A call from a stranger may elicit myriad responses – panic, confusion, maybe even excitement – but it turns out that hippos have a rather more corporeal reaction: they spray dung. Researchers studying hippopotamuses in Mozambique have revealed that the creatures not only react to the vocalisations of other hippos, but that the calls act as an identity signal. In other words, they allow hippos to tell the difference between a familiar individual and a stranger. “Hippos are quite talkative. They have a repertoire of different calls: wheeze honks, grunts, bellows, squeals,” said Prof Nicolas Mathevon, of the University of Saint-Etienne in France, a co-author of the study. “However, the function of these calls has not been studied experimentally. Our study is the first to test experimentally the function of a hippo call.” Writing in the journal Current Biology, Mathevon and colleagues report how they studied the loud “wheeze honk” calls of hippos – a sound not unlike a growling laugh. The team recorded calls from individual hippos within groups – or pods – living in the same or different lakes in the Maputo special reserve. Five pods were then played back calls from an individual in their own group, a neighbouring group at the same lake, and a distant group of hippos that were strangers to them, while two pods were played calls from their own group and a distant group. The team found that hippos responded to the calls by calling back, approaching the sound or by spraying dung. The latter, however, was more common when the call was from a stranger than from a hippo of the same or neighbouring group. “When we played back familiar calls … the reaction was not aggressive. Basically, they just called back,” Mathevon told the Guardian. The team add that the hippos’ responses were stronger for calls from individuals that were less familiar. “Our experiments suggest that in hippos, the arrival of a stranger individual is perceived as more threatening than that of a neighbour,” the team write. While Mathevon said it was not a surprise that hippos use vocalisations to communicate – noting they are an excellent way to send information – he said the results show that hippo groups are territorial. The team add that their experiments show wheeze honks can travel more than 1km (0.6 miles), suggesting hippos would be familiar with the calls of others living on the same lake. “The most interesting thing to come out of this study is that hippos may have a fine-grained knowledge of the voices of all the individuals around them, and that this knowledge can help them navigate in their social network,” said Mathevon. The team said the findings could have implications for conservation, particularly when it comes to relocating individuals. “It may be possible to get the local hippos used to the voice of the new ones before they arrive and vice versa,” said Mathevon. “Of course, we are not saying that this measure will be sufficient to suppress all aggression, since other sensory signals are certainly also involved, but it may help.”