A call from a stranger can elicit myriad responses: panic, confusion, maybe even excitement, but it turns out hippos have a rather more bodily reaction: they spray dung.
Researchers studying hippos in Mozambique have revealed that the creatures not only react to the vocalizations of other hippos, but that the calls act as a signal of identity. 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: squawks, growls, bellows, screeches,” said Professor Nicolas Mathevon, from the University of Saint-Etienne in France, co-author of the study. “However, the function of these calls has not been studied experimentally. Our study is the first to experimentally test the function of a hippo call.”
Writing in the journal Current Biology, Mathevon and colleagues report how they studied hippos’ loud “whistling” calls, a sound similar to growling laughter.
The team recorded calls from individual hippos within groups, or herds, that lived in the same lake or in different lakes in the Maputo special reserve.
The calls of five pods from an individual in their own group, a neighboring group in the same lake, and a distant group of hippos that were strangers to them were then played, while two pods heard calls from their own group and a distant group. .
The team found that hippos responded to calls by calling back, moving closer to the sound, or spraying dung. The latter, however, was more common when the call was from a stranger than from a hippopotamus from the same group or from a neighboring group.
“When we played family calls…the reaction was not aggressive. They basically just called back,” Mathevon told The Guardian.
The team added that the hippos’ responses were stronger for calls from people who were less familiar.
“Our experiments suggest that in hippos, the arrival of a strange individual is perceived as more threatening than that of a neighbor,” write the team.
While Mathevon said it was not a surprise that hippos use vocalizations to communicate, noting that they are a great way to send information, he said the results show that hippo groups are territorial.
The team adds that their experiments show that squawks can travel more than 1 km (0.6 miles), suggesting that hippos would be familiar with the calls of others living in the same lake.
“What’s most interesting about this study is that hippos may have detailed knowledge of the voices of all the people around them, and that this knowledge can help them navigate their social network,” Mathevon said.
The team said the findings could have conservation implications, particularly when it comes to relocating individuals.
“It may be possible to get local hippos used to the voice of new ones before they arrive and vice versa,” Mathevon said. “Of course, we are not saying that this measure will be enough to suppress all aggression, as other sensory cues are certainly involved as well, but it can help.”
George is Digismak’s reported cum editor with 13 years of experience in Journalism