecics2833. ese scars may orig-
inate from rough interactions between or within sex and age classes but, causes of their occurrence likely vary in
nature and among populations.
In the North Pacic Ocean several populations of both sh-eating and mammal-eating killer whale exist34.
Among those that prey on marine mammals (Bigg’s killer whales) are a number of genetically distinct populations
in the Gulf of Alaska35 as well as the West Coast Transient (WCT) population which is primarily found in coastal
waters and along the continental shelf edge from central California to southeastern Alaska36. is population is
currently considered reatened in Canada37 but has been growing at an average rate of about 3% per year since
1975 and now consists of several hundred individuals38,39. Social units in this population usually contain two
to six individuals and are typically composed of a reproductive female and her ospring. Both male and female
ospring are known to permanently or temporarily disperse as they mature but post-reproductive females oen
travel with their adult sons continuously40.
Field studies on WCT killer whales conducted by researchers aliated with the Pacic Biological Station in
British Columbia, Canada began in 197341. Aer more than 5300 photographically documented encounters with
WCT killer whales, an encounter on 2 December 2016 provided the rst direct observations of lethally agonistic
behaviour between individuals in this population as an adult male and his mother killed a neonate belonging
to a female that was not maternally related. e events leading up to and following the infanticide incident are
presented chronologically below and the occurrence of this phenomenon within WCT killer whale society is
discussed in context of the various hypotheses for this behaviour.
At 09:59 WCT killer whale vocalizations were detected on a remote hydrophone station located near Robson
Bight in Johnstone Strait. By 10:10 vocalizations were also heard on two other hydrophone stations about ten km
to the west, indicating a westward movement of the whales (Fig.1). Between 10:06 and when the last vocalization
was recorded at 11:01, two or more individuals repeated several discrete and aberrant pulsed calls (Supplementary
Recording S1; Supplementary TableS1). At 10:55 a research boat was dispatched from Alert Bay to intercept the
Figure 1. Map of study area (created with ArcGIS v10.5 - e locations
and acoustic range of hydrophone stations, place names, and positions of the encounter beginning, end and the
infanticide event are provided.
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whales and they were spotted heading west in western Johnstone Strait at 11:07. Adult female T068 (age 46 yr,
thus post-reproductive) and her adult son T068A (age 32) were travelling together about 200 m behind a young
mother T046B1 (age 13), her ospring T046B1A (age < 2 yr) and her sister T046B4 (age < 3 y r) (Fig.2). e three
T046B individuals were traveling rapidly between 11 and 17 km/h. T046B4 had fresh wounds from the teeth of
another whale on both anks and a noticeable kink on the dorsal surface of the ridge posterior to the dorsal n
that was associated with broken skin (Fig.3A). e wounds on the le ank were still bleeding (Fig.3A). e
T068s were keeping a steady pace behind and o to the side of the T046Bs. At 11:29, the mother of T046B1 and
T046B4, T046B (age 28 yr), was found west of Alert Bay about a kilometer ahead of the other whales with her
other daughters T046B2 (age 8 yr) and T046B3 (age 5 yr) and a neonate of unknown sex with visible fetal folds
and a dorsal n that was not yet entirely erect (T046B5) (Figs2 and 3B). ey remained ahead of the rest of the
T046Bs until 11:55 when they all came together near Haddington Island. e T068s remained roughly 200 m
behind. All whales were making a steady pace to the west.
Nearing Ledge Point at 12:18, a portable hydrophone was deployed ahead of the whales, but no vocalizations
were heard. A few minutes later, aer the whales had passed about 500 m to the west, erratic movements and
splashing suggestive of a predation event were observed. Upon arriving on scene at 12:27, T068A was approx-
imately 150 m from the other whales and moving away from them. e other whales followed. ey all came
Figure 2. Genealogicalschematic of present individuals. e identities and where known, sex and birth year of
all whales present and their maternal relations to each other (see original schematics in Towers et al.30) displayed
in colour coded statuses. Purple: post-reproductive female, Blue: reproductive male, Fuscia: reproductive
females, Pink: juvenile females, Yellow: juvenile of unknown gender, Gray: neonate of unknown gender.
Figure 3. Observations leading to infanticide. (A) Fresh wounds on le ank and kinked spine anterior to
dorsal n on T046B4. (B) T046B with ospring T046B2 and (neonate) T046B5. (C) T068A surrounded by
T046B, T046B1 and T046B1A. (D) T046B ramming T068A from below sending spray and blood into the air.
