WCT killer whales may be well adapted behaviourally for performing infanticide because of their experience killing other small cetaceans. Individuals typically dispatch these prey by ramming or crushing them with their rostrums or tails36. Observations of infanticidal behaviour towards young common bottlenose dolphins, Indo-Pacific humpback dolphins and a Guiana dolphin by conspecifics have included repeated ramming and crushing actions but also forced submergence4,5,6,7,8,9. Although we did not observe the first 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 surface and breath due to his consistent forward motion. Although both T068 and T068A were observed with fresh teeth scars after the incident, the only ramming behaviour we observed was performed by T046B toward her neonate’s killer. This 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). The ramming occurred approximately ten minutes after the incident began, at which point the neonate would have been either close to or freshly drowned, which is probably why the fight did not persist. The brevity of this physical aggression might be typical of agonistic interactions between members of the WCT killer whale population. This, 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 reflect 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. This may have caused the T046Bs to separate into two groups to benefit the neonate and the other young animals through their disassociation from it, but vocalizations between them and their ultimate re-grouping may have revealed their locations to the T068s.

The 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. This benefits inclusive fitness of the female because a positive relationship exists between reproductive success and age in male killer whales45,46. This 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 offspring suggests that T068’s active involvement in this event was sexually selected, especially considering that T068A is of a reproductively mature age30,46. The sexual selection hypothesis requires that the infanticidal male does not kill his own offspring, that the event provides a near future mating opportunity with the infants’ mother and that he sires offspring with her12. Although it has been confirmed in many other mammal species that infanticidal males are not the fathers of their victims48,49,50,51,52,53,54,55 and that they subsequently sire offspring with the infants mothers13,15,51,53,55,56,57, 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 fitness. For example, young offspring are often 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 fitness61. 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 offspring, so its death along with loss of her neonate could have benefitted her condition by reducing any prohibitive physical costs associated with caring for either of them. This would help restrict parental investment towards any forthcoming offspring of T068A if he successfully mated with T046B. Furthermore, infanticidal males are typically not familiar with the infant’s mothers13 and although unrelated matrilineal groups of WCT killer whales have often 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. The apparent low levels of association between these individuals suggest that they are not well known to each other and this may be reflective of inherent sexual conflict between their differing 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 behaviour. 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 decades63,64,65,66,67,68,69. These increases in prey availability have been commensurate with increasing social activity70 and higher recruitment within the WCT population39 suggesting that it is not nutritionally stressed. Thus, 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 maturation in this species. Thus, this incident is not easily explained by the resource competition hypothesis. Finally, there is little support for interpreting infanticide as a non-adaptive behaviour other than when it occurs by accident 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 field observations to confirm that it does occur in cetaceans and more specifically, in killer whales. This species shares many life history traits with other mammals that are known to commit infanticide (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.