The hippopotamus is a surprisingly mysterious beast. Although it's the third largest land mammal, can run a spritely 30 km/h (19 mph) over short distances, and is far more dangerous to humans than a grizzly bear, we know relatively little about its social communication.
A new study working to rectify this lack of knowledge has now discovered that hippos don't like strangers – to the extent that they'll spray the area with poop at the mere sound of strange hippo voices.
"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," says one of the study authors, University of Saint-Etienne bioacoustician Nicolas Mathevon.
"In addition to showing that hippos are able to identify [other hippos] based on vocal signatures, our study highlights that hippo groups are territorial entities that behave less aggressively toward their neighbors than toward strangers."
The team travelled to the Maputo Special Reserve in Mozambique and began by recording calls from seven hippo (Hippopotamus amphibius) groups across three lakes. They focused on the 'wheeze honk' – the most common hippo vocalization, which can be heard over long distances and is thought to be part of the animals' social cohesion communications.
"Studying the behavioral biology of hippos in the wild is complicated. It is difficult, if not impossible, to identify and mark individuals and sometimes highly challenging to locate them. Hippos feed on land mainly at night and are rather solitary," the researchers write in their paper.
"During the day they gather in groups in the water. Hippo groups are socially structured around a dominant male, a variable number of females and their young, and some peripheral males. However, it is unclear whether individuals in a pod form a stable group defending a territory or pods are organized in a fission-fusion manner with individuals moving from one pod to another."
After the researchers recorded the calls, they then started playing the wheeze honk back to the hippos to find out how they would react to their own group, their neighbors on the same lake, or a stranger group from a different lake.
The team found that the behavioral responses such as calling back, approaching the sound, or marking their territory by spraying poop occurred in response to all three of the sounds – but strangers incurred much higher levels of pooping.
You can see what this looks like in the fabulous video below:
"Whereas individuals responded to calls from any group, marking behavior (dung spraying) is modulated by the category of the calls," the team writes.
"Stranger group calls induce more marking than calls from an individual of the same group, while there is no significant difference between reactions to the calls from the same group or from a neighboring group."
This is interesting for a couple of reasons. Firstly, although hippos look pretty relaxed and zoned out in their watering holes, their quick responses to the calls mean the animals are still paying close attention to their surroundings.
Secondly, this also suggests that hippos are less aggressive to their neighbors than they are to strangers. This doesn't always work this way, as some territorial animals will be nastier to their neighbors than strangers.
Finally, this also might provide a way to help conservation efforts when moving hippos around.
"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 says.
"Reciprocity, in which the animals to be moved become accustomed to the voices of their new neighbors before they arrive, could also be considered."
The research has been published as a correspondence in Current Biology.