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together aer 1–2 minutes and began circling vigorously (Fig.3C). It was realized that the neonate T046B5 was
not surfacing next to its mother T046B or in the vicinity and then T068A swam close past the research boat
and the uke of the neonate could be seen in his mouth with the body intact trailing underneath his lower
jaw. Movements of all whales were erratic but the neonate’s mother T046B appeared to be chasing the male
T068A while his mother, T068 attempted to manoeuvre between them (Supplementary MovieS1). Intense vocal
activity could be heard through the hull of the boat so the hydrophone was deployed. A wide variety of excited
discrete and aberrant pulsed calls (Supplementary TableS1), whistles, and percussive sounds were recorded
(Supplementary Recording S2). Underwater video obtained during this time shows T068A with an object par-
tially white in colour in his mouth, closely anked by T068 (Supplementary MovieS2). At 12:35, T046B rammed
T068A near the surface with sucient force to cause a noticeable undulation through his body, sending blood
and water into the air (Fig.3D).
At 12:43 the activity began to subside. Fresh teeth wounds that were not present before the incident were
visible on T068A’s le and right anks (Fig.4A), rostrum, and base of dorsal n and on T068’s melon. T068A
and T068 began to withdraw from the others and slowly travel west. T068A still had the neonate by the uke in
his mouth (Fig.4A). At 13:07, his mother T068 was lmed and photographed packing the neonate by the uke
(Supplementary MovieS3; Fig.4B). e T046Bs remained 200–300 m behind and o to the side of the T068s, sur-
facing together and travelling slowly. At 13:42 all whales were o Pulteney Point and the dorsal n of the neonate
was seen above the surface on T068’s right side as she surfaced, indicating she was holding its le pectoral ipper
in her mouth. At one point she pushed the neonate along in front of her and its head was visible above the surface
before sinking back down into the water. A few minutes later the T046Bs slowly approached them from behind.
T046B came out ahead of the others within about 50 m of the T068s but then reversed direction and re-joined
her group. Aer this the T046Bs remained grouped up within a kilometre of the T068s for the duration of the
encounter. At 14:20, underwater footage of the T068s showed neither of them to be holding T046B5, but just prior
to this they had begun to circle at the surface aer making long dives indicating that the neonate may have been
below them. At 14:50 it was visually conrmed that the T046Bs were not in possession of the neonate. Aer not
seeing T046B5 since before 14:00, T068A surfaced at 16:00 with its dorsal n visible o the le side of his head
(Fig.4C), indicating he held its right pectoral ipper in his mouth. He was photographed surfacing rostrum to
rostrum with the dead neonate a few minutes later (Fig.4D). Its body appeared to be intact. Aer unsuccessfully
attempting to acquire a biopsy sample of T068A, observations ended at 16:15 due to failing daylight. No evidence
of feeding behaviour such as oil slicks or birds becoming interested in the whales were observed at any point.
Figure 4. Observations following infanticide. (A) T068A with the uke of T046B5 in the le side of his mouth.
Fresh scars from the teeth of another whale can be seen on his le ank and rostrum. (B) T068 surfacing with
the uke of T046B5 visible in the le side of her mouth. (C) T068A surfacing with the dorsal n of T046B5
visible o the le side of his melon. (D) T068 rostrum to rostrum with T046B5 approximately 215 minutes aer
its death.
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WCT killer whales may be well adapted behaviourally for performing infanticide because of their experience kill-
ing other small cetaceans. Individuals typically dispatch these prey by ramming or crushing them with their ros-
trums or tails36. Observations of infanticidal behaviour towards young common bottlenose dolphins, Indo-Pacic
humpback dolphins and a Guiana dolphin by conspecics have included repeated ramming and crushing actions
but also forced submergence49. Although we did not observe the rst few minutes of this incident closely enough
to determine the details of interactions that took place, it appeared as though T046B5 was quickly immobilized by
having its tail gripped in the teeth of T068A and subsequently drowned because there was no opportunity to sur-
face and breath due to his consistent forward motion. Although both T068 and T068A were observed with fresh
teeth scars aer the incident, the only ramming behaviour we observed was performed by T046B toward her neo-
nates killer. is combative behaviour is similar to actions directed towards infanticidal males by the mothers of
targeted infants in several rodent, primate and carnivore species (see review in13). e ramming occurred approx-
imately ten minutes aer the incident began, at which point the neonate would have been either close to or freshly
drowned, which is probably why the ght did not persist. e brevity of this physical aggression might be typical
of agonistic interactions between members of the WCT killer whale population. is, in addition to the fact that
most exchanges between individuals take place below the surface, may help explain why none of this nature have
previously been observed. Similarly, anomalistic calls are not commonly made by whales in this population42, but
as many animals are known to produce atypical sounds under stressful situations43, the high number of aberrant
and discrete pulsed calls and other excited sounds recorded before and during the infanticide event likely reect
the levels of intensity in the complex interactions taking place between individuals. For example, the fresh trauma
observed on T046B4 at the beginning of this encounter indicates that other rough interaction between the whales
occurred prior to our arrival. is may have caused the T046Bs to separate into two groups to benet the neonate
and the other young animals through their disassociation from it, but vocalizations between them and their ulti-
mate re-grouping may have revealed their locations to the T068s.
