Like a football player who just scored a touchdown, male white-flippered penguins (Eudyptula minor albosignata) perform triumph displays after defeating an opponent. Now, researchers in New Zealand have found that those victory dances, complete with a braying, donkeylike call and flipper waving, make it less likely that nearby penguins will challenge the winner.
"Scientists have spent a lot of time studying antagonistic interactions, but quite often, they turn the camera off after the fight, so they miss a lot," says Tom Sherratt, an evolutionary ecologist at Carleton University in Ottawa, who was not involved in the current study. Researchers have investigated the effects of triumph displays on the loser, but because this is a fairly recent field of study, the new research is probably the first published account of its effects on nearby birds, he added.
Explanations for the function of triumph displays include browbeating an opponent so that he doesn't forget who beat him and signaling to an audience not to mess with you.
Researchers at the University of Waikato in Hamilton, New Zealand, wanted to know whether penguins near a fight picked up on those signals. So they studied the effects of triumph displays on nesting white-flippered penguins in a colony at Flea Bay in New Zealand. These penguins were ideal study subjects because they're really aggressive and squabble frequently, says Joseph Waas, a behavioral ecologist and a co-author on the study. Only about 5% to 10% of their aggressive encounters lead to a full-fledged fight, complete with slashing bill hooks and flipper bashing, he says. But it's not unusual to see male penguins that are missing an eye.
The penguins are also territorial, very noisy, and live in burrows organized into loose colonies, says Solveig Mouterde, lead author of the study and a visiting doctoral student at the University of California, Berkeley. Their aggressive nature, combined with the proximity of burrows, results in frequent dustups near an audience—an ideal study situation for the researchers.
The researchers edited audio recordings of fights among two sets of white-flippered penguins foreign to the Flea Bay colony into two fight sequences, each ending with a triumph call. They tested a total of 43 bystander penguins by playing these recordings for individuals inside their burrows at Flea Bay, using speakers set 5 meters away. Once the "fight" finished, the team waited 5 minutes, then played the winner's victory call again from speakers placed 2.5 meters away from a burrow to trick the bystander penguin into thinking the victor was coming closer. In other trials, researchers played a call from the loser to simulate his approach to the burrow.
To measure the bystander penguins' responses to the fight and to the subsequent approach of the winner or loser, the scientists temporarily swapped out their eggs with an infrared egg. The fake eggs measured the brooding penguins' heart rate, which indicated their stress levels.
The team found that both male and female bystanders were stressed while listening to fights, with females similarly stressed regardless of whether a winner or loser approached their burrows. Males were more stressed if a winner, rather than a loser, approached their burrows. They were also more likely to challenge an approaching loser by calling, the authors report in this month's issue of Animal Behaviour.
"I thought it was a fascinating paper," says Ryan Earley, who studies animal behavior at the University of Alabama in Tuscaloosa and wasn't involved in the study. He especially liked the technique of measuring heart rate with a fake egg. Collecting the data as the stress response occurs provides more convincing evidence that the penguin is reacting to the triumph calls than if researchers had caught the penguins after the fight sequence to measure stress levels, Earley says.
"This is a really nice, carefully done study," Sherratt adds. The most compelling result was the effect of triumph displays on nearby penguins, he says. "That helps pin down the message that the signal is serving to change behavior."
ScienceNOW, the daily online news service of the journal Science