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Hippos release poop tornado in response to stranger danger

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The thunderous sounds of ordinary hippos may 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 frequently replies by spraying a dramatic stream of poo.

Hippos (Hippopotamus amphibius) spray excrement to mark their territory, but scientists didn't realize that an unfamiliar hippo's call may cause another hippo to do the same. Researchers observed hippos at the Maputo Special Reserve in Mozambique and discovered 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, according to a new study published Monday (Jan. 24) in the journal Current Biology.

Hippos could tell a known hippo from a stranger by the sound of their "wheeze honk," the animals' distinctive call.

"The wheeze honk is recognized as the hippo's characteristic call, 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 but was not involved in the study. The initial "wheeze" rises fast and loudly, like a trombone, and the "honks" that follow sound like a deep, throaty laugh.

Hippos frequently emit wheezing honks at the same time as others in their social group, as if in chorus, and appear to sound the cry in reaction to changes in their environment, according to Maust-Mohl in an email to Live Science. As a result, the wheeze honk is regarded to be an important part of hippo social communication, according to Maust-Mohl and her colleagues in a 2015 study published in The Journal of the Acoustical Society of America. The latest research indicates how wheezing honks may aid hippos in defining social groups and distinguishing friend from foe.

"As a territorial animal, it would not be surprising for hippos to have acquired this ability to identify and react differently to hippos that are more familiar vs those that are more of a threat," Maust-Mohl explained. "The study's findings show that the wheezing honk may help hippos assess the presence and identification of other hippos nearby, as well as communicate within and across groups in their shared habitats."

Although the study's findings are intriguing, 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, believes "the sample size is a bit tiny." He believes the study should be repeated with larger groups of hippos, in different habitats, and at different times of the year, because hippos' distribution fluctuates between the wet and dry seasons. "It will undoubtedly lead to additional research."

Hippos feed at night, devouring up to 80 pounds (35 kilograms) of grass per evening, and during the day, the huge herbivores congregate in bodies of water, forming groups that normally include a dominating male, a variable number of females and juveniles, and a few periphery males.

"Several groups, or pods, can coexist on the same lake," 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 chose to study hippos, we instantly wondered if they could recognize each other by sound."

The scientists captured wheezing honks from seven separate groups of hippos dwelling on lakes in the Maputo Special Reserve to solve this question. The group sizes ranged from three to 22 people. With these recordings in hand, the team set up speakers near each hippo group's habitat, around 230 to 295 feet (70 to 90 metres) away from the animals, and played the noises back.

The hippos responded to the recordings by making their wheezing honks, approaching the speakers, or marking their territory with faeces, or a combination of these actions — but their responses differed depending on which tape was played, according to the team.

The hippos replied the least to calls from members of their group and reacted slightly more strongly to members of neighboring groups in the same lake, according to the researchers. These answers were typically limited to wheeze-honking and approaching the speaker, with little to no marking. The animals, on the other hand, consistently demonstrated the biggest reactivity to calls from strangers and marked far more frequently in response to these sounds.

Fritsch explained that it makes sense for hippos to be less aggressive towards hippos they know. The size of hippos' social groupings grows and shrinks with the seasons; as water resources become limited during the dry season, tiny groups of hippos congregate at a single water source and merge into a single, huge group, according to Fritsch. "They have some idea of who is around them. As a result, it stands to reason that they are less hostile toward those persons "He stated.

It will be intriguing to watch if and how these social dynamics alter over time as hippo groups migrate about and concentrations thin out or increase, he added. It would also be fascinating to repeat the experiment with hippos living in diverse ecosystems other than lakes, such as rivers and floodplains. The new study, albeit relatively limited, opens the door to further research into these additional topics, he says.

In the long run, these findings could help conservationists better protect hippo populations, according to Mathevon. For example, if conservationists need to transplant hippos to a new habitat, "it may be feasible to get the local hippos habituated to the new ones' sound before they arrive," Mathevon added. Of fact, even though hippos recognize the sound of their voice, they may be startled by the appearance or smell of an unfamiliar hippo. However, he believes that introducing the voice ahead of time may still be beneficial.

"Although hippos are not classified as endangered, their populations are fast diminishing," Maust-Mohl remarked. "Future research of their behavior and communication will allow us to better understand the nature of their social groups, which will aid in the management and conservation of this species."