Off the Pacific coasts of the US and Canada, orca mothers provide food for their sons long after they reach adulthood, even though this restricts them from having more surviving offspring.
Orcas, also known as killer whales (Orcinus orca), give birth after 15 to 18 months of gestation and nurse their young for about two years. However, in the southern resident populations that live off the coasts of Washington state and British Columbia, only about a third of pregnancies end in a live birth, and many newborns don鈥檛 survive the first year, says at the University of Exeter, UK.
快猫短视频s had already noticed that southern resident males stayed close to their mothers even as adults, and that the mothers continued to care for them. So Weiss and his colleagues wondered if that long-term care might be affecting the mothers鈥� reproductive success.
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To find out, they examined data on 40 southern resident orca females from 1982 until 2021 and 54 calves that survived at least the first year of life. They then created computer models to analyse the effects of male and female offspring on subsequent births.
The researchers found that in each year after giving birth to a male calf, mothers were 70 per cent less likely to bear a healthy, surviving calf again, compared to those that had born a female calf. That wasn鈥檛 just due to higher lactation demands 鈥� as males might require more milk from their mothers than females. Even after their sons were weaned and were fully grown, the mother鈥檚 chances of successfully rearing another calf didn鈥檛 go back up again.
This seems to be related to the way mothers hunt and share food with their adult sons. 鈥淪ons specifically need this help because of their size: they鈥檙e huge,鈥� says Weiss. Their bulk increases their food requirements while making them poor hunters for their primary prey, Chinook salmon (Oncorhynchus tshawytscha). 鈥淭hey鈥檙e less manoeuvrable, which could potentially make them less able to capture these relatively small fish,鈥� he says.
The mothers might choose to help their adult sons, but not their adult daughters, because healthy daughters will reproduce within the same pod and create competition for resources, whereas the males鈥� offspring will be born in a different pod, Weiss says.
鈥淗is offspring are in some other group that are not your problem, and [the mother] still gets the benefits of [her] genes passing on to the next generation,鈥� he says. 鈥淭here seem to be a lot of evolutionary incentives to keeping your son alive as long as you can, basically.鈥�
The findings represent the first confirmed evidence of such a lifetime maternal investment in any animal capable of reproducing more than once, Weiss says.
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