IN THE narrowest evolutionary sense, reproduction is the sole measure of success. Menopause, then, poses a bit of a riddle: in a handful of species, including humans, females cease breeding decades before they die. This is puzzling because it limits the number of offspring they can produce. However, two recent studies suggest that, in very different ways, the explanation lies in the extraordinary value of having a grandmother.
Evolutionary success is not just a mad dash to procreate; it also requires as many descendants to survive as possible. So a period of infertility can leave a female free to shepherd a greater number of grandchildren into the next generation. Despite the popularity of this “grandmother hypothesis”, it has been hard to prove decisively, not least because among mammals the menopause seems to have evolved only in humans and some toothed whales.
To study why this is so, Rufus Johnstone of the University of Cambridge used computer models to estimate the benefit of helping groupmates. According to his results, published inProceedings of the Royal Society B, the much-vaunted joys of grandmotherhood are no certain thing.
In many species, for example, males move away upon reaching maturity, whereas females stay. As a result, with each new generation, immigrant males contribute new genes; on average, older females become less genetically related to the group. This reduces the appeal of helping to rear others’ offspring. In contrast, when newly matured females leave to join a clan of males (as ancestral humans may have done), they become increasingly related to groupmates as their offspring make up a growing proportion of the community. When both sexes seek outside mates but return to bring up offspring in a social group (as whales do), ageing females also derive an increasing evolutionary benefit from offering help. Dr Johnstone’s data, therefore, show that despite their differences, both whale and human societies favour a transition to sterile nannyhood.
Indeed, whales and humans have even evolved a similar nurturing style of grandparenting. For humans, senior females provide extra foraging power and child-rearing skills. Though less studied, cetacean matriarchs may shepherd kin to lush feeding grounds along migration routes.
But grannies can also be warriors, says a recent study of Japanese aphids led by Keigo Uematsu of the University of Tokyo and published in Current Biology. Each aphid community is founded by a single, asexually reproductive female at the end of winter. By mid-spring, her brood can number thousands of genetically identical descendants. So by helping her colonymates survive and procreate, an aphid ensures that her own genes are passed on to future generations.
The aphid colonies grow inside galls, abnormal outgrowths on plants. As a gall matures, a small pore opens, enabling fertile aphids to emerge and disperse—and predators to enter. The researchers noticed that invaders were often mobbed by older, wingless aphids. On close inspection, these defenders were found to be sterile females, whose reproductive organs were displaced by an abdominal stockpile of sticky wax. They defend the pore by gluing themselves to intruders, blocking the entrance and protecting the younger, fertile insects cloistered within.
To test the import!ance of these militant matrons, Dr Keigo and his colleagues introduced predatory ladybug larvae to the plants. If the elderly sentries were first tweezed off, more than 63% of the larvae gained entry. If they were left in place, only 23% got through. For some grannies, late-life duties may involve providing cuddles; for others it’s suicidal warfare.