e motivation for T068A and T068 to make this attack is of particular interest, but that they carried it
out cooperatively is not surprising because bonds between maternally related killer whales can be particularly
strong34. In sympatric populations, post-reproductive female killer whales increase the survival of adult sons
by sharing ecological knowledge and prey with them44,45. is benets inclusive tness of the female because a
positive relationship exists between reproductive success and age in male killer whales45,46. is combined with
a prediction by Connor et al.47 that post-reproductive female killer whales may play a role in acquiring mates for
their adult male ospring suggests that T068’s active involvement in this event was sexually selected, especially
considering that T068A is of a reproductively mature age30,46. e sexual selection hypothesis requires that the
infanticidal male does not kill his own ospring, that the event provides a near future mating opportunity with
the infants’ mother and that he sires ospring with her12. Although it has been conrmed in many other mammal
species that infanticidal males are not the fathers of their victims4855 and that they subsequently sire ospring
with the infants mothers13,15,51,53,5557, in this incident the neonate’s carcass could not be recovered, T068A was not
biopsied for DNA analysis and T046B has not yet had another calf.
However, further evidence also suggests that the infanticidal teamwork of T068 and T068A was motivated by
potential to increase their inclusive tness. For example, young ospring are oen selected for infanticide because
new mothers return to estrus much quicker than those that have been lactating for longer periods of time12. In
this case, we know that T046B5 was less than a few days old and that suckling stimulates lactation in female killer
whales whose postpartum return to estrus is much longer (5–32 months) when lactating than when not (1–4
months)58. Secondly, females in some animal societies copulate more with coercive males59,60 that restrict parental
investment and improve the female condition by committing infanticide to increase their tness61. In this case,
although it is not clear if the attack on T046B4 was attempted juvenilicide, this whale (although third youngest
in the group) was T046Bs second youngest ospring, so its death along with loss of her neonate could have ben-
etted her condition by reducing any prohibitive physical costs associated with caring for either of them. is
would help restrict parental investment towards any forthcoming ospring of T068A if he successfully mated with
T046B. Furthermore, infanticidal males are typically not familiar with the infant’s mothers13 and although unre-
lated matrilineal groups of WCT killer whales have oen been known to spend several weeks together40 in what
have generally been perceived to be mutual associations, T068A, T068 and T046B had only been documented
together on two previous occasions – once in 2005 and again in 2007. e apparent low levels of association
between these individuals suggest that they are not well known to each other and this may be reective of inherent
sexual conict between their diering sex and age classes.
Sexually selected infanticide in WCT killer whales has important implications for our understanding of the
social behaviour and evolution of this species (Supplementary Discussion S1) and although in this case, the
hypothesis cannot be proven, there is more supporting evidence for it than other hypotheses for this behav-
iour. For example, WCT killer whales are well known for their dramatic hunting behaviour and protracted kills
that typically result in immediate vocal and social activity, including division and consumption of mammalian
prey36,62. However, in this incident the vocal and social activity subsided once the kill was made and although the
dead neonate was kept in the possession of its killers for at least 220 minutes, there was no indication that it was
dismembered or consumed. Additionally, most prey species that WCT killer whales target have been steadily
increasing in coastal waters over the last few decades6369. ese increases in prey availability have been commen-
surate with increasing social activity70 and higher recruitment within the WCT population39 suggesting that it is
not nutritionally stressed. us, this incident is not easily explained by the predation hypothesis. Furthermore,
although the WCT population has more than doubled in size since 199039, there is little evidence available to
suggest that individuals within it compete for habitat or prey71. Regardless, these and the availability of other
resources such as mates would not easily be threatened by an infant of either sex due to the slow rate of matu-
ration in this species. us, this incident is not easily explained by the resource competition hypothesis. Finally,
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Scientific RepoRTS | (2018) 8:4366 | DOI:10.1038/s41598-018-22714-x
there is little support for interpreting infanticide as a non-adaptive behaviour other than when it occurs by acci-
dent or under unnatural conditions13,53,72 and contrary to accidental or pathological explanations, the infanticidal
behaviour of T068A and T068 appeared goal oriented because their chase led to an attack that was maintained
until a particular outcome had been achieved. Once it had, their behaviour immediately changed.
