Maternal care of the extreme kind: Orca mothers take care of their adult sons so intensively that their own reproductive success suffers, a study shows. Ultimately, however, this can be worthwhile for the mothers: The special investment in the male offspring probably increases their chances of having more grandchildren, the scientists explain.
Apparently selflessly, parents sacrifice themselves for the well-being of their offspring: This concept is also known to us humans and it is widespread among mammals. It is dedicated to the goal of promoting the survival and reproduction chances of the offspring, in order ultimately to ensure that one’s own genes are passed on. Maternal care in particular can take on very distinctive forms. In some species, the females dedicate themselves to the well-being of their offspring even into adulthood. A particularly interesting example of this can be found in the orcas (Orcinus orca), also known as killer whales.
Special support for the sons
These toothed whales are known for their high intelligence and complex social behavior. They live and hunt in groups led by experienced females. The mother animals initially take care of their children of both sexes intensively. But as studies on an orca population on the Pacific coast of North America have shown, something special occurs after they reach adulthood: while the young females become independent, the mothers continue to look after their sons. Above all, they provide them with food, as observations show: if a killer whale has caught a salmon, for example, she typically splits it in half, eats one and passes the other half on to her son.
Gender-specific care is also clearly noticeable at the population level: “We have already shown that sons have a significantly higher chance of survival if their mothers are present,” says Michael Weiss from the University of Exeter. “In the current study, we wanted to clarify to what extent this support has a price for the females”. Weiss and his colleagues evaluated data from the intensively researched Pacific killer whale population from 1982 to 2021. Various information flowed into the statistical evaluations in order to uncover connections.
At the expense of their own reproduction
As the researchers report, the results reflected that the investment in the sons weighed heavily on the mothers. The study found that each surviving son cuts the annual chance of his mother’s successful rearing of another calf by about half. This is probably because the mother whales do not eat enough themselves in favor of their sons. “The magnitude of the expense that females incur in caring for their adult sons was really surprising,” Weiss said. This extreme form of nurturing, in which mothers indefinitely sacrifice their own reproductive success specifically to nurture their sons, is highly unusual and possibly even unique, say the scientists.
But how can such a concept, which at first glance appears to be disadvantageous, develop in an evolutionary manner? This has to do with the fact that individuals are not “interested” in the well-being of their species, but only in the most effective way possible to pass on their own genes. The researchers explain that by investing in her sons, a female orca is better able to ensure that she leaves her legacy than by having more offspring of her own. Specifically: mothers who care for their sons benefit from the fact that they can mate with numerous females and thus provide for a comparatively large number of grandchildren. “Females have evolutionary advantages when their sons are able to reproduce successfully, and our results seem to indicate that these advantages are enough to outweigh the high direct costs,” says Weiss.
Ultimately, however, the concept may have a negative impact on the orcas’ overall reproductive rate, and thus be important for their conservation, the researchers say. This is particularly true in the case of the population studied, which appears to be very isolated and struggling with human-caused food shortages. Co-author Daniel Franks of the University of York says: “This strategy of sacrificing one’s reproductive success in order to promote sons may have been beneficial in its evolutionary past, but now it potentially threatens the future viability of this small population,” according to the scientist.
Source: Cell Press, University of Exeter, Article: Current Biology, doi: 10.1016/j.cub.2022.12.057