When mothers and daughter whales breed at the same time, calves from the older whales are more likely to die than calves from the younger whales.
Killer whales are one of only three species that scientists know go through menopause. But what they didn’t know was why.
A recent report in Current Biology explains that the reason older female killer whales stop reproducing is due more to conflict than to cooperation between mothers and their daughters.
The only three species that are known to go through the process of menopause are humans, short-finned pilot whales, and killer whales. The females of these species lose their ability to reproduce, but the animals live on for decades. Female killer whales start reproducing at the age of 15 and stop in their 30s or 40s, but they can live past the age of 90.
An international research team from the Universities of Exeter, Cambridge, and York in the United Kingdom; the Center for Whale Research in Friday Harbor, Washington; and Fisheries and Oceans Canada in British Columbia set out to unravel the menopause mystery. To do so, they studied the family ties of wild resident orcas from the Pacific Northwest.
The team set out to test the “reproductive conflict” hypothesis, which proposes that women in ancestral social groups left their homes when they found a partner and lost ties with their blood family. Over time, they became more closely related to their new family, leading older women to stop reproducing later in life to invest fully in helping their younger kin.
The hypothesis builds on the idea that female orcas past reproductive age take on a “grandmother” role within the pod in which they share their knowledge of where and when to find food in order to help the family survive.
The study data showed that older female whales are more closely related to their kinship group than are younger females. The investigators determined that when older and younger females breed side by side, natural selection almost always favors the younger females.
Food sharing is one of the main reasons behind the so-called reproductive conflict among mother and daughter whales. They forage together and share food, with both male and female offspring relying on their mothers for food, typically for many years. Behavioral ecologist Darren Croft, lead author of the current study, says that “females of many species act as leaders in late life but continue to reproduce,” but this new research explains that older females loser out “in reproductive competition with their daughters.”
This evidence states that it’s more challenging when older females reproduce alongside their daughters. When mothers and daughters breed at the same time, calves from the older whales are 1.7 times more likely to die than the calves of the younger whales.
Investigators say it’s better for the older females to focus all their time and energy on helping their daughters reproduce, rather than to continue reproducing themselves.
This new understanding of how whale families interact may help better explain their survival and reproductive success—even why some species are endangered and at risk for extinction.
The next step in the research process is to incorporate the use of drones to look more closely at behavioral interactions of killer whales.
Croft says that getting a bird’s eye view will “transform our understanding of the social lives of these amazing animals.”
While the rarity of menopause in killer whales is now better understood, there remain several other quirks and behaviors that need this same understanding.