Call them oracles of the orca world. Female orcas may go through the menopause so that they can use their accumulated experience and wisdom to guide their entire pod to food and survival, especially when the going gets tough.
"This is the first study showing these post-menopausal orca females act as repositories for important knowledge," says Darren Croft of the University of Exeter, UK. "They essentially store important survival information."
The results come closest yet to explaining why female orcas often live into their nineties, even though they typically stop breeding at 40. In contrast, males often only live to 50. The only other species known to go through a menopause and live so long without reproducing are humans and short-finned pilot whales.
Salmon hunting
Croft and colleagues watched 750 hours of video of orca family pods. Over 100 individually recognisable orcas were filmed in the coastal Pacific waters off British Columbia and Washington since 1976.
The team found that post-menopausal females were 32 and 57 per cent more likely than non-menopausal adult females or adult males respectively to lead the group. They were also significantly more likely to lead the group in years when their staple food – chinook salmon – was in short supply.
"It's probably accumulated experience," says Croft. "Anyone who fishes for migratory trout or salmon will tell you that timing is key, that the fish return in particular cycles of tides and times of the year. Post-menopausal females probably get to know where to look and when."
Earlier research by the same team showed that males are highly dependent on their mums for survival. If their mother died, sons were almost 14 times more likely to die within a year than sons in pods who still had their mothers. Daughters are at lower risk, 5.4 times more likely to die within a year of losing their mothers.
Wise matriarchs
This, says Croft, underscores the importance of mothers to survival, and supports the argument that their wisdom and know-how is crucial to the survival of their younger pod-mates. It may also explain why in the latest research, sons tended to stay closer to their mums, following immediately behind 29 per cent more often than daughters did.
Croft argues that the sacrifice of reproduction in order to assist family survival makes evolutionary sense because of the unusual family structures of orcas – structures not unlike those in human hunter-gatherer communities.
In most known animal species, males rapidly leave their parents, becoming completely independent. Male and female orcas, by contrast, stay in a family unit for life, with the males occasionally exchanging pods temporarily to breed. The upshot, says Croft, is that if females survive for many decades, breeding for the first three or four, their pod becomes increasingly replete with their descendants. Therefore, it becomes more and more in their own interests to safeguard the survival of the pod, and thereby their own genetic legacy.
"There's a tipping point where they stop reproducing and help their offspring instead, as do grandmothers in the human context," says Croft.
The findings seem to support the "grandmother hypothesis", the idea that older women in hunter-gatherer communities evolved to go through the menopause so that they could carry on passing on their wisdom and experience about food sources and other survival tips without the added costs of having more children themselves.
Mama's boys
"It seems male orcas are mamas' boys, either for their safety or their food. It also fits with theoretical predictions that sons should receive more help than daughters as their offspring are elsewhere and thus they are not competing with other family members," says Ruth Mace, at University College London
But there are still a few things left unexplained. For example, she asks: "Why can't males learn about fish and impart knowledge to family members? Are they impeded from learning because they die young or do they die young because they don't have any evolutionary reasons to help their family?"
And she says that given the differences between us and orcas the evolutionary basis of each species' menopause is not likely to be identical.
Journal reference: Current Biology, DOI: 10.1016/j.cub.2015.01.037
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