Inside the killer whale matriarchy - Darren Croft
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Southern Resident killer whales live in life-long family groups called matrilines. These matrilines form larger, looser groups of more distantly-related whales called pods. Whales in these pods share a common dialect, which are a unique set of vocalizations. These dialects are learned from other family members at a young age, and help the whales identify family members. It likely also helps the whales avoid inbreeding when looking for a mate. Interested in hearing killer whale vocalizations? Click here.
So why don’t either sons or daughters disperse? One reason why killer whales do not leave the family group is that offspring depend on their older mothers to help them find food. Around 80% of the resident killer whale diet is one species of salmon, known as the Chinook salmon. But salmon can be very hard to find. The older females take on leadership roles, using their knowledge to help their families find food, particularly when times are hard and salmon is scarce. The shortage of salmon is a major contributing factor to mortality in this population and so the benefits of older females knowing when and where to find salmon are considerable.
To learn more about leadership in killer whales, check out this article.
Explore this site to find out why salmon abundance is crucial for killer whales.
Female killer whales share over 90% of the fish they catch with their family members. This sharing is not equally distributed and mothers preferentially support their adult sons. Males mate outside the group, so mothers can ensure the spread of their genes without increasing in-group competition by preferentially helping their sons. Males continue to receive food from their mothers their entire life. In fact, male killer whales are over eight times more likely to die the year after their mother’s death. Learn more about why males are dependent on their mothers here.
This sharing of knowledge and other helpful behavior from older individuals may help explain an evolutionary mystery: killer whales, along with their close relatives, short-finned pilot whales, are the only non-human mammals known to go through menopause. Much like humans, female killer whales cease reproduction around the age of 40, but may live another 40 years or more past this age.
The benefits that older female whales may provide for their families can explain why they have evolved to live so long—but why do they stop having calves? The same close family ties that the whales rely on for food and support can also lead to intense competition. When older females have calves, they run the risk of breeding at the same time as their daughters—and, in turn, competing for food and other resources needed for reproduction. This competition means that there is another mouth to feed and calves can face an increased mortality risk. Because older females have more close relatives in the group—as they have had more offspring—evolution allows them to avoid this competition by halting reproduction and focusing on helping their family group. A similar mechanism may have led to the evolution of menopause in humans.
To learn more about the evolution of menopause in killer whales, listen to this BBC Radio 4 documentary here. You can also watch a short video here explaining reproductive competition in killer whales.
Killer whales are found in all of the world’s major oceans. However, not all killer whales feed on fish; many feed on other marine mammals. The social lives of different ecotypes of killer whales are very different to that of the resident killer whales; their offspring disperses from social groups. Scientists don’t currently know if these other ecotypes of killer whales also go through menopause. This provides a very exciting area for future research. To learn more about other killer whale ecotypes, click here.
Unfortunately, the Southern Resident killer whales are at risk of extinction. Without an increase in their food supply, their likelihood of recovery is slim. If we lose these whales, we not only lose a unique population of animals, but we lose a society and culture that has given us a unique insight into our own evolutionary history.
To find out how you can help protect the Southern Resident killer whales, click here.
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