Killer whales are normally associated with Sea World or Free Willy, not with the science of reproduction. And yet, they are among the few species in the world, including humans, that undergo menopause, and enjoy a long lifespan thereafter. The origins of killer whale menopause are still hazy, but a recent study, making use of a 40-year killer whale census, applies human evolutionary hypotheses to killer whale data, and in turn finds that conflict for food might be at the root of killer whale menopause.
Menopause is not a foreign idea to humans, as we are familiar with its occurrence in females of a certain age. “However, it is actually striking and weird that humans, in comparison to other primates, are healthy for long after menopause,” says Dr. Kristen Hawkes, who researches human lifespan at the University of Utah. After all, the point of reproduction is to spread your genes as much as possible. Thus, if you discussed menopause with a brown bear, dog, or salmon, they would look on in horror and back away slowly, because ceasing reproduction so early in life seems wasteful.
Brown bears, however, do not grasp the utility of grandmothers. In humans, grandmothers help raise children, especially through cooking food that young children are unable to prepare for themselves. These “old ladies” are so helpful that they have a major impact on child survival, says Dr. Hawkes. A similar relationship holds true for killer whale grandmothers. They help with hunting and also pass on hard-won knowledge of the best locations to find food. This phenomenon is termed the grandmother hypothesis, and it states that menopause evolved specifically to allow grandmothers to care for their grandchildren, thus providing an evolutionary advantage for not reproducing past a certain age.
However, the grandmother hypothesis does not fully account for the evolution of menopause, says researcher Darren Croft of the University of Exeter. If grandmothers simply help raise grandchildren, menopause should occur later in life than it does. Why might menopause occur so early in human and killer whale females? A recent study, led by Dr. Croft, suggests that the answer, at least in killer whales, may lie in “reproductive conflict” for resources between older and younger mothers.
For his study, Dr. Croft used a killer whale census accumulated over 43 years. Using these data, he compared the survival rates of the children of young and old (but still pre-menopausal) killer whale mothers. The difference is striking: children born to older mothers die much more often than those born to younger mothers. The results of other analysis in the paper may explain why: because of the social structure of killer whale society, older females are more related to the other whales of their family group than younger females are. As a result, the researchers predict that older mothers spend less effort per offspring, because of increased food sharing and guiding other whales to food. This, in turn, could cause the decrease in survival for children of older killer whale mothers. Therefore, since younger females are free to focus solely on their offspring, it is evolutionarily advantageous for older females to stop reproducing and assist other areas of killer whale society, instead of investing smaller amounts of energy in offspring that are less likely to survive.
Killer whale menopause may be profitable for grandmothers in other ways, as well. A recent study led by Brianna Wright of Fisheries and Oceans Canada examined food sharing behavior based on the age and sex of killer whales. In her study, Wright determined that killer whale grandmothers not only share food with their grandchildren, but that they share a large portion of their food with their adult sons as well. Wright speculates that keeping their sons well-fed may help encourage their reproduction, further spreading the grandmother’s genes.
Wright’s data are not directly related to the evolution of menopause, but they further illustrate the yin and yang of the grandmother hypothesis and reproductive conflict, and how these forces may have shaped killer whale evolution. On the one hand, menopause may have evolved because food sharing, whether it is with sons or grandchildren, is extremely profitable for the grandmother who shares the food—the essence of the grandmother hypothesis. Conversely, menopause may have evolved because at the right stage in the life of a killer whale, reproduction costs more than it’s worth, as suggested by the reproductive conflict hypothesis.
These studies illustrate an often overlooked aspect of evolution: its subtlety. Neither the grandmother hypothesis nor reproductive conflict is enough to explain the evolution of menopause or long post-reproductive lifespans. However, the subtle influence exerted by both forces in concert may just be the reason that killer whales experience menopause. These studies further demonstrate the influence that subtle evolutionary pressures can have on establishing the fine balance of ecosystems, and of nature itself.
The Inner Life of Life 