When post-menopausal female orcas are present, calves in their family group have better survival rates, according to a study published Monday (December 9) in the PNAS. This “grandmother effect” has been observed in humans, and scientists have suspected it exists in orcas for some time, but the current study is the first to quantify how beneficial it is for calves to have a grandmother.
Orca pods are matriarchal, with females reaching around 90 years old, which means that after they go through menopause around age 45, they have another 45 years of life in which to pass along critical information to the rest of the pod about the location of food.
“In killer whales, what granny knows is really, really important,” Hal Whitehead of Dalhousie University who was not involved with the study, tells Science.
The team of researchers, led by Daniel Franks of the University of York, analyzed 40 years of census information on northern and southern resident orca populations. When a grandmother orca dies, they found, a calf has a more than four-fold greater risk of dying within the next two years than calves that have their grandmothers around. The effect is magnified when salmon populations are in short supply.
Chinook salmon, the primary prey of orcas, do not swim in schools. When orcas hunt, they often need to dive down more than 150 meters and can spend 5–10 minutes finding and chasing down a fish. Calves, which spend the first two years of life nursing, must wait near the surface when it is the mother’s turn to hunt. Grandmother orcas are known to babysit and help guard the calf from being picked off by predators. Both mothers and grandmothers have been observed sharing fish with youngsters after a hunt.
Although mother orcas tend to share food with their male offspring well into adulthood, while females are cut off around the time of sexual maturity, the benefits coming from a grandmother affect males and females equally.
“The grandmother effect that we have shown also appears to impact whales for their entire lives,” Franks tells The Washington Post. Even at 20 years old, orcas have higher survival outcomes when there is a living grandmother.
Additionally, Franks tells the BBC, because the grandmothers aren’t breeding, there is reduced competition for resources and attention. If the grandmothers were able to breed until the end of their lives, they might be too preoccupied with rearing their new calf to take the time to care for and mentor a grandwhale. Calves with grandmothers that have gone through menopause have better outcomes than those with grandmothers that are still breeding, the study found.
Lisa Winter is the social media editor at The Scientist. Email her at firstname.lastname@example.org.