Did you know that menopause is one of nature’s greatest mysteries? Even more surprising, only three species go through menopause: humans, pilot whales and killer whales.
Life-history theory predicts that no species would have menopause because once reproduction stops, there is no need for selection for survival. Yet studies have shown that humans in hunter-gatherer communities live beyond 60 years of age, well past reproduction. Many researchers believe menopause evolved through inclusive fitness (for success, a gene must leave behind the maximum number of copies of itself in the population) because postreproductive females help their relatives to survive. Researchers also speculate that menopause occurs because postreproductive females carry a wealth of knowledge about the world the group lives in, and this knowledge helps out the community in times of hardship. But, how exactly the females were helping their kin remained a mystery.
Now, a new study in Current Biology by a team of researchers from University of Exeter, University of York and the Center for Whale Research has provided the first evidence that the wisdom from postreproductive females can drive selection and survival for the community.
Researchers analyzed 751 hours of video footage of 102 killer whales in the coastal waters of British Columbia and Washington. The video footage was shot over the course of nine summers during the whales salmon migrations (when the whales travel in groups to food sources). Both killer whales and pilot whales rely on food sources that are scarce. Killer whales rely predominantly on salmon, which makes up 90% of their diet during the summer. From observing the whales during these migrations, researchers discovered that adult females were more likely to lead the group compared to adult males. And, among the adult female leaders, postreproductive females (35 years or older) led the most and always in times when the food source was scarce.
Knowing who led the group, the researchers next asked the question: Who follows the leader? “All offspring remain with mom, and by extension, with grandma,” says Dr. Kenneth Balcomb, one of the authors and a researcher at the Center for Whale Research. This means, the mother’s presence is very important to the group. Previous Studies have found that after a mother’s death, the risk of death in daughters increases 2.7 fold and 8.7 fold in sons.
Though both daughters and sons stay with their mother, researchers observed that it is best overall for the group if the mother helps the sons more than the daughters. Sons can reproduce outside of the group, and since offspring stays with their mother they do not join their father’s group and are not in competition for food sources. Females also live longer than males, surviving into their 90s, whereas males rarely survive beyond 50. This makes the males even more reliant upon the females for survival. The research team observed this to be true as the footage showed males were more likely to follow their mother than the daughters, and were even more likely to follow postreproductive females who were not necessarily their mother, but were still part of the group (i.e. adult sisters and grandmothers).
This study is the first to provide evidence that there is a benefit to a long life past reproduction, and that postreproductive females carry a wealth of information about the world that is valuable to the group, especially in times of hardship. Though males were more likely to follow than females, both males and females preferred to follow postreproductive females, especially in years with low resources of salmon. “As one of the only other menopausal mammals, and as a species that does not transfer information via written communication, killer whales may provide insight into how menopause evolved in preliterate humans,” say the authors.
Many animals share their experience with their offspring, so why is menopause so rare? The authors believe “menopause will only evolve when inclusive fitness benefits outweigh the costs of terminating reproduction.” In humans, pilot whales and killer whales, the benefits of helping increases with age, which researchers believe to be the reason these three species are predisposed to menopause. “Our study animals present an extremely social specialization that requires very longterm learning,” says Balcomb. “I have been involved in this demographic study for forty years, and we are now seeing confirmed known-age females entering the post-reproductive cohort. With the study continuing for a few generations of these whales, we can better answer questions; but, that may be a hundred years from now, if these whales are not driven to extinction.”