Killer whale mothers’ love for their sons comes with huge costs: Study

Killer whale mothers, not unlike human moms, go to extremes to provide for their sons, well into adulthood, sacrificing their future reproductive success, reveals a new study published in Current Biology.

The study shows that by caring for a son, the killer whale’s chances of having another offspring are reduced by 50 per cent. The study provides direct evidence of lifetime maternal investment in an iteroparous (multiple reproductive cycles) animal.

The authors of the paper titled Costly lifetime maternal investment in killer whales studied the ‘southern resident’ population of killer whales in the coastal waters of Washington state in the US and British Columbia in Canada.

This population is critically endangered and has only 73 individuals left. For the purpose of the study, the authors collected data on 40 females of the population from 1982 to 2021. They studied 54 calves who survived their first year of birth for the research.

The study aimed at finding whether male and female offspring reduced their mother’s future reproductive success. While the researchers did not find evidence that daughters influenced their mother’s chances of having more offspring, the sons imposed significant reproductive costs on their mothers.

Michael N Weiss — the lead author of the of the study — told Science Daily that while they had known for almost 10 years that adult male killer whales relied on their mothers to keep them alive, it had never been clear whether mothers pay a cost to do so. Weiss is currently working with University of Exeter, United Kingdom and the Center for Whale Research in the United States.

The authors state in the paper that two process could lead to such high costs to the mother for continued maternal investment towards the son, even when he has attained adulthood.

As males have greater energetic requirements and are less likely to perform cooperative behaviours, it is possible to draw correlations between the number of surviving sons and reproductive output of the mother.

The sons do not become less costly as they grow older. The paper adds that raising an offspring to weaning is itself energetically costly and females may bias this early investment towards sons.

The study found a strong negative correlation between females’ number of surviving weaned sons and their annual probability of producing a viable calf. 

‘If this early investment has long-term consequences, this could generate a negative correlation between females’ number of sons and annual reproductive success, even in the absence of continued investment,’ read the paper.

The findings of the paper also have important conservation implications as they highlight a previously unrecognised determining factor leading to low reproductive rates and reproductive success of the ‘southern resident’ population of killer whales. This can may help to in estimating future population viability analyses.