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Killer whale study sheds light on the ‘grandmother hypothesis’

CC/Flickr/RayS
A new study found that having a post-menopausal mom around greatly increases the survival rate of grown male killer whales.

An intriguing new study on whales and menopause was published this week.

It found that having a post-menopausal mom around greatly increases the survival rate of grown male killer whales.

This finding is of interest to human as well as marine biologists. Why menopause happens so early in humans (around age 50) has long been a scientific puzzle. The females of only two other species, pilot whales and killer whales, are known to stop reproducing relatively early in their lifespan. Among all other species, including other primates, females tend to die soon after losing their ability to produce young.

The leading theory about why human females survive so long after menopause is known as the “grandmother hypothesis.” It makes the Darwinian proposition that long life after menopause enables women to continue caring for their children — and, of course, their children’s children — thus ensuring that their genes survive.

It now appears that older female killer whales perform a similar role in their pods, or family groups — although mostly for their sons. Here’s British science writer Ed Yong’s description of the study, which was conducted by a team of U.K. researchers and published today in the journal Science.

Killer whales, or orcas, become infertile during their 30s or 40s, but they can live well into their 90s. Individuals stay within the pod they were born in, which gives older mothers plenty of chances to help their children and grandchildren. … The question is: Does this actually matter?

There’s no better place to get an answer than the Pacific North-West. Since the early 1970s, when Mike Bigg discovered that individuals could be identified from photos of their fins, scientists have conducted a thorough census of all the whales swimming off Washington state and British Columbia. Led by Ken Balcomb, they have recorded the lives of 589 individuals (and the deaths of around half of them). They have even deduced the whales’ family ties.

By tapping into this rich vein of data, [researchers] found that a mother’s presence does help her offspring survive, even if they are full-grown adults. If sons are 30 or younger at the time of their mother’s death, they are 3 times more likely to die themselves in the next year. If they are older than 30 when mum dies, they are 8 times more likely to die.

It’s clear that mothers who had been through menopause were just as useful to have around, and probably more so, than those who are still fertile. On average, a 30+ male is 8 times more likely to die in the next year if his mother passes away, but his odds actually go up by 14 times if mum had gone through menopause. This confirms that mothers are helping their sons well into adulthood, since older orcas actually benefit from mum’s presence more than young ones. Perhaps she helps them to hunt, or maybe she watches their backs during fights with rivals.

Interestingly, adds Yong, “killer whale daughters don’t depend on their mothers in the same way. If mum died, younger daughters were fine, and older ones were just 3 times more likely to die in the next year.”

You can read Yong’s full discussion of the study on his Not Exactly Rocket Science blog on the Discover magazine website. (I strongly recommend bookmarking his blog if you’re at all interested in science and the natural world.) Unfortunately, the killer whale study itself is behind a paywall at Science.

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