Monkeys living on an island without big cat predators do not show any particular alarm when recorded tiger growls are played to them, according to research by a UC Davis graduate student. The pig-tailed langurs do, however, flee in a hurry from the sound of human voices.
"This contributes to a growing literature on how animal behavior changes under relaxed selection pressures," said Jessica Yorzinski, a graduate student in animal behavior at UC Davis, who authored the study with Thomas Ziegler of the German Primate Center in Göttingen, Germany.
Pig-tailed langurs are medium-sized monkeys that spend most of their day sitting in trees in small groups eating leaves. Their close relatives on the mainland of Indonesia are prey for tigers and leopards, but on the Mentawai islands, the monkeys have been isolated from big cats for about half a million years.
Yorzinski played recordings of tiger and leopard calls and growls, as well as the sounds of elephants (another animal unknown to the monkeys), pigs and birds (animals they know, but which do not eat monkeys) and people talking in the local language. People do hunt the monkeys for food.
On hearing the noises, the monkeys would look around and at each other and might leave the area. They did not show any greater alarm at hearing big cat noises than at hearing an elephant, and would flee in about four to five seconds. But on hearing recorded human voices, the monkeys would flee within a second. They did not flee from bird or pig noises.
Yorzinski, of course, had to take care to stay out of sight when locating monkeys and setting up speakers. "We couldn't do the experiment if the monkeys saw us first," she said.