Orcs who still have their maternal grandmothers survive longer than others, especially when the fish is scarce, scientists report on Monday in a study that adds a stone to the knowledge of a rare phenomenon in mammals: the menopause.
Female orcs cease to reproduce after 30 or 40 years, but still live decades beyond. Only humans, killer whales and three other species of cetaceans (beluga, narwhal and pilot whale) experience menopause.
One hypothesis is that this evolution allows postmenopausal females to help their offspring, which is called the "grandmother effect" in humans. But the theory had not been observed scientifically in whales.
"This is the first non-human example of a grandmother effect in a menopausal species," says lead author Daniel Franks of York University at AFP. The study was published in the American scientific journal PNAS.
"This has also been observed among elephants, but they can reproduce until the end of their life," adds Daniel Franks. It is not known why cetaceans have menopause and not elephants or other species.
For this work, scientists have used more than 40 years of census data on two groups of killer whales living off the northwest coast of the United States and British Columbia in Canada.
The researchers knew how to identify individuals by the shape of the fins, the spots near the fin and the various scratches of their bodies. Sex was revealed by the pigmentation around the genitals and the adult size of the fin. Family ties were identified by group observation.
Of the 378 grandchildren studied, those whose maternal grandmothers had died in the previous two years had a mortality rate 4.5 higher than the others in that period.
The link was even stronger when salmon was scarce in the region.
How to explain it? Scientists had already seen that older killer whales led their group towards more fish-rich waters. "We've already seen them share their food with the younger ones, and they're also suspected of babysitting," says Daniel Franks.
One hypothesis to explain the usefulness of menopause is that it reduces the competition between mothers and girls for reproduction. But that would not explain why elephants, who do not experience menopause, also help their grandchildren.
The team will continue its research to observe more precisely the operation of whale groups, using drones in particular.
© 2019 AFP