In conclusion, given that infanticide is so rarely observed in terrestrial mammals it is not surprising that it has
taken many years of directed eld observations to conrm that it does occur in cetaceans and more specically, in
killer whales. is species shares many life history traits with other mammals that are known to commit infanti-
cide (see1), such as a high lactation to gestation ratio58, a society where individuals live in stable mixed sex groups
where calves are born year round71 and a breeding system that is monopolized by a minority of mature males46.
As a result, this phenomenon might be expected to occur in more killer whale populations than just the WCT. In
any case, additional study is required to investigate any impacts that this phenomenon may have on the evolution
of the social structure of this population and the behaviour of individuals within it.
Underwater acoustic data were transmitted live from three xed hydrophone stations maintained by OrcaLab in
or adjacent to Johnstone Strait to monitors in Alert Bay, Telegraph Cove and on Malcolm Island. Vocalizations
were digitally recorded and classied through visual and aural comparison using Audacity spectrograms and call
type catalogues for this population provided in Ford73 and Deecke62. Calls were considered aberrant if they were
similar but not identical to stereotyped calls. Vocalizations that were impossible to identify due to signal strength
and/or interference were classied as unidentied. Digital audio, video and images were obtained in the eld
while observations were taking place from a 5 metre motor vessel. e individual killer whales and their geneal-
ogies were identied using Towers et al.30. Data on age, sex, associations and observations from other encounters
were obtained from the database on this population maintained for the Pacic Biological Station (Fisheries and
Oceans Canada).
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We thank Shari Willmott, Eva Stredulinsky, Natalie Barefoot and the late Twyla Roscovich for helping prepare
materials, David Cannamore for helping to record acoustic data and take notes during this encounter, Volker
Deecke for assistance with call classication, and James Pilkington for reviewing a dra of the manuscript. is
research and preparation of this paper has been funded in part through the Fisheries and Oceans Canada Species-
at-Risk Program and the Annenberg Foundation.
Author Contributions
Observational data on the infanticide event featured in this article were collected by J.R.T., M.J.H. and G.J.S.
under Canadian marine mammal research license MML-01. Acoustic data were collected by H.K.S., A.B.M., P.S.,
J.P.B. and J.R.T. e acoustic data were analyzed by H.K.S. Funding was directed to this project by J.K.B.F. and
J.R.T. e article was draed by J.R.T. with input from all authors.
Additional Information
Supplementary information accompanies this paper at
Competing Interests: e authors declare no competing interests.
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... One notable observation of infanticidal behavior occurred in mammal-eating killer whales off British Columbia, Canada (Towers et al. 2018). In 2016, an adult male and his postreproductive mother engaged in a long chase and eventual killing of a neonate. ...
... In one case, a female who could have been the calf's mother was involved in both aggressive and helping behaviors (Ramos et al. 2022), which is unusual but difficult to interpret without confirmation that the female was the mother. In the most conspicuous example of a female participating in an infanticidal attack, in which an adult female killer whale assisted her adult son, the exception may prove the rule (Towers et al. 2018). Postreproductive killer whale females provide fitness benefits to their adult offspring, particularly their sons (Foster et al. 2012;Brent et al. 2015). ...
... Previous social association between the infanticidal individuals and their victim's mothers is often not known, but when it is reported, infanticidal males appear to be socially unassociated with the female and unlikely sires of the targeted infant (Table 10.1). In several cases, the males were not frequent associates of the mother (Robinson 2014;Zheng et al. 2016;Díaz López et al. 2018;Towers et al. 2018), and in others, the attackers and the mothers seemed to be from different subpopulations (Ronje et al. 2020;Ramos et al. 2022). The conspicuous exception to this is a case in the Amazon river dolphin, in which one attacker was the maternal brother of the targeted calf (da Silva et al. 2021). ...
Full-text available
Infanticide by adult males is a striking example of sexual conflict; males can increase their reproductive success by killing an unrelated infant and accelerating the mother’s return to breeding condition. Reports of infanticide in cetaceans have quadrupled in the past decade, and infanticide has now been documented in six species of toothed whale, including multiple populations of common bottlenose dolphins ( Tursiops truncatus ). Evidence of infanticide in these species is consistent with the sexual selection hypothesis; perpetrators are predominantly adult males and targets are neonates. Toothed whales have long lactation periods that suppress estrus, making infanticide potentially adaptive for adult males. However, it remains unclear if infanticidal males are likely to sire the mother’s subsequent offspring. Here, we provide an overview of infanticide in cetaceans, evaluate the evidence for the sexual selection hypothesis, and propose a framework to predict infanticide risk in this clade. Toothed whales do not typically have dominance hierarchies, stable social groups, or monopolizable mating opportunities, all hallmarks of infanticide risk in terrestrial species. Instead, we hypothesize that infanticide risk in toothed whales is modulated by encounter rates with unfamiliar males.
