Cooperative behaviour as a population-shaping factor in northern resident killer whales

Source https://tse1.mm.bing.net/th/id/OIP.9g_Fnl4Sp3X6E22Xbke6_gHaE8?pid=Api; on 29/10/2025

The northern resident killer whale (Orcinus orca) is an ecotype of orca in the northeastern North Pacific Ocean. They live off the coast of British Columbia, Canada, southeastern Alaska and norther Washington state in the United States. There are three clans that consist of several pods with one or more matrilines in each pod. Matrilines are highly stable, multigenerational and closed to immigration which means that all offspring, male and female, stays within their matriarchy throughout their life. In biology this is called philopatry.

Philopatry is promoted when cooperative behaviours provide indirect fitness benefits to individuals staying with their natal group through kin selection. Fitness describes an individual’s contribution to passing on their genes to the next generation either by producing and supporting their own offspring or close relatives. Fitness is the ultimate endgame in the animal kingdom.  So, in simpler words, staying with the family and helping out can in some cases be a very good situation to leave your genetic mark.

Staying in the group for life also comes with costs. Usually, there is a competition for food, mates and territories which is why for most species at least one sex leaves the family to seek these resources elsewhere. The biggest risk is that staying with the natal group increases the chances of mating with kin. If no sex permanently leaves the group, animals tend to avoid inbreeding by mating with visiting groups or during temporary multigroup associations. This requires the ability to recognise relatives or non-relatives, which resident killer whales do through social familiarity and vocal dialect recognition.

Avoiding the problem of competition over food is where cooperative behaviour comes into play within this population. Northern resident killer whales are an exclusively fish-eating ecotype, and they are specialised on salmon, preferably Chinook salmon (Oncorhynchus tshawytscha). They have been observed to share prey which reduces food competition within the matriarchy and inhibits dispersal. Fish is broken apart before sharing and all species of salmon are shared regardless of size. This is remarkable because prey could easily be consumed by just one individual and there is no evidence that prey is cooperatively hunted. When prey capture and handling require the participation of multiple individuals, food sharing creates an immediate payoff for cooperation. Sharing individually caught prey, however, is costly and only happens if the sharer’s energetic loss is eventually compensated.

In a 12-year study (2002-2014), Wright et al. (2016) documented the cooperative prey sharing behaviour by northern resident killer whales to investigate the potential connection between prey sharing and group stability to find out if that could explain the lack of dispersal from natal groups in this population. They also hypothesised that prey sharing would be more common among more closely related individuals based on potential indirect fitness gain.

Their findings showed that prey sharing happens almost exclusively within the same matriline, even when killer whales foraged in mixes associations with multiple matrilines. Prey sharing is also more common with members that are more closely related. Individuals that were most likely to share food were adult females, likely to provide for their offspring (maternal care). Mothers providing for their young, for example subadults that may still lack the foraging skills to meet their energetic needs, increase offspring survival which equals direct fitness benefits. However, they can also gain indirect benefits by sharing food with maternal relatives. It is possible that resident killer whales exchange food for alloparental care (i.e. babysitting). However, all age and sex classes engaged in prey sharing with close maternal relatives which supports the importance of indirect fitness benefits over direct compensation.

Postreproductive females were the only demographic group that shared regularly with adult males. As males will mate with several females, adult sons have a higher reproductive potential than adult daughters. Therefore, postreproductive females likely experience greater indirect fitness benefits through increased production of grandoffspring by supporting sons. In turn, sons that share prey with their postreproductive mothers are actually performing a self-serving behaviour that directly improves their own fitness. Generally, adult males were much less likely to share prey. Their offspring will belong to different matrilines which means their potential to gain indirect fitness benefits from prey sharing within their matriline is therefore lower, which may explain why they are less likely to share prey.

Overall, the study provides strong evidence for cooperative behaviour as the driving factor of philopatry in northern resident killer whales due to increased fitness benefits through prey sharing between close relatives. Leaving the group at the expense of proximity and social bonds with maternal kin, would mean losing both the indirect fitness benefit of sharing with relatives and the direct fitness benefit of receiving extra food. This shows how important cooperative behaviours can be for the ecology of certain populations.

Source: Wright, B.M., Stredulinsky, E.H., Ellis, G.M. and Ford, J.K., 2016. Kin-directed food sharing promotes lifetime natal philopatry of both sexes in a population of fish-eating killer whales, Orcinus orca. Animal Behaviour, 115, pp.81-95.

By Hannah Carstens