Posts Tagged ‘adaptiveness of post-reproductive fem’

Older Females as Leaders

Wednesday, April 22nd, 2015

A killer whale pod is a spectacular, complex, cooperative hunting group.

Cooperative hunting as a way of getting food is hard at best, and demands considerable intelligence and a good memory. It has evolved in a few mammals, and among these are killer whales and the closely related pilot whales.

A resident killer whale pod on the coast of BC (bcwhalewatchingtours.com)

A resident killer whale pod on the coast of BC (bcwhalewatchingtours.com)

The theory of natural selection long ago was extended to include cooperative behavior, where individuals could increase their own fitness by helping close relatives. In every case, each hunting group consists of close relatives. As with most animals, adults die not long after they reproduce for the last time, but killer whales, at least in their ‘resident’ eco-morph, are the a remarkable exception: though they are able to reproduce until about 40 years old, and males rarely live much past that age, females may live for decades after the end of their reproductive lives.

Resident pods of killer whales hunt for salmon in the Salish Sea - the coastal interior waters of southern BC and northern Washington (timescolonist.com)

Resident pods of killer whales hunt for salmon in the Salish Sea – the coastal interior waters of southern BC and northern Washington (timescolonist.com)

The best data come from a resident pod that hunts mostly for Chinook salmon in the Salish Sea (BC/ Washington State) and that has been followed intensively since 1976. Every individual and its relationships to all the others in the pod is well documented. Though the age of the oldest female is not definitively known, it is in the range of 103. And that is amazing.

What aid might she and other older females provide to her pod that younger and possibly stronger adults could not more easily provide? An enticing possibility is that older females might be able to provide ecological information about when and where to hunt for fish, particularly in times of environmental stress when fish are hard to find. In hopes of testing this hypothesis, a group of scientists studied pod leadership over the years, in times of both salmon abundance and salmon scarcity. They found that in times of salmon scarcity, older females were more likely to lead the hunt.

When fish are scarce, older females (red in this cartoon) are more likely to lead the hunt to find them (cell.com/current-biology)

When fish are scarce, older females (red in this cartoon) are more likely to lead the hunt to find them (cell.com/current-biology)

This isn’t exactly proof, but it provides tantalizing support for the hypothesis that older females are valued and useful as repositories of ecological knowledge. Of course older females may help the pod in other ways – perhaps assisting others in the pod, mediating conflicts among pod members, providing familiarity with other groups – but these hypotheses are so far too hard to test.

In any case, the more we know about resident pod behavior, including the roles of older, post-reproductive females, the more we can ensure we don’t wreck the ecosystem the killer whales depend on. We should at least be able to do that.

And yes, there is one other species we know of where older post-reproductive females play a critical role in the social success of the group: us. Let’s hear it for grandmothers.

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