Human females are very strange in the animal kingdom, because they are one of the few species with a long postmenopausal lifespan. Even our genetically similar chimp relatives do not seem to always experience menopause — though there has been much debate on both sides of that argument. The only other species known to have long post-reproductive lives are killer whales and pilot whales, and a recent study published in the journal Science shows this life change positively impacts killer whales’ adult offspring.
Female orcas go through menopause in their 30s or 40s, though live through to their 90s. Biologically speaking, it doesn’t make much sense for an animal to stop reproducing so early in life unless there is some other benefit, so Emma Foster and her team of researchers decided to look for that benefit.
The researchers looked at census data from the past four decades to determine the impact of older female whales on the rest of their pods. Most of the data was collected from the Center for Whale Research in Friday Harbor, Wash., and from the Pacific Biological Station in Nanaimo, British Columbia, both of which collect data on local resident whales.
The team’s major finding was the significant impact mothers seem to have on the survival rates of their children, particularly male children. On average, a male orca 30 or younger is 3.1 times more likely to die the year after his mother’s death, and chances increase to 8.3 times more likely if he is older than 30, indicating that adult whales are actually helped more by their mothers than younger whales. Though reasons remain unknown, Foster’s team believes the mother whales likely help their adult offspring forage and manage encounters with other orca groups.
The data shows an even stronger impact when focused only on mothers that had gone through menopause. After a post-reproductive mother’s death, sons older than 30 were almost 14 times more likely to die within one year. This difference may suggest postmenopausal mother whales actually assist their offspring more than mothers who are still able to reproduce, though it is unclear why this may be the case.
Increased mortality was also noticeable among older daughters whose mothers had recently died, though to a much lesser extent. The researchers posit the stronger connection between sons and mothers is due to the fact that sons mate outside of the family group, and the grandmother wants to ensure her genes are passed on by keeping her son healthy. The team aims to next focus on learning the exact ways postmenopausal whales assist their sons.
Click here to listen to Emma Foster discuss the research on last week’s Science Podcast.
Main photo credit: Robert Pitman/NOAA