As humans, we often take grandmothers for granted. Dispensing baked goods and well-intentioned advice, grandmothers are an integral part of our human societies. However, human grandmothers represent an incredibly rare phenomenon among all living things: they are post-reproductive. The vast majority of plants and animals do not live past their reproductive years. Dramatic examples of a single reproductive bout followed by death within days, such as occurs in Pacific salmon, are far more common in the animal kingdom than the curious case of post-reproductive individuals. The rarity of post-reproductive individuals has led many researchers to ponder, what is the purpose of life after reproduction?
Human females have the longest post-reproductive period in the animal kingdom, but female killer whales are close runners-up. Female killer whales cease reproducing at approximately 30 or 40 years of age, and yet can live for approximately 90 years, with a post-reproductive period that rivals humans. In a 36-year long-term study of over 500 resident killer whales off the coast of Washington, USA, and British Columbia, Canada, a group of researchers have uncovered convincing evidence that there is an evolutionary benefit to this extended post-reproductive period.
Led by Emma Foster from the University of Exeter, UK, the group of scientists from Exeter, the University of York, UK, the Center for Whale Research, USA, and the Pacific Biological Station, Canada, tracked family groups of whales for multiple generations. The scientists identified mother–offspring pairs by recording small calves with their mothers. As there is no dispersal in killer whale family groups, the disappearance of an individual from a family group indicates that the individual has died. From this long-term data set of family trees and family member survival, the researchers were able to assess the consequences of a mother's death on the survival of her sons and daughters.
The group of scientists found that for an adult female killer whale, there is no benefit of having her mother around. Once grown, female survival is independent of whether her mother is still alive. However, having a surviving post-reproductive mother significantly increased survival for sons, even for 35-year-old, fully grown male killer whales. Killer whale mothers help their grown sons forage, and they also form alliances with their sons, defending them during fights with other whales. It seems that with mum around to help feed and defend her sons, these males have a better chance of long-term survival even as adults.
From an evolutionary perspective, this favouritism may have arisen because resident killer whales live in permanent matrilineal family groups, where grandmothers, mothers, sons and daughters live and hunt together. However, males will mate with females from other family groups. Thus, although a grandmother must incur the costs of raising her daughter's offspring within the family group, other killer whale families raise the offspring of her sons. For a mother, the fewest costs and greatest benefits occur when she ensures that her sons survive and reproduce, rather than by producing more offspring of her own or by favouring her daughters. For these adult males, it seems that there are distinct benefits of having a post-reproductive mother around and being a mummy's boy.