Playing favourites

We know of only three species that enter menopause: short-finned pilot whales, orcas and human beings. Scientists have long been baffled by the phenomenon. After all if, as evolutionary biology suggests, the purpose of life is to pass on your genes, why give up reproducing before the end of your lifespan?

Previous studies suggested that the biological drive behind menopause – at least in orcas – lies in their complex social structure. Research showed that matriarchal killer whales continue to care for their pods long after reaching menopause (they become infertile in their 30s or 40s but can live for more than a century) and that the whole group benefits from their continued survival as ‘repositories of ecological knowledge’. The same is true for humans and has been termed ‘the grandmother effect’. Now, a new study that examined the same resident orcas in the Pacific Northwest has taken the theory one level deeper – and darker.

The research suggests that it is not just the altruistic nature of grandmothers that drives menopause, but rather the favouritism exhibited by their daughters. The research team, from the University of Exeter in the United Kingdom, examined 43 years of data, during which 525 calves were born. In 161 of these births, older females produced calves at the same time as their daughters did. And in about 31% of these cogeneration births, the calves died, with the mortality of the older females’ calves being 1.7 times higher than that of the younger females’.

‘It’s a tug of war between young and old females, and the young pull harder,’ behavioural ecologist Darren Croft told Hakai Magazine.

One of the key dynamics of resident orcas is that a female will stay with the pod for her entire life, eventually bringing her own offspring into the fold. As the family grows over time, an older female will find that her own gene pool has spread increasingly wide within the pod. While a young mother, with fewer offspring, will fiercely prioritise her own calf, older females are less inclined to show favouritism towards their youngest progeny, choosing rather to ‘spread the love’ among all their relatives. Thus, for these orcas – and perhaps for human beings as well – a rare and sophisticated family structure that leads to both cooperation and breeding competition between generations has made menopause a sensible evolutionary strategy.