Whale researchers say they’ve known for some time that male orcas demand more of their mothers’ attention than female ones, but they only recently learned what impact that extra neediness has on reproduction.
In a study of 40 southern resident killer whale mothers published Wednesday (Feb. 8), the Center for Whale Research found when the mothers birthed a son rather than a daughter their annual childbearing success was cut in half.
The study’s researchers believe part of this is because while mother orcas stop feeding their daughters once they reach reproductive age, they continue to provide for their sons into adulthood. The more sons around, the less food available for moms, the theory goes.
“Female reproductive success in this population is highly dependent on prey availability and females’ nutritional state, and thus reductions in food intake due to prey sharing are likely to have significant impacts on females’ reproductive success,” the study reads.
Mother orcas also choose to invest more energy into their sons than daughters.
“The evolutionary benefit of helping their male offspring survive into maturity is that the sons will mate, passing their genes to future generations,” lead-author Dr. Michael Weiss said in a news release.
It’s a strategy that researchers say could do more harm than good, though, for the southern resident whales as their numbers dwindle.
“A strategy of females reducing reproduction to increase male offspring longevity only works if a sufficient number of female calves are born, survive, and reproduce,” Weiss said.
There are 73 orcas remaining in the population, which has struggled to recover from marine park captures in the 1960s and 1970s. Since then, the endangered population has battled declining food sources, boat noise, pollution, and acidifying and warming oceans.