Killer whales abandon their female offspring as they mature but continue to care for males
Mothers forgo having more offspring and share the food they hunt with their sons for life but neglect their daughters after a certain age
Killer whales, like all other cetaceans in the Delphinidae family (dolphins, killer whales, pilot whales), are among the most maternal mammals. Mothers nurture their offspring until the young reach sexual maturity, while grandmothers participate in caring for their granddaughters. Now, a group of ecologists has discovered a surprising wrinkle in orca maternal behavior: a mother will sacrifice having more offspring to care for her male offspring all her life. But she does not do the same for her daughters. Scientists believed that this strategy was quite logical until orca whales began to lack food.
A particular killer whale population lives on the west coast of the United States and Canada. Unlike most of their congeners, which are migrants, the so-called southern resident orcas do not move from this part of the Pacific. As a result, they are science’s most-studied cetaceans. They have been key in investigating the role of menopause in the few mammals that experience it, as well as in studying the impact of competition with humans for the same resources (orca whales only feed on salmon). Now, with data from 50 years of observing 40 mothers, a group of scientists has discovered something totally unexpected in an animal as maternal as orca whales: they forgo having another calf to take care of the one they have, even if it has been an adult for years.
The results of the research, published in the scientific journal Current Biology, show that when a mother orca has a calf, the probability that she will become pregnant again in the following years is reduced to less than half. Like humans and rabbits, killer whales are iteroparous animals, that is, they can procreate many times during their lifetime. The opposite strategy is called semelparity (spawning only once). The best-known animal that practices such extreme behavior is the Pacific salmon; after a year at sea, it goes up the river to where it was born, then mates and dies moments later. It turns out that the southern resident orcas have been practicing semelparity, which had not been seen until now.
We knew that adult male killer whales depended on their mothers to stay alive, but it was never clear whether the mothers paid a price to do so.Michael Weiss, an ecologist at the Center for Whale Research (USA) and the Centre for Research in Animal Behaviour at the University of Exeter (UK)
The study’s lead author Michael Weiss of the Center for Whale Research (USA) and the Center for Research in Animal Behaviour at the University of Exeter (UK) said: “For more than a decade, we have known that adult male killer whales depended on their mothers to keep them alive, but it was never clear whether the mothers paid a price to do so.” They do. “The magnitude of the cost that females take on to care for their weaned offspring is truly staggering. While there is some uncertainty, our best estimate is that each additional surviving offspring reduces a female’s chances of having a new brood in a given year by more than 50 percent. That’s a huge price to pay to care for already adult offspring!” added Weiss, who is also a professor at the University of Exeter (UK).
But the study yielded even more surprising information. The authors also found that mothers sacrifice more for their sons than for their daughters. They care for males and females equally while the offspring are dependent on the mother’s milk and during their youth, but that changes afterward: “Mothers seem to stop sharing food with their daughters when they reach sexual maturity, around the age of 12. In contrast, [mother orcas] never seem to stop [sharing food] with their sons,” Weiss said. Each time they catch a salmon, half goes to the son. It should be remembered that there is a marked sexual dimorphism among these cetaceans, meaning that males are larger and need much more fish than females. However, this differential pattern does not change even when the male orca ceased to be a calf years ago.
Darren Croft, Weiss’s colleague at Exeter and co-author of the study, argued that “mothers gain an indirect biological benefit: helping their offspring survive and reproduce improves the chances that their genes will be passed on to future generations.” To explain this strategy, the study’s authors noted that, in this population of killer whales, both males and females live and die within the group in which they are born. Males mate with females from other groups, but they always return to their mothers.
Mothers gain an indirect biological benefit: helping their offspring survive and reproduce improves the chances that their genes will be passed on to future generations.Darren Croft, a biologist at the University of Exeter (UK)
Weiss highlighted other advantages of this unique maternal investment: “First, offspring become more reproductively successful as they age, and the oldest males in the population get the most mating opportunities. So, making sure that a son survives to old age is a good way to ensure that her genes are well represented in the next generation.” Meanwhile, their daughters pose a threat: “The offspring of sons are born in other groups, while those of the daughter are born within the group itself. This implies a potential cost of the daughters’ reproduction, since her offspring will require additional resources, while the son’s offspring do not represent a cost to the group.”
The low birth rate of orca whales is a direct consequence of this strategy. Over the past 50 years, the 40 mothers that make up the various groups of the southern resident killer whale population have had 67 calves; 54 of them have survived their first year. When food is available, the strategy of taking the risk to raise a calf seems to be a good one. The problem is that there are fewer salmon off the north Pacific coast, which makes feeding them more difficult. In addition, these whales have never moved from this area, and they do not mate with the orcas that do migrate. As a result, the population has been declining steadily for years.
Dan Franks, the biology professor at the University of York (UK) who confirmed the hypothesis that orcas live longer when they are cared for by their grandmothers, said: “This strategy of indefinitely sacrificing future reproduction to keep their sons alive may have been beneficial in their evolutionary past, but it now potentially threatens the future viability of the southern resident killer whale population.” The southern resident killer whale population, which numbered nearly two hundred in the middle of the last century, is now critically endangered; in 2022, only 73 of them remained.
The study’s authors are not aware of this type of maternal investment occurring among other animals or even among other killer whale populations. But the scientists assume that it does. In fact, they believe that other cetacean species whose daughters and sons live with their mothers forever and whose males mate outside the group, such as pilot whales or black killer whales, could also follow this rare reproductive strategy.
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