As any schoolkid in the playground can tell you, fights don’t just involve those trading blows, but those watching too. Like spies, these bystanders observe, obtaining useful information about the individuals in the fight that they may be able to use to their advantage in future aggressive situations.
Biologists interested in aggression have previously reported that animals also “eavesdrop” when fights occur, and use the information in the future. Now, an evolutionary biologist from the University of Cambridge, Rufus Johnstone, has made a mathematical model of fighting that allows the possibility of eavesdropping. His paper was published Wednesday in the journal Proceedings of the National Academy of Sciences.
Johnstone was aware of some elegant experiments on Siamese fighting fish by Peter McGregor of the University of Copenhagen. Watching the fish doing what they’re named for, McGregor noticed that other fish in the tank were hanging around watching the fight, like spectators at a boxing ring. When McGregor set up fights between bystander fish and contestants, he found that bystanders were wary — they took four times longer to approach the winner of the fight they’d just seen than they did to approach the loser.
It seems that fighting fish make decisions about whether or not to fight according to how strong they feel. When they feel weak, they save their resources and live to fight another day. What’s more, females are in on the act too. When McGregor and his colleagues set up fights that females could watch, they found they visited the winner first, and spent more time with him than with the loser.
Similar behaviors have been seen in great tits and nightingales. It was thought that since eavesdropping can provide animals with more information, it might lead to more cautious behavior throughout the population, but Johnstone’s model predicts something quite different.
Evolutionary biologists interested in aggression use “game theory,” borrowed from economics. This is a way of studying theoretical contests between animals playing a different role, or strategy. If the game was janken, for example, there would be three possible decisions: paper, scissors or stone. A mathematical model of these games can help tell you which strategy is best to play, according to what the other players are doing.
The hawk-dove game is the classic biological game. This posits players who solve disputes in two different ways, by fighting or displaying. The model predicts that individuals exhibiting aggressive, hawklike behavior will be biologically more successful than those of a more cautious, dovelike bent — but only when hawks are rare. When there’s lots of hawkish behavior in the population, there’s more chance of fights breaking out — and then it’s better to be dovelike. Therefore, says the model, through these inbuilt checks and balances it’s possible for hawks and doves to coexist.
Johnstone’s model is a hawk-dove game that incorporates eavesdropping behavior. Hawks and doves always play the strategy for which they are named, but eavesdroppers play according to the outcome of previous encounters.
The model showed, contrary to expectations, that eavesdropping promoted aggression. Even though the eavesdropper can use information picked up by watching fights to avoid a dangerous situation, it seems that when an audience is present, more aggression is better. The model may also be useful in understanding human aggressive behavior. “Where reputation is important,” said Johnstone, “the presence of an audience can favor increased aggression.”
One animal with a reputation to establish is a silverback gorilla. These dominant males must always be on their guard for other males who are after their females. Another paper published today in Nature dramatically shows how wild western-lowland gorillas intimidate their rivals.
Richard Parnell and Hannah Buchanan-Smith, of the Scottish Primate Research Group at the University of Stirling, studied lowland gorillas in the swamplands of northern Congo. Unlike mountain gorillas, which live in dense jungle, in the open swampland gorillas are visible from long distances. They take advantage of the unrestricted view to make a scene when a rival male shows himself. Males were seen running and leaping into pools, and striking the water with their hands. Both actions sent up large plumes of water.
In most cases, these “splash displays” were made by male silverbacks, and were performed when another male was present. Tool use is common in apes, but using water as a means to amplify a signal is rare in wild primates. (“Dive bombing” and “cannonball runs” are well-known displays performed by adolescent human males at swimming pools, and probably have a similar function to the silverback display.)
Toshisada Nishida, of Kyoto University, is the only other scientist to report on water use in primates: Chimpanzees in Tanzania have been seen to throw rocks in streams, apparently as threats. The gorilla report will no doubt stimulate more research on the subject — research which cannot come too soon, says Parnell.
“Gorillas may be extinct long before their behavior is fully understood,” he said. “It is estimated that at the current rate of hunting, these animals, one of the world’s most magnificent creatures, will be extinct in the wild within 50 years.”