Orca pod (photo credit: NOAA)
A recent study published in Science looks at a question that has long vexed biologists: why do many female animals live years–often decades–after they are able to reproduce? It could be that a long life is just a side effect of being a healthy animal. Or older animals may increase the chances of survival and/or reproductive success of their offspring (thus increasing the likelihood that their genes will be passed on to grandchildren) by living past reproductive age. Killer whales are excellent for examining these ideas since they stop reproducing in their 30 and 40s and yet often live into their 90s. According to the article this is the “longest post-reproductive life span of all non-human animals.”
This study used photographic census records of killer whales living off the coast of Washington state and British Columbia to examine the impact a mother whale’s death had on her offspring. The results showed that a mother’s death had the largest effect on male offspring. In fact, for sons over the age of 30, the death of a post-reproductive mother increased the likelihood of dying by 13.9-fold (compared to 5.4-fold in females over age 30) in the first year after the mother died.
How does having a post-menopause mother whale around help older sons? This question wasn’t directly addressed by the paper although the authors mention that the mothers may help with foraging and defense. A more interesting question is why the mothers help their sons more than their daughters. There are a few considerations that might be at play here. One is that the offspring of sons are raised by other pods thus requiring less investment of time and energy into helping to raise grandchildren. More important, perhaps, is the fact that sons can reproduce their entire lifespans and thus have the potential to sire many more offspring than daughters.
Side note: There is interesting research into the “grandmother” hypothesis in humans. This study found that maternal grandmothers helped the survival of grandchildren in rural Gambia. And this study presents a model for how the benefits of grandmothering combined with the conflict of having multiple simultaneously reproducing generations could explain why women often live twice as long as their reproductive lifespan.