SEATTLE — A new study from the Center for Whale Research found that when female killer whales - orcas - have male offspring there are life-long consequences.
The study, published in Current Biology on Feb. 8, used data gathered from the Center for Whale Research’s ORCA SURVEY which consists of data from a 40-year period (1981-2021) and used 40 female whales from the Southern Resident orca population in the Pacific Northwest. There were 67 documented births, with 54 of those calves surviving to the first year of life.
Throughout the study, Research Director Dr. Michael Weiss and his team saw that mother orcas focused their energy on ensuring that their male offspring thrive and are able to reproduce themselves.
The study found statistically significant evidence that male offspring "impose biologically significant reproductive costs." The data was clear that sons, and only sons, had a negative effect on their mothers' reproductive success.
Producing a male offspring reduces a mother orca's annual breeding success by about 50%.
Furthermore, the study was able to conclude that sons are costly despite their age. Female orcas continue to care for their male offspring past the time that the calf has reached adulthood. This means that sons do not just negatively impact their mothers reproduction rate until they reach adulthood, but well into it.
While maternal involvement investment is known to take place in animals who give birth annually, this study says that it may provide the first documentation of lifetime maternal investment.
The maternal investment of a mother orca in her son is so intense that many continue to feed their sons throughout his life. A practice which stops for their daughters once she reaches maturity.
This continued providing of nutrients may be one of the factors that causes the decrease in mothers of male orcas to successfully reproduce. Since the mothers are sharing the food that they have, their intake of food is less. This may make it harder for them to successfully reproduce.
So why do female orcas pay their male offspring so much attention, especially if it costs them? The researchers suggest that it might be because it helps the survival of the species. Within orca pods, daughters reproduce within their mother's pods. This can led to reproductive tension amongst some females in the group, possibly causing mothers to give preferential care to sons.
"The indirect fitness benefits of boosting adult son survival likely contribute to selection for longer lifespans in female killer whales, while late-life reproductive conflict with daughters exerts selective pressure against extended reproductive lifespans," reads the study. "Our results expand this picture of killer whale life history evolution, suggesting that the indirect benefits of improving sons’ survival are significant enough to outweigh substantial costs to females’ reproductive success across their lifespan."
Researchers expect these findings to extend to other species of whales.