... Killer whales share several life history traits with other species known to commit infanticide and this behavior has been documented in the Bigg's population (Towers et al., 2018). However, it is unlikely that SRKWs use porpoises to practice infanticide, due to the style of interactions compared to the more aggressive attacks documented during cetacean infanticide (Díaz L opez et al., 2018;Kaplan et al., 2009;Towers et al., 2018;Zheng et al., 2016). ...
... Killer whales share several life history traits with other species known to commit infanticide and this behavior has been documented in the Bigg's population (Towers et al., 2018). However, it is unlikely that SRKWs use porpoises to practice infanticide, due to the style of interactions compared to the more aggressive attacks documented during cetacean infanticide (Díaz L opez et al., 2018;Kaplan et al., 2009;Towers et al., 2018;Zheng et al., 2016). ...
Full-text available
Endangered Southern Resident killer whales (Orcinus orca) are fish-eaters that preferentially prey on adult Chinook salmon (Oncorhynchus tshawytscha). Despite being salmon specialists, individuals from all three killer whale pods (J, K, L) have been observed harassing and killing porpoises (family Phocoenidae) without consuming them. Retrospectively , we identified and analyzed 78 episodes of Southern Resident killer whales harassing porpoises between 1962 and 2020, of which 28 resulted in the porpoise's death ("phocoenacide"). Fifty-six episodes involved harbor porpoise (Phocoena phocoena), 13 involved Dall's porpoise (Phocoenoides dalli), and the porpoise species was unreported for nine episodes. Southern Resident killer whales often targeted young porpoises that were similar in size to adult Chinook salmon. † These authors contributed equally to the manuscript.
... Across the 50 years of data, the observed average group size of Bigg's killer whales in the Salish Sea has increased from 4.4 to 6.1 individuals [35,36,47], coinciding with several factors: an increase in the availability of Bigg's prey [63][64][65][66], a rapid increase in the population size in the last approximately 30 years [34,67] and an increased presence of the Bigg's ecotype in the Salish Sea [36]. These factors appear to be related in that recovering prey populations correspond with an increase in available energy intake per individual, which subsequently drives increased group size, reproduction rates, overall abundance, and range use in Bigg's killer whales. ...
... Future modelling should focus on exploring the individual-level changes to female average relatedness under such dispersal patterns to better understand the selection for intergenerational helping and competion under such social conditions. The mating strategy of Bigg's killer whales is still unknown [67]. Yet, the high relatedness between group members suggests that mating occurs with individuals outside of the group perhaps as a mechanism of inbreeding avoidance, similar to resident killer whales [82], although both within-pod and within-matriline mating occurs in the southern resident killer whale population [20]. ...
Full-text available
Age-related changes in the patterns of local relatedness (kinship dynamics) can be a significant selective force shaping the evolution of life history and social behaviour. In humans and some species of toothed whales, average female relatedness increases with age, which can select for a prolonged post-reproductive lifespan in older females due to both costs of reproductive conflict and benefits of late-life helping of kin. Killer whales (Orcinus orca) provide a valuable system for exploring social dynamics related to such costs and benefits in a mammal with an extended post-reproductive female lifespan. We use more than 40 years of demographic and association data on the mammal-eating Bigg's killer whale to quantify how mother-offspring social relationships change with offspring age and identify opportunities for late-life helping and the potential for an intergenerational reproductive conflict. Our results suggest a high degree of male philopatry and female-biased budding dispersal in Bigg's killer whales, with some variability in the dispersal rate for both sexes. These patterns of dispersal provide opportunities for late-life helping particularly between mothers and their adult sons, while partly mitigating the costs of mother-daughter reproductive conflict. Our results provide an important step towards understanding why and how menopause has evolved in Bigg's killer whales.
