A new study in press at Animal Behaviour shows that hyenas can count. This study builds on work done in other animals, including work that I did on chimpanzees, and finds similar results: animals that fight in groups don’t like to pick fights if they seem to be outnumbered.
Game theory predicts that animals should use numerical assessment when deciding whether to get into a fight with a rival group. You generally don’t want to fight you can’t win. I found this to be the case with chimpanzees, and other studies have found this to be the case for lions and howler monkeys too. Game theory predicts that hyenas should do this too.
To test this prediction, Sarah Benson-Amram and others on Kay Holekamp‘s research team at Michigan State University did playback experiments with hyenas at Masai Mara in Kenya. A write-up of this study in Nature discusses how a particularly nice feature of this study is the way they set up the experimental stimuli. Previous studies have simulated larger groups by playing back recordings of multiple individuals calling at once. This is, in fact, what often happens in the wild – members of groups of lions, chimpanzees, howler monkeys, wolves, hyenas and such often call at the same time, probably to announce to the world that they are in a big group, so watch out. But experimentally, this raises the question of whether the animals are responding to the number of individuals, or just the total amount of noise in the vocalization. This study very elegantly disentangles these variables by always playing three non-overlapping whoops in each playback. In the single-intruder playback, the three whoops are by one individual. In the three-intruder playback, the three whoops are by three different individuals. And on hearing these playbacks, the hyenas responded differently to whoops by different individuals. When they heard the same individual calling repeatedly, their vigilance response decreased. But when they heard a new individual calling, they looked longer at the speaker.
So for hyenas to respond differently to the different numbers of callers, they have to be keeping track of the different individuals that call. This is trickier than simply distinguishing between a single caller and a chorus of callers – but it makes sense that hyenas should be able to do this, since their livelihoods depend on defending group territories and keeping track of how many rivals they face, even if they’re not all calling at once.
This method has the happy byproduct of also demonstrating that hyenas can tell individuals apart from their calls. It makes sense that they should be able to do this – but it’s nice to have experimental support for this prediction.