A study of 600 orcas revealed that a mother’s presence increases a son’s change of survival significantly. The mothers are driven by an innate drive to ensure that they have as many descendants as possible, and this might compel them to look out for their adult young.
The evolutionary benefit of keeping watch over their descendants might explain why female killer whales live decades beyond their reproductive prime, just like humans and pilot whales. The scientists published their findings in the journal Science.
This idea dates back to almost 50 years when studies in hunter-gatherer societies were accomplished. Evolution seems to favor menopause and a prolonged post-reproductive lifespan. Female killer whales live up to their 90s, but females stop bearing young between 30 and 50. Killer whales are part of an elite group of organisms on Earth that go through menopause. The group includes humans and pilot whales.
Orcas live in complex social groups that can include sons and daughters. Data on 589 orcas was analyzed using the algorithms used by insurance companies to calculate morbidity. Losing a mother was a liability for sons. Young males were three times as likely to die the year after their mother’s death as males whose mothers were still alive. Males over 30 were even more vulnerable. Their risk of death increased eightfold.
Young daughters had no ill effects of a deceased mother, but older ones were 2.7 times as likely to die. Either mothers fight off aggressive males or assist in hunting. There could be other reasons for this significant change in survivability.
Reference: “Adaptive Prolonged Postreproductive Life Span in Killer Whales” by Emma A. Foster, Daniel W. Franks, Sonia Mazzi, Safi K. Darden, Ken C. Balcomb, John K. B. Ford and Darren P. Croft, 14 September 2012, Science.