... Infanticide is a type of delayed sexual coercion in which adult males kill young that they are unlikely to have sired; this behavior may increase a male's reproductive success by causing postpartum females to return to estrus more quickly (Connor et al. 2000;Lukas and Huchard 2014). Although observed in other delphinid species (McEntee et al. 2023, this book), only one observation of infanticide has been reported in killer whales; a mature male Bigg's killer whale and his post-reproductive mother killed the neonate of a maternally unrelated female from the same population (Towers et al. 2018). With no other observations of infanticidal behavior, it is unclear whether infanticide is an aberrant behavior in killer whales or whether it serves an adaptive function by increasing a male killer whale's mating opportunities. ...
Full-text available
We present the current available knowledge about sexual behavior and mating systems in the killer whale, Orcinus orca , focusing primarily on the most well-studied ecotype, the Residents of the eastern North Pacific. Resident killer whales display lifetime natal philopatry of both sexes and thus form stable social groupings organized along maternal lines. Inbreeding is minimized via exogamous mating during multigroup aggregations that are diffusely seasonal. Male reproductive success in Residents is highly skewed, likely as a result of female mate choice or possibly intrasexual contest competition (or both), with only the largest and oldest males siring offspring. Female choice for males with large body and appendage sizes likely explains why killer whales as a species have evolved a high degree of sexual dimorphism. Scarring patterns and dorsal fin injuries indicate that breeding-aged males may occasionally engage in aggressive intrasexual competition to secure mates. Young male Residents appear to practice courtship behaviors by engaging in sociosexual play with other males. Coercive mating or mate-guarding in this species has not been observed and appears to be unlikely. Relative testes size, penis length, and penis tip morphology indicate that some sperm competition presumably exists in killer whales and likely co-evolved with female polyandry. Thus, both pre- and postcopulatory sexual selection may occur simultaneously in killer whales, and this species probably has a polygynandrous mating system in which both males and females can breed with multiple partners. Genetic pedigree studies of killer whale ecotypes besides Residents are needed to provide currently lacking but valuable information about mating patterns in other populations.
... The influence of maternal presence on male reproductive success in cetaceans is largely unexplored. However, a female killer whale cooperated with her adult son in killing an unrelated female's calf (Towers et al. 2018), potentially to increase his own reproduction. In order to aid their sons, females may hinder other males from mating or bring their sons in proximity to estrus females as observed in bonobos (Surbeck et al. 2011). ...
Full-text available
The primary purpose of sex is reproduction. However, because not all mating events result in fertilization and only a small number of species provide biparental care to their young, successfully reproducing individuals can rarely be identified from behavioral observations alone. Genetic tools permit reliable identification of an individual’s parents and thus of successfully reproducing individuals, because each parent passes on half of their genetic material to their offspring. In cetaceans, genetic tools are required to identify a female’s already weaned offspring and to detect successfully reproducing males due to the absence of paternal care. To date, relatively few studies have investigated variables linked to reproductive success in this taxon, owed to the difficulty of sampling entire cetacean populations. We summarize currently known factors that are linked to successful reproduction in whales, porpoises, and dolphins, as well as in terrestrial mammals with comparable life histories that give birth to single young.
... Based on the sexual selection hypothesis, male mammals commit infanticide to enhance their fitness, since after having killed a calf the female stops lactating and may become fertile again, which enhances mating opportunities, and in addition infanticide removes unrelated males from the gene pool (Hrdy 1979). In cetaceans, conspecific infanticide is relatively rare, but it has been described in various dolphin species: common bottlenose dolphins (Patterson et al. 1998;Dunn et al. 2002;Kaplan et al. 2009;Robinson 2014;Perrtree et al. 2016;Díaz López et al. 2018 (Towers et al. 2018;McEntee et al. 2023, this book). ...
Full-text available
Risso’s dolphins ( Grampus griseus ) in the Azores maintain a sexually stratified community with males forming stable clusters, whereas females tend to associate in temporally stable units when calving and nursing. Large relative testes size indicates a mating system based on sperm competition. Small sexual size dimorphism and long interbirth intervals may facilitate male cooperation. We describe mating tactics observed in a resident population of Risso’s dolphins based on data collected over 8 consecutive years. We distinguished groups by sex and age class and analyzed peaceful and aggressive behaviors using an ethogram. Males displayed a variety of mating tactics described in cetaceans, spanning display, contest, endurance, and scramble competition, in the assumed context of sperm competition. We observed rare behaviors including male care for neonates but also intense aggression toward newborn calves, leading to possible infanticide. Females’ mating tactics included evasive behaviors and signal discrimination. We hypothesize that male mating tactics of Risso’s dolphins are related to age class. For females, the polygynandrous mating system may serve to improve fertility, reduce sexual harassment, and reduce the risk of infanticide. Adult females seem to prefer older over younger male groups, suggesting that some level of female mate choice exists in this species.
