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WORLD HIPPO DAY

There are two species of hippos, the common and the pygmy hippo. The common hippo lives in East Africa, south of the Sahara and the pygmy hippo in the forests of West Africa.


Common hippos are the third-largest living land mammal in the world, after elephants and rhinos, and they are perfectly adapted to life on land and in the water. Their feet have webbed toes which help them to paddle in the water, and they are spread out to distribute their weight when moving on land.


Both species of hippos have very thick skin with just a little bristle-like hair on their heads and tails. Their outer skin layers are quite thin however, which means that they are prone to wounds when they fight with other hippos.


Hippos have enormous mouths, and their powerful jaws can open to 150 degrees. Inside their mouths, hippos have huge incisors. These teeth are not for eating though, as hippos are herbivores and only eat plants. Their large incisors are used for territorial fights, as well as to frighten off predators such as humans.


Hippos can keep their teeth sharp by grinding them together. They then use them to first threaten other hippos, by yawning and showing off their teeth as a threat. Then, if a fight ensues, the giant teeth can be used to block another hippo’s attacks, as well as to attack one another. Hippo males are usually covered in scars from these territorial battles.


Social lives

Hippos generally live in herds of up to 40 animals, presided over by a territorial male. But they have flexible social systems defined by food and water conditions, with periods of drought forcing them to congregate in large numbers around a limited water supply.


Due to their social nature, they are very good communicators.


Hippos produce a ‘wheeze honk’ call that can be heard over long distances and conveys information to other hippos. Hippos are able to recognize each other's voices based on their vocal signatures, and they respond differently to calls from their own group members, neighbouring group members and those of strangers. Showing less aggression to the call of a neighbour than the call of a stranger, with a stranger's call instigating territorial behaviours such as dung spraying.


Hippos can also communicate via clicks. Their closest living relatives are cetaceans from which they diverged some 55 million years ago, and it appears that they may have retained this feature of communication known to cetaceans. The clicks are spread far and wide and express information to other hippos within their vicinity.


Underneath the water, the vibrations made by the sound travel through layers of fat in the hippo’s neck and travel out into the water. Hippos above ground receive sound through the air as normal, but underwater hippos close their exterior ears and thus have no way to listen to these sounds using their ears. Therefore, they listen, through their jaws. The connections from their jaws to their skull is very thin, and a portion of the jaw is connected to the middle ear. The vibrations then travel through the body and into the ear, where it is translated into sound.


Hippos and crocodiles

Hippos share both the land and the water with crocodiles. Generally, crocodiles leave hippos alone as they are much bigger and stronger than they are. Hippos take advantage of this, and they will even use crocodiles as living teethers.


Hippos, especially baby hippos, have been seen licking crocodiles around the base of their tail. It is thought that they are getting mineral salt from the crocodile’s skin. Baby hippos have even been seen to be chewing on the tails of crocodiles, using the rough scales as a living teether. The crocodiles know better than to fight back, and they will let them do this. Although, when baby hippos are left alone, they can be preyed upon by crocodiles if they are not under the fierce watch of their mother.


Hippos rescuing other animals

Hippos have been observed to do some peculiar behaviours, including chasing crocodiles away from wildebeest and buffalos, allowing them to escape. Could it be that they have empathy for these animals, and that they are rescuing them? This has not as yet been determined, but there is certainly much more to learn about these impressive animals.


Hippo poo crucial to the health of functioning ecosystems

The common hippo may spend up to 16 hours a day immersed in water, lumbering out at night to graze on tropical grasses and consuming 80 to 100 pounds in a single meal. By daybreak, having eaten their fill, they return to the water to rest, digest and excrete and it is this natural process that has been determined as a crucial part of the health of certain aquatic ecosystems, with millions of tons of hippo poo entering Africa's waterways every year. This organic matter is a source of nutrition for a variety of river fish and aquatic insects, and hippos play a vital part in transferring this energy source across terrestrial and aquatic ecosystems, with fish and insects absorbing nutrients from hippo dung as part of their diet. The rate of absorption is highest during the dry season when the organic matter is more readily available as it is not being as easily diluted with fast flowing waters.


Hippos have even been described as silicone pumps’ for their crucial role in transferring this particularly important element from the land back into waterways. The grass that hippos eat contains silicon and this has been absorbed by the grass from the groundwater. Hippos absorb silicone through the plants they eat and at least some of this is returned to the water via their faeces.


Silicon is vital for certain organisms such as the unicellular algae known as diatoms that produce oxygen and form the basis of the food chain in many water ecosystems. If a lack of silicon occurs, the diatomaceous algae population can collapse, with harmful consequences for the entire food web in the lake or river concerned. If the diatoms do not get enough silicon, they will be replaced by other species which can lead to depleted oxygen supplies and the associated death of fish.


This demonstrates the importance of hippos in the aquatic food web, and the impact that their declining numbers is likely to be having on the overall health of the riverine environments in which they live.


Populations of the common hippopotamus have undergone widespread decline as a result of habitat conversion resulting in valuable hippo grazing land being turned into farmland, they are also hunted for bushmeat and, increasingly, for their ivory. They once had a broad distribution from the Nile in North Africa down to the Cape at the Southern tip, but due to habitat loss and hunting they only survive in eastern central and southern sub-Saharan Africa, where their populations are in decline. The hippo is classed as vulnerable due to its declining numbers in the wild.


With hippos playing such an important part in the health of their local ecosystems, this decline in numbers is also likely to be having a far greater impact on the health of aquatic animal populations that have for many years relied upon the hippos poo to obtain their essential nutrients for life. Thus the fate of the hippo is intimately linked to the fate of whole food webs and to the functioning of entire ecosystems.


Hippos really are amazing animals!


Hippo welfare:

Hippos are complex animals, and require specialist care when they are kept in captivity. Research has shown that most hippos in North American zoos are not kept in optimal conditions, and so they are suffering as a result.


Hippos are social animals, and in the wild they will live in groups of around 40 individuals. In North American zoos, most hippos are kept in groups of less than three.


Hippos are also most active at night. This means that they perform most of their natural behaviours at night and need their housing to reflect this. Sadly, most zoos keep hippos in holding areas overnight, which means that they cannot perform their normal behaviours, and have poor welfare as a result.


Hippo conservation:

The common hippo is listed as vulnerable, and the pygmy hippo is endangered.

The main threats to hippos are a loss of habitat and deforestation. This is a big problem for the forest dwelling pygmy hippos, and there are now only 2000-3000 individuals left in the wild.


The common hippo is also threatened by hunting. Hundreds of hippos are shot every year to minimise human-wildlife conflict. There are many programmes in place to deter hippos from encroaching on human areas and farms, but the hunting still occurs. This may be because hippos are also hunted for their meat, fat and ivory tusks.


Sadly, hippos join elephants and rhinos in being targets for ivory poachers. Hippo teeth are traded around the world at an alarming rate. Hippo ivory is similar to elephant ivory, and currently there is a loophole in terms of trading it, which has left hippos vulnerable to poaching. In 2018, there were 12,847 hippo teeth traded globally, and this is a dramatic increase from previous years. There are plans in some countries, like the UK, to close this loophole which will help to protect the hippos, but further action is needed.


Hippos once had a broad distribution from the Nile in North Africa down to the Cape at the Southern tip, but due to habitat loss and hunting they only survive in eastern central and southern sub-Saharan Africa, where their populations are in decline. The hippo is classed as vulnerable due to its declining numbers in the wild


For more information about conserving the pygmy hippo see:


For more information about conserving the common hippo see:


Written by Dave Neale, Animal Welfare Director, Animals Asia Foundation

Updated - 30/01/23





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