Post-menopausal killer whale grandmothers improve the chances of survival for their grand-calves, new research has found.
The study found that grandmothers who were no longer able to reproduce had the biggest beneficial impact on the survival chances of their grand-offspring. This may be because grandmothers without calves of their own are free to focus time and resources on the latest generation, the researchers suggest.
The research team also found that grandmothers had a particularly important role in times of food scarcity, as the impact on a calf of losing a post-menopausal grandmother was highest in years when salmon was scarce.
Previous research has shown that post-reproductive female killer whales are the most knowledgeable and provide an important leadership role for the group when foraging in salmon grounds.
These benefits to the group may help to solve the long-standing mystery of why the menopause has evolved in some species of whales and in humans, the authors of the study say.
“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.”
In resident killer whales, both sons and daughters stay with their mothers for life, but they mate with individuals from a different family group. Male killer whales typically have a shorter lifespan than females with many not surviving beyond 30 years. Females usually stop reproducing in their 30s-40s, but just like humans they can live for many decades following menopause.
“We suspect when breeding grandmothers are supporting their own calves, their movement and activity patterns are constrained and they are not able to provide support and leadership in the same way as post-menopausal females. Also, grandmothers with their own calves will be busy caring for their own calves, and be able to invest less in their grand-offspring, compared to post-menopausal grandmothers.
“Our new findings show that just as in humans, grandmothers that have gone through menopause are better able to help their grand offspring and these benefits to the family group can help explain why menopause has evolved in killer whales just as it has in humans.”