Male-to-male combat among elephant seals are loud, violent, bloody events. These gargantuan seals, with their inflatable noses, can weigh up to 2,000 kilograms. But whether they’re the winner or the loser, these fights take a toll. To prevent unnecessary, costly confrontations, northern elephant seals (Mirounga angustirostris) have learned the calls of their rivals so that when they encounter them again, they can fight or flee – depending on their status in the dominance hierarchy. The findings are published in Royal Society Open Science this week.
Social status is initially determined through physical confrontations that consist mostly of ritualized displays: “clap threats,” slamming their chests into the sand, and what’s known as posturing, when they rear up to show an elevated profile. Alpha males defend their harem from challengers, and beta males react to competitors based on social status. "There is a very structured social network among the males in a given location. Although the rate of conflict between males is very high, the rate of actual physical aggression is very low," Colleen Reichmuth from the University of California Santa Cruz says in a statement. After all, during the noisy, crowded breeding season, they might fast for up to three months.
To figure out what sorts of information are coded within male vocalizations, Reichmuth and colleagues combined observations with experiments in Año Nuevo State Park from December through February from 2009 to 2013 and San Simeon State Park during breeding season from 2011 to 2012. They recorded and analyzed calls made by males during challenge displays directed at other males. You can see a young male vocalizing to the right. And please watch this awesome video of a researcher recording the vocalizations.
Unlike males from many other species – who assess their rivals through "honest signals" that convey their size and strength – elephant seals learn the calls of other males. When they recognize the vocal signatures of familiar opponents, they act accordingly: Males responded aggressively to the calls of subordinate males, but when they heard the call of a dominant male, they quickly and quietly moved away.
Not only are their individual calls distinct, they’re always the same, no matter what the situation. "It's impressive that they could stay that stable, which tells us how important it is for them to have an identity signal – a special vocal signature," says Caroline Casey, lead author of the study. One male could engage with as many as 43 opponents in just one breeding season. But sustained fights, the researchers found, occur in less than 2% of the interactions, and they always involve males of similar dominance status who haven’t previously fought that season.
When the team played recordings of unfamiliar rivals to the males, they didn’t attack or retreat: They just sat up and looked in the direction of the speaker. "They either ignored the speaker or seemed hesitant to respond," Casey adds, "like they were looking for more information."
Images: A. Friedlaender.
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