Having children is demanding. For female orcas, however, raising sons is particularly exhausting. At least that’s what a study in the journal says Current Biology vicinity, for which data from 40 female orcas of the so-called southern resident population in the northeast Pacific were analyzed. According to this, orca sons place such extreme demands on their mothers throughout their lives that the annual probability of having more offspring is roughly halved with every male killer whale born.
The team led by Michael Weiss from the British University of Exeter was not able to give a conclusive answer as to why this is the case. One assumption has to do with the competitive behavior between mothers and daughters: Orcas – also known colloquially as killer whales – live in groups. Orca daughters usually reproduce within the mother’s group. This makes them direct competitors of their mothers when looking for a partner.
Orca sons, on the other hand, seek out a new group to mate. The behavioral researchers conclude: In order not to raise their own rivals with maternal care at the same time, orca mothers prefer their sons and go to great lengths to ensure their well-being. If their sons reproduce, their mothers have two advantages: their place in the group was not questioned and their genetic material was passed on. The indirect reproductive competitive advantage that mothers gain by favoring their sons ‘appears to outweigh the lifelong consequences for their reproductive success,’ the research team said.
“It can be assumed that male orcas stress their mothers equally at all ages”
Another explanation for this behavior would be that male orcas generally have a higher energy requirement than their female siblings. In order to cover that, the mothers have to give up more of their loot than their daughters would ask for. So much – that ultimately too little is left for them to have enough energy for successful reproduction.
Unlike other mammals, the sons’ burden does not decrease as they get older. The study states: “It can be assumed that male orcas are equally demanding on their mothers at all ages.” The phenomenon that mothers continue to spend their entire lives looking after their offspring is extraordinary. The researchers therefore describe their study as the first direct evidence of this behavior among iteroparous animals – i.e. those that reproduce several times in their lives.
However, this behavior of the orca mothers, which may have been advantageous in the course of evolution, is now endangering the survival of the population. The Southern Resident population is considered critically endangered with only 73 remaining in the surveyed area. 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.