Giving birth to a son can be a costly experience for mother killer whales, also known as killer whales — and it comes at the expense of giving birth to additional offspring, researchers have found.
IN recent study, researchers report that male offspring from a population known as southerners — found along the Pacific coast of the US — depend on their mothers to such an extent that they reproduce less. (Daughters, however, do not appear to affect reproduction.) These adult sons impose “biologically significant reproductive costs” on their mothers, according to the study. The researchers used data on forty women from the population dating back to the 1980s to reach their conclusions.
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“We’ve known for quite some time that mother killer whales off the Pacific Northwest help their offspring survive throughout their lives,” said animal behaviorist Michael Weiss, research director at the Center for Whale Research and one of the authors. of the article.
Female killer whales and sons
Previous studies have shown that the mortality rate of male killer whales increases when their mothers die. It is also known that within this population – which is heavily dependent on Chinook salmon — female killer whales share their bounty with their sons by splitting the fish in half.
One hypothesis is that because of this practice of salmon sharing, mothers may not be able to carry a pregnancy to term, Weiss says.
Orca offspring
So far, this mother-son dependence of killer whale offspring is known to occur only in the southern population. “We’re interested to see if it’s present in these other populations with strong mother-offspring bonds,” says Weiss.
The new study reveals that orca mothers still feed their adult sons, a relationship that may come at the expense of further offspring. (Credit: Center for Whale Research)
About Julia Sutherland, researcher from Division of Marine Mammal Research at the University of St. Andrews, who was not involved in the recent study, found such comparisons “intriguing,” highlighting how different orca populations may function.
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“For mammals that forage on the coast of Scotland, for example, we have field observations of males actively hunting and even providing younger members of the group,” she says. “We see permanent pairs and individual roving males that successfully hunt alone. The findings of this paper offer an interesting case for comparison and contrast with population dynamics elsewhere in the world.
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What these findings mean for population longevity and broader conservation is still unclear, Weiss says, and further research is needed. Last year, only 73 individuals remain in this population.
Orca conservation status
“We know from previous modeling that the growth of this population in the future will be determined mostly by the reproductive rate of the population,” Weiss adds. Whether female killer whales have sons will be an important factor to consider moving forward in such modeling based on these findings.
“We’re interested in how this might affect extinction risk, but what drives the population toward extinction is the lack of the primary food source,” Weiss says. Food shortages, boats and pollution are all these known to affect the population, leading to their endangered status. “They are specialist Chinook salmon that are in decline or greatly reduced from historical levels.”
“But at the end of the day, from a conservationist’s perspective, the thing we need to do is pretty clear,” Weiss says. “It’s not easy, but [we need to] to rebuild their prey populations.”
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