February 15th is Hippo Day! That’s right, while you may still be trying to recover from Valentine’s Day, try to muster up some celebratory feelings for the hippos, as today is their special day.
“Hippopotamus” is Latin for “river horse” and it’s obvious why this name was chosen. They look just like horses! Wait…no they don’t. You’d be hard pressed to confuse these rotund, nearly hairless, wide-lipped, short-legged mammals with horses, but perhaps the Ancient Greeks thought differently. Though horses and hippos are both ungulates (“hoofed mammals”), horses are odd-toed ungulates (of the order Perissodactyla), while hippos have four toes on each foot, and are thus members of the even-toed ungulates (order Artiodactyla).
The full species name of the common hippo, Hippopotamus amphibious, means “river horse that lives in both land and water”. And indeed, the hippopotamus is what’s known as a “semi-aquatic” mammal. This means that it spends a lot of its time in the water, but is not exclusively confined to water like whales, dolphins and manatees. Seals, otters and polar bears are other examples of semi-aquatic mammals. Hippos live in and around the rivers of Africa, although their current range is much smaller than it once was.
Hippos are quite formidable, and are considered one of the most dangerous animals in Africa. Their large tusks can cause serious damage, and they are known to be aggressive, so they are often given a wide berth by predators and humans alike. Hippo skin can be up to two inches thick and is very difficult for claws and teeth to penetrate, making them unappealing as prey. However, the outermost layer of skin is very sensitive, and dries out quickly if it isn’t kept wet. Additionally, hippos will often wallow in mud to cover their skin and protect it from parasites and biting insects.
There are currently two recognized living species of hippopotamus. Most people are familiar with the common hippo, but its smaller cousin the pygmy hippopotamus (Choeropsis liberiensis) is less well-known. Both species are fairly similar, with rotund bodies that are nearly hairless, with short legs and large wide heads with broad, heavy-lipped mouths. They both have large canine teeth, and tusks in their lower jaw. At first glance, the pygmy hippo looks like a baby common hippo, but on closer inspection there are some key differences.
The pygmy hippo has proportionately longer legs in relation to its body size, while its head is proportionately smaller. It also has less prominent eyes and nostrils than the common hippo. Most of these differences are the result of the living habits of the pygmy hippo, which spends more time on land than its larger relative. While hippos eat mainly grass, pygmy hippos eat ferns, plants and fruits they forage in their forest habitat.
While hippos resemble rhinoceroses and pigs in many ways, their closest living relatives are actually the whales and dolphins of the infraorder Cetacea. While whales are now fully aquatic animals, they are thought to have evolved from an even-toed land mammal in the distant past, around 50 million years ago. The hippo lineage is thought to have split from the group that would become whales about 5 million years before that.
Both species of hippopotamus are endangered. The main threat to the pygmy hippo is habitat loss, as its range is much more restricted than its cousin. The common hippo faces threats of poaching, as it is illegally hunted for both its meat and its ivory tusks. There are many organizations out there fighting for the hippos, including the African Wildlife Foundation and the World Wildlife Fund. In addition to learning about hippos today, visit their sites to see what they’re doing to help, and how you can participate!
Bernie’s Bonus Fun Fact: Hippos lack sweat glands, but they do secrete a red-orange liquid called hipposudoric acid. It is commonly called “blood sweat” due to its color, although it isn’t actually sweat or blood. It serves many purposes, including protecting the hippo as a sort of natural sunblock.