... The west coast transient population has been growing at approximately 3.5% since 1975, seeTowers et al. (2018). ...
Understanding the evolution of menopause presents a long-standing scientific challenge1,2,3-why should females cease ovulation prior to the end of their natural lifespan? In human societies, intergenerational resource transfers, for example, food sharing and caregiving, are thought to have played a key role in the evolution of menopause, providing a pathway by which postreproductive females can boost the fitness of their kin.4,5,6 To date however, other late-life contributions that postreproductive females may provide their kin have not been well studied. Here, we test the hypothesis that postreproductive female resident killer whales (Orcinus orca) provide social support to their offspring by reducing the socially inflicted injuries they experience. We found that socially inflicted injuries, as quantified by tooth rake marks, are lower for male offspring in the presence of their postreproductive mother. In contrast, we find no evidence that postreproductive mothers reduce rake marking in their daughters. Similarly, we find no evidence that either reproductive mothers or grandmothers (reproductive or postreproductive) reduce socially inflicted injuries in their offspring and grandoffspring, respectively. Moreover, we find that postreproductive females have no effect on reducing the rake marks for whales in their social unit who are not their offspring. Taken together, our results highlight that directing late-life support may be a key pathway by which postreproductive females transfer social benefits to their male offspring.
Here, we describe the epimeletic behavior of an adult Risso’s dolphin towards a deceased newborn calf of the same species across several days with photographs, acoustic monitoring, and video recordings. Boat-based observation surveys were conducted from April 2014 to October 2021 along the southern coast of Galicia (northwest Spain) as part of a longitudinal study on the ecology of cetacean species. On September 21, 2020, an adult Risso’s dolphin was observed carrying a dead newborn calf in coastal waters. This presumed mother was further sighted with her deceased calf on September 23 and 24 (20 km and 40 km from the first observation site, respectively) and with other adult individuals on September 30, probably after having lost or finally abandoned the carcass. The presumed mother supported the corpse for at least 5 days, which may have had potential consequences for the female health. As direct observations are scarce in the wild, this type of report provides valuable information to better document the occurrence of care-giving behavior in highly mobile marine top predators.
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The social organization of mammal-eating "transient" killer whales (Orcinus orca) was studied off southern Vancouver Island from 1985 through 1996. Strong and long-term associations exist between individual transients, so sets of individuals with consistently high association levels, termed pods, can be delineated. Pods consist of individuals of mixed ages and sexes, and typically contain an adult female and one or two offspring (averaging 2.4 individuals). The mother-offspring bond remains strong into adulthood for some male (and less often for female) offspring. Other males disperse from their maternal pod and appear to become "roving" males, spending some of their time alone, and occasionally associating with groups that contain potentially reproductive females. These males appear to have no strong or long-term relationships with any individuals, and adult male - adult male associations occur significantly less often than expected by chance. Females that disperse from their natal pod appear to be gregarious (having high average association rates) but socially mobile (having low maximum association rates). Differences in social organization from the sympatric fish-eating "resident" killer whales (where no dispersal of either sex occurs) likely relate to differences in foraging ecology. Transient killer whales maximize per capita energy intake by foraging in groups of three individuals, whereas no such relationship has been documented for resident killer whales.
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Infanticide is considered a conspicuous expression of sexual conflict amongst mammals, including bottlenose dolphins. Although reported previously in this species, confirmed cases of infanticide and associated epimeletic behaviour are very rare and their socio-behavioural context remains poorly understood. Here, we provide evidence of epimeletic and infanticide behaviours in free-ranging bottlenose dolphins in Galicia, NW Spain. After describing the observed events, we include a complete description of the post-mortem examinations (where the carcasses were recovered) in order to confirm the cause of death. With evidences of blunt trauma in two of the presented cases, we confirm that the calves were intentionally killed by adult individuals. The aggressive interaction between adult individuals and the neonates together with the observed ante-mortem injuries bore a strong resemblance to the behaviours and traumatic injuries described in other cases of violent dolphin interactions in other parts of the world. The circumstances under which these infanticides occurred at our site fit the conditions proposed under the sexual selection hypothesis. The difficulties for researchers to observe this type of behaviour in the field and to find carcasses in good enough condition to determine the cause of death, emphasizes the importance of this type of study.
