Female killer whales usually breed and become mothers between the ages of 12 and 40, but they can live well into their nineties. (Male killer whales, by comparison, rarely make it past 50.) Few animals outlive their reproductive lives by that much. Now, researchers working with the striking, toothy cetaceans have discovered why some post-reproductive females live that long: Their experience allows them to become good leaders of the pod, especially when food is hard to come by. The findings are published in Current Biology this week.
"Menopause is one of nature's great mysteries," Lauren Brent from the University of Exeter says in a news release. If natural selection is mostly about food, sex, and survival, why live for decades more after you’re no longer able to pass on your genes directly? In fact, other than humans, killer whales (Orcinus orca), and short-finned pilot whales (Globicephala macrorhynchus), most other animals die around the same time they stop reproducing.
To investigate, Brent and colleagues analyzed 35 years’ worth of data collected by the Center for Whale Research and observed 102 killer whales in the wild off British Columbia and Washington. The researchers examined their births and deaths as well as their genetic and social relationships. Additionally, they also looked at data from fisheries on the abundances of salmon like Chinook salmon (Oncorhynchus tshawytscha), which makes up as much as 90 percent of the whales' diet during some months.
Older females are strong leaders, they found, directing younger members to the best spots for feasting on salmon (pictured to the right). The foraging knowledge they’ve accumulated over the years enabled these females to become especially key leaders during years with low salmon availability. Salmon shortages are a major contributing factor to mortality of killer whales in this population.
In this way, post-reproductive aged females help boost the survival and success of their kin—and their sons in particular. "Killer whale mothers direct more help toward sons than daughters because sons offer greater potential benefits for her to pass on her genes," study co-author Daniel Franks from the University of York explains in a statement. "Sons have higher reproductive potential and they mate outside the group, thus their offspring are born into another group and do not compete for resources within the mother’s matriline.” The team did notice that males follow their mothers more closely than daughters did.
"Our study is the first to demonstrate that the value gained from the wisdom of elders may be one reason female killer whales continue to live long after they have stopped reproducing,” Brent adds. Turns out, they're repositories of ecological knowledge.
Images: David Ellifrit, Center for Whale Research