Killer whale mothers maintain a very intensive relationship with their sons – even when they are already grown up. However, their enormous care also brings disadvantages for the females.

Orca mothers take great care of their full-grown sons and to a large extent do without further offspring of their own. The researchers write in the journal “Current Biology” after analyzing data from 40 female orcas of the so-called Southern Resident population in the northeast Pacific. The more surviving weaned sons an orca mother had, the less likely she was to give birth to a viable calf within a year. Daughters, on the other hand, had no noticeable influence on the mother’s reproductive success.

“The price a female pays for caring for her weaned sons is enormous,” said Michael Weiss of the University of Exeter in Britain. “We estimate that each additional male calf reduces a female’s chances of having a new calf in any given year by over 50 percent.”

Parental care is widespread in the animal kingdom: for a certain period of time, the parents, or at least one parent, take very close care of the offspring in order to increase their chances of survival. During this phase they even do without further offspring and thus reduce their own reproductive success. In some animal species, the offspring stay with their mother for life. But whether this lifelong care can bring disadvantages for the mother was previously unclear.

In primates – including us humans – it is assumed that a long-term mother-child relationship offers mutual benefits: mother and child support each other, which increases not only the child’s chances of survival, but also those of the mother and other siblings. Thus, the care that mothers invest in their children pays off later.

To find out whether this survival strategy also occurs in other animal species, researchers led by Weiss studied the orcas – also known as killer whales – in the northeast Pacific. “Not only do the Southern Residents exhibit an extreme form of maternal care that daughters, but especially sons, benefit from throughout their lives,” Weiss said. “This orca community is also one of the best-studied wild mammal populations in the world.”

The Center for Whale Research has been monitoring the population since 1976. It turned out that adult male orcas are still dependent on their mother: For example, she takes the lead when searching for food and shares her prey with her sons. To determine whether this intimate mother-son relationship also benefits the mother, Weiss and his team analyzed data from the years 1982 to 2021.

Even with the increasing age of the sons, the effect that the mother has fewer further offspring persisted. The observation cannot be explained by the male pups being breastfed for longer or by a different group composition with more male animals. So it seems that simply caring for sons for a long time leads to reproductive disadvantages. According to the researchers, this is the first direct evidence of lifelong maternal investment and a previously unknown strategy.

However, the scientists suspect that this behavior also has an evolutionary advantage for the orca mothers. “Maternal support increases both sons’ chances of survival and reproductive success,” explains co-author Darren Croft. “In this way, the females ensure that their genes are passed on to the next generation.”

What has established itself as an advantage over the course of evolution could now be fatal for the killer whales. Because the population of the Southern Residents consists of only 73 individuals and is therefore threatened with extinction. In addition, the salmon stocks that the Southern Residents feed on are also critically endangered. Whether the orca population can recover also depends on the survival and reproductive success of the females.