The thunderous calls of common hippos can be heard from more than half a mile (1 kilometer) away — and when a hippo hears the booming call of an unfamiliar hippo, the animal often responds by letting loose a dramatic spray of dung.
Hippos (Hippopotamus amphibius) spray dung as a way to mark their territory, but until now, scientists didn't know that an unfamiliar hippo’s call could provoke this behavior in another hippo. In a new study, published Monday (Jan. 24) in the journal Current Biology (opens in new tab), researchers observed hippos at the Maputo Special Reserve in Mozambique and found that the animals recognize and respond differently to the calls of different hippos in the area, depending on whether those hippos live in the same territory, a neighboring territory or somewhere farther away.
Specifically, the hippos could differentiate a familiar hippo from a stranger by the sound of their "wheeze honk," the animals’ signature call.
"The wheeze honk is recognized as the characteristic call of the hippo, consisting of a higher pitched 'wheeze' followed by several 'honks,'" said Maria Maust-Mohl, an associate professor in the psychology department at Manhattan College in New York City, who studies animal communication and was not involved in the study. That initial "wheeze" swells quickly and loudly, like the sound of a trombone, and the "honks" that follow sound sort of like a deep, throaty laugh.
Hippos often produce wheeze honks at the same time as others in their social group, as if in chorus, and seem to sound the call in response to changes in their environment, Maust-Mohl told Live Science in an email. For this reason, it's thought that the wheeze honk may be a key element of hippo social communication, Maust-Mohl and her colleagues reported in a 2015 study published in The Journal of the Acoustical Society of America (opens in new tab). Now, the new study reveals how wheeze honks may help hippos define their social groups and distinguish friend from foe.
"As a territorial species, it would not be unusual for hippos to have adapted this ability to distinguish and react differently to hippos that may be more familiar versus those that may be more of a threat," Maust-Mohl said. "The findings of this study suggest the wheeze honk may serve to help hippos determine the presence and identity of other hippos nearby, as well as communicate within and between groups in their shared habitats."
But although the study offers new insights, "I think the sample size is a bit small," said Camille Fritsch, a postdoctoral researcher at the University of KwaZulu-Natal in South Africa, who studies hippo behavioral ecology and was not involved in the study. The research could be repeated in larger groups of hippos, in different habitats and at different times of the year, since the distribution of hippos shifts between the wet and dry seasons, he said. "It will definitely lead to further study."
Hippos feed at night, consuming up to 80 pounds (35 kilograms) of grass each evening, and the massive herbivores gather in bodies of water during the day, forming groups that typically consist of a dominant male, a variable number of females and juveniles, and a few peripheral males.
"On the same lake, several groups, or pods, can cohabit," Nicolas Mathevon, a co-senior author of the study and director of the Sensory Neuro-Ethology Team at the University of Saint-Etienne in France, told Live Science in an email. "When we decided to study hippos, a question immediately arose: can they recognize each other by voice?"
To answer this question, the team recorded wheeze honks from seven different groups of hippos living on lakes at the Maputo Special Reserve. The group sizes ranged from three individuals to 22; hippos on the reserve often live in groups of 10 to 25, depending on the area, Miguel Gonçalves, the reserve's park warden, told Live Science in an email.
With these recordings in hand, the team then set up speakers near each hippo group’s habitat, positioned about 230 to 295 feet (70 to 90 meters) away from the animals, and played back the sounds.
The hippos reacted to the recordings by producing their own wheeze honks, approaching the speakers or marking their territory with feces, or by displaying a mixture of these behaviors — but their responses varied depending on which recording was played, the team found.
The hippos showed the least reaction to calls from individuals within their group, and reacted slightly more strongly to individuals from neighboring groups at the same lake, the team found. These responses typically only included wheeze-honking and approaching the speaker, and included little to no marking. But the animals consistently showed the strongest reaction to calls from strangers and marked far more often in response to these sounds.
It makes sense that hippos would show less aggression towards hippos they know, Fritsch said. The size of hippos' social groups grow and shrink with the changing seasons; as water resources become scarce in the dry season, small groups of hippos gather at one water source and merge into a single, large group, Fritsch said. "They have some understanding of who's around them. And therefore, it would make sense that they are less aggressive towards those individuals," he said.
It would be interesting to see if and how these social dynamics change over time, as hippo groups move around and their densities thin out or grow more concentrated, he said. It would also be interesting to repeat the experiment with hippos living in different habitats, such as rivers and floodplains, rather than lakes. The new study, although somewhat limited, opens the door to investigate these additional questions, he said.
In the long term, this line of research could help conservationists better protect hippo populations, Mathevon said. For example, in the event that conservationists need to relocate hippos to a new habitat, "it may be possible to get the local hippos used to the voice of the new ones before they arrive, and vice versa," Mathevon said. Of course, hippos may still be provoked by the sight or smell of an unfamiliar hippo, even if they recognize the sound of their voice. But introducing the voice ahead of time may still help, he said.
Gonçalves agreed that such studies of hippo communication could help inform translocation strategies, should that ever be needed at the reserve. The research could also be useful for estimating hippo population sizes, by someday allowing scientists to estimate the density of a hippo group based on the amount of sound it produces, for instance, he said.
"Although hippos are not listed as endangered, their populations are declining rapidly," Maust-Mohl said; thankfully, within the confines of the Maputo Special Reserve, estimates suggest that the local hippo population is currently increasing, Gonçalves noted. "Future studies of their behavior and communication can help improve the management and conservation of this species by allowing us to better understand the nature of their social groups," Maust-Mohl said.
Editor's note: This article was updated on Jan. 25 with comments from Park Warden Miguel Gonçalves. The story was first posted on Jan. 24.
Originally published on Live Science.