The presence of female orcas that live on long past child-bearing age boosts their grandchildren’s chances of survival, according to new research.
The menopause occurs only in orcas, three other species of toothed whales and humans, and it has long been an evolutionary mystery why females of these species stop reproducing well before the end of their lives. The scientists believe that their new study might provide the answer.
They found that post-menopausal orcas made the biggest contribution to the survival of their offspring’s offspring, possibly because, without direct parental responsibility, these females were free to devote their time and resources to the latest generation.
Post-menopausal grandmothers were found to have a particularly important role to play when food was scarce. Previous research had already shown that such orcas, with their accumulated life experience, tended to act as group leaders when foraging.
Marine biologists from the UK’s Universities of York and Exeter analysed 36 years of data gathered by the USA’s Centre for Whale Research and Fisheries & Oceans Canada on two populations of resident orcas living off the north-west Pacific coasts of North America. These populations, which included several pods made up of multiple family groups, had a diet of Chinook salmon.
“The study suggests that breeding grandmothers are not able to provide the same level of support as grandmothers who no longer breed,” said senior author of the study Dr Dan Franks from the University of York. “This means that the evolution of menopause has increased a grandmother’s capacity to help her grand-offspring.
“The death of a post-menopausal grandmother can have important repercussions for her family group, and this could prove to be an important consideration when assessing the future of these populations. As salmon populations continue to decline, grandmothers are likely to become even more important in these killer-whale populations.”
Orca sons and daughters stay with their mothers for life in such populations, but mate with individuals from different family groups.
Females tend to live longer than males, which often die at around 30, and usually stop reproducing in their 30s to 40s, though they can live for many decades after the menopause.