Sunday, 4 July 2010

Whales can be grandmothers too

Pilot Whales from Sea Fijii Travel

It is no secret that the pro-whaling lobby likes to characterize all cetaceans as 'fish'.
The Swedish Wire reported last month that:

Kristjan Loftsson, Iceland's millionaire whaling king, doesn't really see the difference: "whales are just another fish," he said at a crunch meeting of the International Whaling Commission.....If they [whales] are so intelligent, why don't they stay outside of Iceland's territorial waters?"

However, the fact that whales are social mammals often living in close kinship groups is well-known and research continues into why this is so.

According to Victoria Gill at BBC News on Friday 2 July 2010:

Scientists have discovered an evolutionary reason why humans and whales both have grandmothers.
As post-menopausal females age, the researchers say, they become increasingly interested and helpful in rearing their "grandchildren".
This could help explain why female great apes and toothed whales (cetaceans) have lifespans that extend long beyond their reproductive years.

To celebrate a summer of Royal Society science festivities, all Royal Society journal content is free to access until 30 July 2010, so the full research article by Rufus A. Johnstone and Michael A. Cant The evolution of menopause in cetaceans and humans: the role of demography can be downloaded as a PDF file:

Human females stop reproducing long before they die. Among other mammals, only pilot and killer whales exhibit a comparable period of post-reproductive life. The grandmother hypothesis suggests that kin selection can favour post-reproductive survival when older females help their relatives to reproduce. But although there is an evidence that grandmothers can provide such assistance, it is puzzling why menopause should have evolved only among the great apes and toothed whales. We have previously suggested
Cant & Johnstone 2008 Proc. Natl Acad. Sci. USA 105, 5332–5336 (doi:10.1073/pnas.0711911105)) that relatedness asymmetries owing to female-biased dispersal in ancestral humans would have favoured younger females in reproductive competition with older females, predisposing our species to the evolution of menopause. But this argument appears inapplicable to menopausal cetaceans, which exhibit philopatry of both sexes combined with extra-group mating. Here, we derive general formulae for 'kinship dynamics', the agerelated changes in local relatedness that occur in long-lived social organisms as a consequence of dispersal and mortality. We show that the very different social structures of great apes and menopausal whales both give rise to an increase in local relatedness with female age, favouring late-life helping. Our analysis can therefore help to explain why, of all long-lived, social mammals, it is specifically among the great apes and toothed whales that menopause and post-reproductive helping have evolved.....

Our analysis thus implies that females of most social mammalian species will experience a decline in local relatedness with age, but that the two unusual and very different social arrangements that characterize menopausal species (respectively, female-biased dispersal and local mating in ancestral humans, and philopatry of both sexes combined with extra-group mating in pilot and resident killer whales) both give rise to an increase in local relatedness with female age. This build-up of local relatedness over the reproductive lifespan of a female means that the great apes and toothed whales, by contrast with most mammals, are predisposed to the evolution of reproductive restraint and altruistic helping behaviour later rather than earlier in life. The value of an explicit focus on kinship dynamics is that it reveals the underlying similarity between the ape and whale cases, which would otherwise be obscured by the differences in their social structure....

The expected benefits of ceasing reproduction in order to selectively assist close kin will thus increase with age in the ape and whale cases because younger females are less likely than are older females to have close relatives in the group—there is little value to sacrificing the possibility of direct reproduction to become a helper at an age when there are likely to be few or no kin present to help.

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