A killer whale mom and her son.David Ellifrit, Centre for Whale Research
Human and killer whale mothers often live long past menopause and now research suggests their prolonged life spans may be due, at least in part, to never-ending care for offspring -- especially sons.
The discovery could help to explain why both human and killer whale moms often live longer than males. In the case of killer whales (also referred to as orcas), the life span differences are profound, with females frequently surviving into their 90's while males are rarely documented as living much past 50.
"For most animal species, the potential for an individual to increase the propagation of its genes stops when they stop reproducing," co-author Darren Croft told Discovery News. "Our results show that, as with humans, female killer whales can continue to increase the propagation of their genes long after menopause."
"They do this by helping to increase the survival of their older sons, which in turn increases the number of grandchildren fathered by their sons," explained Croft, a senior lecturer in animal behavior at the University of Exeter. "Through this process, evolution favors females that live longer after their menopause."
Croft and his colleagues from the Universities of Exeter and York, the Center for Whale Research and the Pacific Biological Station analyzed records spanning 36 years documenting two populations of killer whales in the North Pacific Ocean off the coasts of the United States and Canada. The unique data set consisted of 589 individually identifiable animals, of which 297 died during the study period.
After statistically modeling births to calculate the probability of survival for any individual whale at any age, the researchers found that whales with long-lived mothers tended to survive longer. This particularly held true for sons.
"Due to the stable social structure of resident type killer whales, when sons mate, their offspring are cared for by the females in another family group," Croft said. "In contrast, when daughters reproduce, the offspring stay in the group, which increases local competition for resources within the family group."
"Theory predicts that in order to have the best chance of spreading their genes, without carrying additional burden, mothers should focus their efforts on their sons," he added. "Our research supports this theory and demonstrates the extent to which older sons are dependent on their mothers for survival."
Killer whale moms may do this by assisting with foraging, providing support during fights and through other means.
Human mothers obviously live in a different societal structure, so caring for sons, daughters, and their grandkids may hold equal importance, at least from an evolutionary standpoint.
Such caregiving might even help to explain why menopause exists in the first place.
"While it is believed that menopause evolved in humans partly to allow women to focus on providing support for their grandchildren, it seems that female killer whales act as lifelong carers for their own offspring, particularly for their adult sons," Croft said. "It is just incredible that these sons stick by their mothers' sides their entire lives."
Michael Cant, an associate professor in evolution and animal behavior at the University of Exeter, Cornwall, told Discovery News, "This new data offers an exciting new insight into the evolutionary puzzle of menopause."
He pointed out that the issue has been difficult to study because, outside of humans and killer whales, only one other mammal -- the pilot whale -- goes through menopause. Whales are inherently difficult to study.
"As in humans, this new killer whale data suggests that post-reproductive lifespan in killer whales evolved because older mothers boost the fitness of their offspring," Cant said. "Unlike humans, however, where both sons and daughters benefit from the presence of grandmothers, in killer whales, only sons benefit."