Female orcas can go up to 20 years without reproducing, a very long period of their lives, considering that they can live up to ninety years. Until now, scientists did not know the reason for this break, but a study suggests that they do it to care for their sons and protect them from attacks by other orcas.
“The aim of the project was to understand how these post-reproductive females help their young and our results show that menopause is adaptive in killer whales,” says first author Charli Grimes, an animal behavior scientist at the University of Exeter.
To reach that conclusion, the researchers studied a group of killer whales that live off the Pacific Northwest coast. These orcas live in matriarchal social units consisting of a mother, her calves, and her daughters’ calves.
Using data from the Center for Whale Research’s annual photographic census of the orca population, the researchers looked for scars on the skin of each orca recorded. In addition, they took into account that these animals have no more natural predators than humans, so it is likely that a tooth mark capable of piercing their skin was inflicted by another orca.
The study, published in the journal Current Biology, found that if the mother of a given male was still alive and was no longer reproducing, that male had fewer teeth marks than his conspecifics without a mother or conspecifics with a mother that was still reproducing.
The authors only circumscribe this support to female orcas, but if they are mothers. “If you have a post-breeding mother who is not your mother within the social group, there is no benefit. It’s not that these females have a general police function but these mothers focus their support on their offspring,” says lead author Darren Croft, an animal behavior scientist at the University of Exeter.
Researchers still can’t say for sure what kind of social conflicts tooth marks cause, or how older females protect their offspring against them. They do not know its origin. What they did observe is that postmenopausal females have the lowest incidence of tooth marks in the entire social unit, suggesting that they do not physically intervene in a conflict.
