A new study published today by three Georgetown researchers in the Proceedings of the Royal Society B could provide fresh insights into the evolution of menopause in long-lived mammals.
In a study of bottlenose dolphins, biology department Ph.D. candidate Caitlin Karniski, post-doctoral researcher Ewa Krzyszczyk and professor Janet Mann found that even though survival of offspring decreases as mothers age, the older mothers invest significantly longer (through nursing) in their final offspring than for their earlier progeny.
“The prominent hypotheses for how menopause evolved in humans rely on extended maternal care, much like we see in the final offspring in bottlenose dolphins,” says Karniski. “Some researchers, for example, maintain that grandmothers helping their daughters and granddaughters could be more beneficial than continuing to reproduce themselves and enough for this post-reproductive stage to evolve.”
Some studies in humans show that the last born child receives more investment from parents than middle-born offspring, the Georgetown author says.
Karniski says the new findings could be interpreted to imply that some threshold of maternal care is needed in order for an extended lifespan to evolve past the ability to reproduce.
While humans, short-finned pilot whales and killer whales are the only long-lived mammals that go through menopause, dolphins continue to reproduce throughout their life.
The new Georgetown study, for which Karniski serves as first author, found that older dolphin mothers use a compensating strategy known as “terminal investment” – nursing their last-born offspring longer than their earlier-born offspring, in some cases for as long as eight years.
“Because it wouldn’t benefit her to reserve any reproductive effort for future offspring, this mother really gives her all to her last born,” she explains. “Nursing also comes at a huge energetic cost to the mother, so this is especially striking if she is in poorer physical condition near the end of her life.”
Aging and Maternal Care
Mann and her research team have studied bottlenose dolphins in Shark Bay, Australia, for more than 30 years.
“This is the first study to show empirically this linear decline in reproduction with age in bottlenose dolphins,” Karniski says. “This is also one of the first studies to consider two different types of reproductive aging in a wild population – both the aging of the reproductive organs, but also how aging affects the ability to provide maternal care.”
The researchers based their findings on more than 34 years of longitudinal data on 229 adult females and 562 calves. They found that calf survival decreased with maternal age, and calves with older mothers had lower survival than predicted by birth order.
Proceedings of the Royal Society B is one of two scientific journals published by the prestigious Royal Society in London; Series B publishes research related to biology.