Recently the Center for Whale Research (CWR) published an exciting new paper about rake marks on killer whales, particularly males, and how the marks’ occurrence is related to post-reproductive mothers.
Photographs © Copyright 2023 Center for Whale Research.
There are multiple sets of rake marks on Southern Resident (SRKW) orca L85’s left side. His mother, L28, died in 1994. (Photograph by CWR’s Mark Malleson during Encounter #49 on August 13, 2022).
What do the rake marks and their appearance on males have to do with their mothers?
RECENTLY THE CENTER FOR WHALE RESEARCH (CWR) published an exciting new paper about rake marks on killer whales, particularly males, and how the marks’ occurrence is related to post-reproductive mothers. What are rake marks? They are killer whale skin injuries or scarring inflicted by the teeth of other killer whales. Why do killer whales rake one another? We don’t know for sure, but rake marks likely occur during rough play and aggressive interactions. Some rake marks can be severe and penetrate deep into the flesh. Any open wound is a potential source of infection, and we know that such infections pose a risk for individuals in the population. And the scars can last for life. These marks actually help us to identify the whales.
What do the rake marks and their appearance on males have to do with their mothers? New research from the CWR shows that post-reproductive females spend time protecting their sons. Males with post-reproductive mothers have approximately 35% fewer tooth rake marks than males with reproductive-aged moms. Put in other words, post-reproductive moms are providing benefits to their sons and reducing the socially inflicted injuries they sustain—they are truly mommies boys. Reproductive-aged mothers don’t seem to offer their sons the same safeguards. We’re not 100% sure why after menopause, females are able to provide these benefits; perhaps they have extra time and energy to help their sons in risky social interactions. Older mothers may also use their broad-ranging experiences to help their sons navigate social encounters with other whales. A worldly-wise mom might intervene when a fight looks likely.
From previous killer whale studies, we’ve learned menopause may have evolved so mothers and their daughters don’t have concurrent offspring, leading to undesirable competition for scarce resources. Post-menopausal whales have proven to spend significant time helping their offspring and grand offspring: food sharing and leading their families to better feeding areas (critical in times of low Chinook salmon abundance). This new research shows another way that mothers can help—by providing social support to their sons. Females didn’t receive the same protection benefit from their post-reproductive mothers.
Any open wound is a potential source of infection, and we know that such infections pose a risk for individuals in the population.
SRKW L87’s left side shows tooth rake marks on his dorsal fin and behind his saddle patch. The zoomed-in dorsal fin image of the rake marks (right) reveals an open wound. L87’s mother, L32, died in 2005.
So why do moms help their sons and not their daughters in this way? Well, we know from previous CWR research that, overall, males get more rake marks than females, so it is likely they are more at risk of getting into fights. But also, mothers can benefit additionally from helping their sons over their daughters. When a son reproduces, he will hopefully mate with females from outside the family group, so there is no cost to the local group in rearing his calf. In contrast, when a daughter reproduces, the calf is born in the local group and is another mouth to feed. Combine this with the fact that sons have the potential to produce multiple calves in a given year (whereas females in the SRKW typically only have one calf every six years). It’s understandable why moms would invest their energy in their sons rather than their daughters—which is what they do.
This new research further emphasizes the striking similarities between the social lives of killer whales and humans. Like humans, it seems that older female killer whales play a vital role in their societies—using their knowledge and experience to provide family and community benefits, including finding food and resolving conflict.
This study adds to the mounting evidence that post-reproductive female killer whales offer numerous life-sustaining benefits to their offspring and grand offspring, with extra emphasis on caretaking the sons. What is concerning for the future of the SRKW population is the number of post-reproductive females has declined over the last two decades.
Take a deeper dive into the Center for Whale Research’s Current Biology-published paper: Postreproductive female killer whales reduce socially inflicted injuries in their male offspring.
Graphical abstract from the Center for Whale Research’s Current Biology-published paper: Postreproductive female killer whales reduce socially inflicted injuries in their male offspring.
Other research publications of interest:
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