Whereas most animal species breed throughout their lives, humans and a few species of whales experience menopause. This is a time when the older females no longer reproduce but rather invest their energy and knowledge in helping their offspring and grand-offspring to survive.
Scientists have long puzzled over why menopause exists and how it evolved. Studies on killer whales have provided new evidence in this field. Killer whales occur in oceans throughout the world but are divided into distinct ecotypes based on differences in their hunting tactics and social behavior.
Previous research on the ecotype known as “resident” killer whales has shown that females spend around 30% of their lives in a post-reproductive state. Resident killer whales feed mostly on salmon and live together in large groups. Females in the group become more and more genetically related to the group over time (since their offspring remain in the group), so it makes sense that “grandmothering” would be a useful evolutionary strategy for mature females.
New research has recently been conducted on a different ecotype, known as Bigg’s killer whales. These particular whales specialize in hunting seals, which is a widely dispersed food item in comparison to a large shoal of salmon. For this reason, Bigg’s killer whales live in small groups and individuals tend to leave the group on reaching maturity.
“Some sons and daughters stay with their mother, but overall we predict a weaker pattern of increased relatedness to fellow group members as a whale ages,” explained study lead author Mia Lybkær Kronborg Nielsen of the University of Exeter. Weaker relatedness implies less evolutionary reason to stop reproducing and enter menopause.
The study used more than 40 years of data on Bigg’s and resident whales, and found that females from both ecotypes were post-reproductive for around 30% of their adult years.
According to Professor Darren Croft, it is not immediately clear why the age at menopause and the length of the post-reproductive lifespan seems to be the same in both.
Further research is needed in order to understand these findings. In future, drones will be used to investigate how “grandmothering” by mature whales contributes to the survival of their offspring and grand-offspring.
“Not only do the results contribute to a better understanding of animal evolution, they have significant implications for conservation by shedding light on the importance of social structure for the recovery of these populations,” explained study co-autor Thomas Doniol-Valcroze.
The study is published in the journal Ecology and Evolution.