Behavior

Science Reveals Sperm Whale Bro-Fest

 

In the mammal world, males tend to not like each other very much. Male-only social groups are rare. The few species who do have boys’ clubs, like chimps, cheetahs, and lions, form them from their relatives and only because they need to protect a shared territory or access to females.

This doesn’t seem to be the case in the mammal that, by name alone, epitomizes the “bro” of the oceanic boys’ club – the sperm whale.

Sperm whales traveling in a pod, by Gabriel Barathieu.

Lady sperm whales live in stable social groups, with their offspring, and most will stay with this group their entire life. The boys, though, will leave their mother’s group some time before sexual maturity (at 6-16 years old) and join a “bachelor school” in colder waters. Research has shown that these males will eventually age out of their bachelor school, to live more solitary lives until traveling to the lower latitudes to find themselves a lady.

Scientists don’t know why male sperm whales choose to live together in these travelling bachelor pods. They live in entirely different territories from the females, so they aren’t forming a blubbery militia to protect a shared harem of hot lady spermies. Even stranger, genetic evidence from mass-strandings of male groups has shown that few, if any, of the males in these bachelor schools are related. They all come from different moms. So it’s not a matter of kinship or indirectly helping to pass some of your DNA on. Why do they create these bachelor groups… and what goes on inside the marine boys’ club?

Researchers from Nagasaki and Dalhousie Universities set out to illuminate the social lives of male sperm whales. The Hokkaido area, Japan, is a summer feeding ground for male sperm whales that also boasts an extensive commercial whale watching industry. Whales can be identified by the shape, colour, and marks on their flukes and dorsal fins, so the researchers were able to track individual males over the course of their time in the Nemuro Strait. Altogether, they tracked 226 individual males between 2006 and 2017.

Males were considered to be “associating with each other” if they were spotted from the survey boat no more than 1hr apart (that’s so formal, I’m just gonna call them flipper-friends). This meant they were at least in vocal range of their flipper-friend (the audible range for the “slow click” sounds that males use is ~60km). While they were typically spotted alone, the researchers did track some whales that liked to hang out together. On average, each male would be in some kind of contact with at least one other flipper-friend over the course of a day. Some males were even observed clustering closely together (within 3 body lengths of each other) where they were in visual and, potentially, physical contact. These distant associations and chest-bump clusters weren’t random, either. Most flipper-friendships lasted at least ~3months, with the average association lasting 968 days (or a little over 2 years). Two individuals, affectionately named PM099 and PM101 (let’s say Chad and Dave), hung out together for 3 years and often gathered to rub and rest together at the surface. Rub and rest! For the boys!

This research illustrates a fascinating social dynamic between male sperm whales that is extremely rare in the animal kingdom. Without the benefits offered by kinship or increased access to females, male sperm whales choose to socialize in their own territories with other males that they may have relationships with for years. The researchers suggest that these bro-bonds may foster co-operative foraging or defence against predators while the males are together, but they stress that more work is needed to understand the full extent of male sperm whale social behaviours and life histories.

Until then, I like to think it’s just because everyone needs a flipper-friend.

Kobayashi H, Whitehead H, Amano M (2020) Long-term associations among male sperm whales (Physeter macrocephalus). PLoS ONE 15(12): e0244204. https://doi.org/10.1371/journal.pone.0244204

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