Killer whale grandmas help calves survive
Doting killer whale grandmothers help their grand calves survive, particularly in times of food scarcity, scientists reported Monday in a paper that sheds new light on the evolutionary role of menopause.
Orca females stop reproducing in their thirties or forties but can continue to live for decades more, a phenomenon known only to exist in humans and four other mammal species, all of which are whales.
It has been suggested that the trait evolved because it allowed post reproductive females to help their wider kin -- referred to as the “grandmother effect” in people, but the theory had not been tested in whales until now.
“This is the first non-human example of the grandmother effect in a menopausal species,” senior author Daniel Franks from the University of York told AFP.
“It has also been shown in elephants, but they are able to reproduce until the end of their lives. We currently know of only five species that go through menopause: the others are short-finned pilot whales, narwhals and beluga.”
Writing in the Proceedings of the National Academy of Sciences, Franks and colleagues examined more than 40 years of census data on two killer whale groups off the coasts of the US state of Washington as well as Canada’s British Columbia.
The individuals were identified by their unique fin shapes, saddle patches and the presence of nicks and scratches, and sexed by the distinct pigmentation around their genitals as well as adult fin size.
Their relations to one another were inferred through observations of social organisation, and mothers identified by their repeated association with their young calves.
The team, which also included scientists from the University of Exeter, the US Center for Whale Research and Canada’s Pacific Biological Station, focused on 378 individuals known to have a maternal grandmother.
They found that those whose grandmother died within the last two years had a mortality rate 4.5 times higher than those with a living grandmother, in the two years following her death.
“In killer whales, when mothers and daughters cobreed, the calves of mothers from (the) older generation have significantly higher mortality,” the paper said.
Comments