Spend a few minutes watching a group of hippos and they can appear surprisingly inactive. During the heat of the day, they rest in rivers and pools with little more than their eyes, ears, and nostrils visible above the water’s surface. Yet beneath this calm exterior lies a complex social world filled with communication.
Recent research has revealed that hippos possess one of the most unusual communication systems in the animal kingdom. Unlike most mammals, which communicate either in air or underwater, hippos can communicate through both at the same time. Their calls travel across the water’s surface and through the water itself, allowing individuals to remain in contact even when largely submerged.
For an animal that spends its life between land and water, this remarkable adaptation helps explain how hippos maintain territories, coordinate social groups, and navigate life in crowded river systems.
A mammal between two worlds
Hippos are often described as semi-aquatic mammals, but a better description might be amphibious. They depend on both land and water to survive.
During the day, hippos remain in rivers, lakes, and pools where the water helps regulate their body temperature and protects their skin from the intense African sun. At night, they emerge to feed, sometimes travelling several kilometres from water in search of grass.
This dual lifestyle creates unique challenges. Hippos spend much of their social time partially submerged, yet still need to communicate with other members of their group. As a result, they have evolved a communication system perfectly suited to life at the boundary between land and water.
How do hippos communicate?
Hippos produce a wide variety of sounds, including grunts, huffs, croaks, bellows, screams, and low-frequency calls.
Researchers studying hippo vocalisations found that many of these calls are produced while the animals are partly submerged. A hippo may keep its eyes and nostrils above the surface while much of its head and throat remain underwater. When it vocalises, sound is transmitted simultaneously through the air and through the water.
This allows nearby hippos to receive information regardless of whether they are submerged or at the surface.
The study found that more than a third of observed calls were broadcast through both media simultaneously, while many others occurred entirely underwater. Few mammals possess such a capability.
For hippos living in densely populated river systems, this creates an efficient communication network where information can travel both above and below the surface.
Why sound works so well underwater
Water is an excellent medium for transmitting sound.
Unlike light, which is quickly scattered or absorbed in murky water, sound travels efficiently and over considerable distances. Low-frequency sounds are particularly effective because they lose less energy as they move through the environment.
For hippos, this is extremely useful. Visibility in rivers is often poor, especially when animals are submerged or clustered together in muddy pools. Sound allows individuals to remain aware of one another even when direct visual contact is impossible.
Communication helps maintain group cohesion, signal aggression, establish territories, and coordinate social interactions.
In many ways, the river functions as a giant acoustic environment, with hippos constantly exchanging information through vocal signals.
Life in a crowded river
Although hippos are often seen resting together, they are not simply passive animals sharing space.
Rivers and pools can contain complex social structures. Adult males may defend territories that include access to water and breeding opportunities, while females and younger individuals move within these areas.
Vocal communication plays an important role in maintaining these social relationships.
A dominant male may use vocal displays to advertise his presence and discourage rivals. Other calls may help maintain contact between individuals or communicate agitation and aggression.
This is particularly important during the dry season, when shrinking water sources force increasing numbers of hippos into smaller areas. As densities increase, so too does the need for effective communication.
More than just sound: the role of scent
Hippos do not rely solely on vocalisations.
Like many large mammals, they also communicate through scent. One of the most recognisable examples is dung scattering, where males defecate and rapidly spin their tails to spread dung over the surrounding area.
This behaviour serves as a form of scent marking, helping establish territory boundaries and communicate information about identity and status.
In effect, hippos communicate through both sound and smell, creating multiple layers of information within their environment.
Why do hippos spend the day in water?
Their communication system is closely linked to their lifestyle.
Despite their enormous size, hippos are surprisingly sensitive to heat and dehydration. They lack the dense fur that protects many mammals and spend daylight hours in water to prevent overheating and excessive water loss.
Remaining submerged also reduces exposure to the sun and helps maintain skin condition.
This explains why hippos are so dependent on healthy river systems. Without reliable water, many of the behaviours that define hippo life become impossible.
The night shift
When darkness falls, the behaviour of hippos changes dramatically.
Individuals leave the water and spread across the surrounding landscape to feed. A single hippo may consume tens of kilograms of grass during a night of grazing.
These nightly movements connect rivers to the wider ecosystem. Hippos follow well-established pathways between water and feeding grounds, often creating visible trails through vegetation.
By feeding on land and returning to water, they also transport nutrients between ecosystems.
Hippos as ecosystem engineers
Hippos do far more than simply occupy rivers.
Their movements, grazing, and waste products influence entire ecosystems. Nutrients from grasslands are carried into aquatic environments through dung, helping shape river productivity and food webs.
Their trails create pathways used by other animals. Their wallows alter water flow and habitat structure. In some cases, hippos can influence vegetation patterns around rivers and wetlands.
Like elephants, they are ecosystem engineers, species whose behaviour affects many others around them.
Understanding hippos therefore means understanding the wider landscapes they help shape.
Challenges facing hippos
Although hippos remain widespread across parts of Africa, they face increasing pressures.
Growing human populations, agricultural expansion, water extraction, and habitat degradation can reduce access to suitable river systems. Climate change may further increase pressure on water resources, particularly in semi-arid landscapes.
As rivers shrink during prolonged droughts, hippos may become concentrated into smaller pools, increasing competition and potentially increasing conflict with people.
Because hippos depend so heavily on water, protecting river ecosystems is central to their long-term conservation.