For female orcas, raising sons is at the expense of their chance of even more offspring, write British biologists into this week Current Biology. An adult son ensures that the orca mother’s annual reproductive success is roughly halved – even if the son is already approaching twenty. Mothers with adult daughters do not show such a clear influence.
It has been known for some time that mothers maintain a close bond with their offspring, and that the males in particular often benefit from their ecological expertise. An orca mother knows well where and how to catch the best prey, and is willing to share it with her offspring. But whether that extensive care was at the expense of the mother herself was unknown.
So the biologists studied data on forty female orcas collected between 1982 and 2021. During that period, 67 young orcas were born, of which 54 survived their first year (and were therefore considered a ‘successful reproductive outcome’ by the researchers). They then analyzed the chance of reproductive success per killer whale. It was greatest in females that had not yet had offspring, slightly lower in mothers of adult daughters and about twice as low in mothers of adult sons.
Unrelated males
The differences were not due to the fact that male orca cubs would cost more energy because they need more mother’s milk, because females who lost their sons had reproductive chances afterwards, while this was not the case if the son lived. The difference could also not be explained by the fact that male group members would cost more energy anyway (because they need more food and are less inclined to cooperate, the researchers say), because female orcas living in a group with unrelated males experienced no reproductive hindrance. It is possible that they share so much prey with their fully grown sons that they themselves do not get enough food and are therefore in a less good condition.
The biologists also found that maternal care continued regardless of the sons’ ages. And so their own chance of new offspring also remained lower. The fact that the effect of daughters on reproductive success is much smaller could be related to the fact that they are pampered less by their mother. Daughters often have offspring within the same group as their mother, while sons usually reproduce outside their own group. The arrival of grandchildren within the group would weaken the position of the older orca female, according to the authors. So it’s to her own advantage if her daughter doesn’t do too well.