Hippopotamuses are large, semi-aquatic mammals native to sub-Saharan Africa. They are surprisingly vocal creatures, using a diverse array of sounds for communication. These vocalizations help them navigate social interactions and maintain their territories.
The Distinctive Sounds of Hippos
Hippos produce a wide range of sounds, from loud, booming calls to more subtle vocalizations, heard both above and below water. Their most recognized call is the “wheeze-honk,” a loud, resonant sound that can travel for several kilometers. This sound has been described as a mix of a growling laugh, a rubbery grunt, or a trumpeting bellow. Individual “wheeze-honks” can reach volumes of up to 115 decibels, comparable to a loud rock concert.
Beyond the prominent “wheeze-honk,” hippos emit various other noises. These include deep grunts, groans, and snorts, often associated with social interactions. They also produce squeals and roars, particularly during aggressive encounters, and chuffing noises. Underwater, hippos create sounds such as clicks, similar to dolphin echolocation, and tonal whines or pulsed croaks. These underwater vocalizations are often faint or inaudible on land, highlighting their specialized nature.
The Purpose Behind Hippo Vocalizations
Hippo vocalizations serve various communicative functions within their complex social structure. Sounds are crucial for territorial defense, with dominant males using grunts, roars, and bellows to assert control over aquatic spaces and warn off rivals. These loud calls travel over long distances, helping establish and maintain boundaries without direct physical confrontation. Hippos also use vocalizations as warning signals, alerting their group to potential threats.
Vocalizations also play a significant role in social cohesion within a hippo pod. Research indicates that hippos can distinguish between the calls of familiar individuals, neighboring groups, and complete strangers. Their responses vary based on this recognition; for instance, unfamiliar hippo calls often elicit a stronger, more aggressive reaction, sometimes including dung spraying as a territorial marker. This ability to recognize individual voices helps them manage their social network. Additionally, vocalizations are involved in mother-calf communication and may serve as mating calls, with males using distinctive sounds to attract females.
The Mechanics of Hippo Sound Production
Hippos produce sounds by expelling air past their vocal cords in the larynx. Their anatomy, including large lungs and a powerful diaphragm, contributes to the substantial volume and resonance of their calls. A unique adaptation allows hippos to vocalize simultaneously both above and below the water’s surface. When a hippo’s mouth is submerged, airborne sounds exit through its nostrils, which remain above water.
For underwater communication, vibrations from vocalizations travel through a fatty layer around their neck and jawbones, which have a density similar to water. Hippos can close their external ears when submerged, but they “hear” underwater sounds through their lower jaws. The jawbone connects directly to their middle ear, allowing vibrations to bypass the outer ear and transmit sound efficiently. This mechanism, along with their ability to produce low-frequency infrasound, enables them to communicate across significant distances underwater, where visibility is limited.