Hippopotamuses can tell the difference between strangers and their neighbors based only on the sound of their voices, according to a study published Monday in Current Biology.

The study indicates that the giant herbivores can identify each other by their signature, noisy "wheeze honk." When hippos hear a familiar call, they're less likely to respond with aggression than when they hear a new call for the first time.

The results suggest that hippos, which are famously territorial, still rely on communication networks and form complex social groups.

"We found that the vocalizations of a stranger individual induced a stronger behavioral response than those produced by individuals from either the same or a neighboring group," one of the researchers, Nicolas Mathevon, of the University of Saint-Etienne in France, said in a press release.

"In addition to showing that hippos are able to identify conspecifics based on vocal signatures, our study highlights that hippo groups are territorial entities that behave less aggressively toward their neighbors than toward strangers."

To study the animals, the team worked in Mozambique's Maputo Special Reserve, a 400-square-mile nature reserve that includes several lakes inhabited by groups of hippos. They recorded "wheeze honk" calls from each group. Then they would play the recording back -- to the same group, to other groups in the same lake and to groups in a distant lake.

The hippos would respond by approaching the sound, issuing their own returning call or spraying dung. They reacted to unfamiliar calls from different lakes with more intensity, researchers found, and were more likely to mark their territory by spraying dung if they didn't recognize the vocalization as belonging to their own family or neighbors.

Researchers on the study hope that their findings can help others learn more about hippo calls, like whether the animals can use them to determine size, sex or age. They also hope that their study will be useful in conservation efforts.

"Before relocating a group of hippos to a new location, one precaution might be to broadcast their voices from a loudspeaker to the groups already present so that they become accustomed to them and their aggression gradually decreases," Mathevon said.

Communication and socialization in the animal kingdom has long been a subject of fascination among researchers. In many species, communication tools are surprisingly nuanced and complex -- dogs, for example, can tell when a human switches between languages, a study published earlier this year found.

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