Female killer whales have the longest menopause of any non-human species. Now scientists think they know why.
If his mommy dies, a male whale is 14 times more likely to die within the year.The research team, from the Universities of Exeter and York (UK), the Center for Whale Research (USA) and Pacific Biological Station (Canada) studied killer whales in the North Pacific ocean over a period of 36 years.
Menopause seems difficult to explain in evolutionary terms: how does surviving well past the age in which reproduction is possible contribute to the survival of a species? Attempts to answer the question have lead to the so-called "Grandmother hypothesis" which surmises that Grandma's contributions to the core family unit help the survival chances of the third generation.
In the case of whales, the mothers themselves remain dedicated to their offspring a lifetime long -- until well after their sons are mature whales in their own rights. According to Dr Dan Franks, a Biologist at the University of York:
Our analysis shows that male killer whales are pretty much mommy's boys and struggle to survive without their mother's help. The need for mothers to care for their sons into adulthood explains why killer whales have evolved the longest post-reproductive lifespan of any non-human animal.
The press release explains the unique social groups of killer whales: "Sons and daughters (stay) with their mothers in a single group throughout their lives. With this close association, older mothers have the opportunity to increase the transmission of their genes by helping their adult offspring survive and reproduce. When sons mate, their offspring are cared for by females in another group, whereas when daughters reproduce the offspring stay in the group, which increases local competition for resources within the group."
Southern resident killer whales. Credit: Center for Whale Research