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Infanticide has been observed in several mammalian taxa and studied in considerable detail in carnivores and primates. Although reported previously in cetaceans, known cases are few and their socio-behavioral context remains poorly understood. We report here on three cases of social coercion directed at mother-neonate pairs of Indo-Pacific humpback dolphins (Sousa chinensis) in the Pearl River Estuary, southeast China. Two of these cases resulted in confirmed infanticide. To aid the interpretation of our field observations, we refer to the results of necropsies of calf carcasses stranded and recovered in our research area between 2003 and 2012, which indicate that in several cases the main cause of death of stranded calves was asphyxia resulting from blunt-force trauma. This is consistent with the aggressive behaviors seen during our field observations. We conclude that male infanticide is the most plausible interpretation of the observed behaviors, never previously reported for the genus Sousa, while the calf-directed aggression is likely a result of socio-sexual harassment by males as part of their reproductive strategy.
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Non-parental infanticide, the killing of infants by conspecifics other than the parents, occurs in a variety of vertebrate and invertebrate taxa. In rodents, infanticide has been noted in the wild or under laboratory conditions in two species of hystricognaths and 35 species of sciurognaths. Our review supports the hypothesis that non-parental infanticide is probably adaptive in rodents. However, its functional significance seems complex and cannot be explained by any one single hypothesis. In some cases, nutritional benefits are gained by females while in other species males may gain nutritional resources. In a few species, infanticide is a mechanism of resource competition. Evidence supporting the idea that individuals commit infanticide to avoid misdirecting parental care to unrelated offspring among rodents is rather limited. The sexual selection hypothesis in which males kill infants they have not sired as a means of reproducing with the victims’ mother remains unclear in rodents; studies that measure fitness benefits in terms of increased mating opportunities or of a reduced latency for the females to sire offspring of infanticidal males under wild or more semi-natural conditions are strongly needed. The nature of the mechanisms by which parents prevent infanticide has been controversial, and future studies need to consider two critical issues. First, information is needed from animals whose behavior is recorded under realistic ecological conditions. Second, alternative hypothesis should be stated a priori: the behaviors that have been suggested to be counter-strategies in rodents have other hypothesized functions as well. Overall, we encourage future investigators to design studies that will simultaneously evaluate multiple functional hypotheses and their specific predictions.
Male primates, carnivores and rodents sometimes kill infants that they did not sire. Infanticide by males is a relatively common phenomenon in these groups, but tends to be rare in any given species. Is this behavior pathological or accidental, or does it reflect a conditional reproductive strategy for males in certain circumstances? In this book, case studies and reviews confirm the adaptive nature of infanticide in males in primates, and help to predict which species should be vulnerable to it. Much of the book is devoted to exploring the evolutionary consequences of the threat of infanticide by males for social and reproductive behavior and physiology. Written for graduate students and researchers in animal behavior, behavioral ecology, biological anthropology and social psychology, this book shows that social systems are shaped not only by ecological pressures, but also social pressures such as infanticide risk.
Male primates, carnivores and rodents sometimes kill infants that they did not sire. Infanticide by males is a relatively common phenomenon in these groups, but tends to be rare in any given species. Is this behavior pathological or accidental, or does it reflect a conditional reproductive strategy for males in certain circumstances? In this book, case studies and reviews confirm the adaptive nature of infanticide in males in primates, and help to predict which species should be vulnerable to it. Much of the book is devoted to exploring the evolutionary consequences of the threat of infanticide by males for social and reproductive behavior and physiology. Written for graduate students and researchers in animal behavior, behavioral ecology, biological anthropology and social psychology, this book shows that social systems are shaped not only by ecological pressures, but also social pressures such as infanticide risk.
I analyse and summarize the empirical evidence in mammals supporting alternative benefits that individuals may accrue when committing nonparental infanticide. Nonparental infanticide may provide the perpetrator with nutritional benefits, increased access to limited resources, increased reproductive opportunities, or it may prevent misdirecting parental care to unrelated offspring. The possibility that infanticide is either a neutral or maladaptive behaviour also is considered. I devote the second half of this article to reviewing potential mechanisms that individuals may use to prevent infanticide. These counterstrategies include the early termination of pregnancy, direct aggression by the mother against intruders, the formation of coalitions for group defence, the avoidance of infanticidal conspecifics, female promiscuity, and territoriality. I evaluate the support for each benefit and counterstrategy across different groups of mammals and make suggestions for future research.
This book brings together a range of scientific perspectives from biomedical research on stress and welfare, and assesses new approaches to conceptualizing and alleviating stress. While much of the focus in on conventional farm animals, there is also consideration of fishes, laboratory animals and zoo animals. The 30 contributors include leading authorities from North America, Europe, New Zealand and Australia. This book is invaluable for advanced students and researchers in animal behaviour, animal welfare, animal production, veterinary medicine and applied psychology. For more information see the CABI Publishing online bookshop (