New research shows that raising sons is hard work, which makes killer whale mothers much less likely to have more babies.
The research discovered that each surviving son halves a mother’s yearly chance of successful breeding (a calf surviving for one year).
And this impact persisted as boys aged, indicating that sons are a burden for their mothers throughout their whole lives.
Killer whale moms are known to offer greater care to boys than daughters, particularly once girls reach maturity, and the data indicated that this support imposes a substantial cost on the mothers.
The Center for Whale Research, together with the universities of Exeter, York, and Cambridge, conducted the investigation.
Dr. Michael Weiss of the University of Exeter’s Centre for Study in Animal Behaviour said that their “previous research has shown that sons have a higher chance of survival if their mother is around.”
“In this study, we wanted to find out if this help comes at a price.
“The answer is yes – killer whale mothers pay a high cost in terms of their future reproduction to keep their sons alive.”
The research used data collected from 1982 to 2021 on forty female “southern resident” killer whales, who inhabit off the Pacific coast of North America.
Male and female resident killer whales remain in the group to which they were born, each of which is commanded by an experienced female. Salmon is the main food source for the resident killer whales in the southern waters. Salmon is often split in half by mothers, who eat half and give the other half to their boys.
They tend to cease feeding their young girls once they reach reproductive age, but they continue to feed their males until maturity.
This study uncovered an uncommon strategy in nature where mothers sacrifice their future reproduction to ensure the survival of their sons, and it may be considered a singular occurrence.
Professor Darren Croft explained that this evolution could be attributed to an “indirect fitness” benefit for mothers: by assisting in the survival and reproduction of their sons, their genes have a greater likelihood of being passed on to future generations.
It is obvious that this tactic has worked in the past. Mothers who put a lot of work into their boys would benefit because their sons could mate with a lot of females and have a lot of grand children.
This tactic, nevertheless, may now pose issues for the population’s long-term survival.
Killer whales that live in the south eat mostly Chinook salmon. Many of the populations of these fish are now vulnerable or endangered, and they have become rare in many areas of the whales’ range. The southern residents are also in danger because they don’t have enough food.
Due to their isolation from other killer whale populations, the 73 remaining southern resident killer whales are in danger.
The number of females and the reproductive output of those females will be a determining factor in the possibility for population recovery for this group, according to Professor Croft.
“A strategy of females reducing reproduction to increase the survival of male offspring may therefore have negative impacts on this population’s recovery.”
“This strategy of indefinitely sacrificing future reproduction to keep their sons alive may have been beneficial in their evolutionary past,,” remarks Professor Dan Franks, from the Departments of Biology and Computer Science at the University of York, “but it now potentially threatens the future viability of the southern resident killer whale population, which is critically endangered with just 73 individuals remaining.”
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