When a stranger calls, hippos regularly generate a poo shower; the equivalent of our urge to put up fences and hedges everywhere.

Hippos are very vocal animals. They can let out huge screams that can be heard from a great distance. Scientists have now re-examined those cries and discovered that hippos can actually respond to them in three ways. They may, in response to another hippo’s cry, approach that hippo, let out a cry of their own or throw poo around. And what the research now shows is that the hippos’ response depends on who yelled first. “We found that the vocalizations of an unknown individual elicited a more powerful behavioral response than the cries produced by hippos from the same or adjacent group,” said study researcher Nicolas Mathevon. For example, hippos were more likely to spray poop—a way of marking their territory—when the cry of an unfamiliar hippo was heard. It shows that hippos can distinguish between known and unknown voices, the researchers write in the magazine Current Biology.

The research

The researchers based their conclusions on a study of hippos that lived in a reserve in Mozambique. They recorded the cries of hippos belonging to different groups and then played them back to hippos belonging to that same group, an adjacent group, or a much further (and thus strange) group. It soon became apparent that the hippos reacted much more fiercely when they heard the cry of a stranger. They also sprayed poop more often when they heard a strange hippo, something they did not do significantly more often when the cry of a hippo from a different but adjacent group was heard. “Our study shows that hippo groups are territorial entities that respond less aggressively to their neighbors than to strangers,” Mathevon concluded.

Quick response

What is also striking is that the hippos reacted very quickly. When bobbing in the water, they seem to be quite passive and take little notice of their surroundings. But when the researchers let out the cries of other hippos, a response followed almost immediately. It shows that the hippos keep a close eye on their environment, the researchers say. “The reactions to the sounds we played were also very clear,” added Mathevon.

Social

The research provides more insight into the way hippos communicate. The fact that their cries can be heard over great distances suggested that the vocalizations were important for the formation and maintenance of social groups. The conclusion that hippos are able to distinguish between cries of acquaintances and strangers supports this.

Displacement

In addition, the findings of this study may also come in handy if scientists are forced to relocate hippos in an effort to maintain hippo populations. “Before a group of hippos is moved to a new location, as a precaution, the voices of those hippos can be recorded and played back for the hippos already present in that area, so that they can add their voices before their new neighbors arrive. get used to it and react less aggressively.” Similarly, the migrating hippos could also be familiarized with the voices of their new neighbors prior to the move.

Even after this research, there is still plenty to discover when it comes to the communication of hippos. For example, it remains unclear how exactly the hippos recognize the voices of acquaintances. Scientists also still do not know exactly what the hippos are telling each other. Future research should therefore show, among other things, whether the hippos reveal more about their age, gender and size through their cries, for example.