Hippos are artiodactyl ungulates that belong to family Hippopotamidae. There are two living genera/species:
1) Hippopotamus amphibious, which is amphibious and can swim and dive well. This species lives in herds of 5 to 30 individuals near ponds and rivers. This hippo can weigh up 4.5 tons, has stubby legs each with four toes, thick leathery skin that is hairless, and can very widely open its mouth. It has wide horny lips. This hippo lives in Africa south of the Sahara Desert. This species can remain just below the water surface because its nostrils are located on the top front of the skull. The nostrils open dorsally, thus the animal does not have to be much above the surface of the water in order to breathe. Hippos are slightly denser than water, thus allowing them to walk on the river bed.
2) The other living genus/species of hippopotamus is Choeropsis liberiensis [=the pygmy hippo], which is less aquatic than H. amphibious and occurs singly or in pairs in moist forests of West Africa.
As shown in the figure below, the dental formula for hippopotamuses is typically 1/1, 1/1, 4/3, 3/3 = 36 teeth, but the number of premolars can be as many as 4, on both the upper and lower jaws. Hippos graze on grasses growing in and around lakes.
Oblique side view of a hippo skull with most of its teeth visible.
A hippo’s incisors and canines grow continuously, but the cheek teeth do not. The lower pair of incisors are longer than the upper pair of incisors. Both the incisors and canines are large. Even though hippos are herbivores (plant eaters), their canines are tusk-like and can be very large (up to 1.5 feet long). They are also continuously sharpened. The canines are used for fighting other hippos. The cheek teeth are flat-ridged, like in most other land mammals. Hippo teeth contain enamel and is referred to as being ivory. Their ivory is harder than elephant-teeth ivory, but hippo-ivory is not as white. American colonists in the 1700s, used hippo ivory rather than elephant ivory for making false teeth.
The fossil record of hippos is poor. Based on fragments of teeth found in Kenya, Africa, hippos probably originated during middle Miocene time in Africa. Their ancestors were most likely Miocene peccaries or the extinct anthractotheres (large pig-like mammals). Hippos are not ruminants but have two accessory stomach sacs (like peccaries). Their digestion is very slow and very efficient, therefore they do have to be grazing all day long. During the Pleistocene, there were hippos that had eye sockets elevated on stalks (allowing the animal to have periscopic eyes, which allowed the whole body to remain submerged (while the eyes were above water.)
References Used:
en.Wikipedia.org
Lawlor, T.E. 1979. Handbook to the orders and families of living mammals. Mad River Inc. Press. 327 pp.
Savage, R.J. and M.R. Long, 1986. Mammal evolution an illustrated guide. British Museum of Natural History. 259 pp.
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