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Even-toed ungulate

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Even-toed ungulate

Even-toed ungulates
Temporal range: 54–0Ma
Early Eocene - Recent
Bones of right fore feet of existing Artiodactyla: From left to right: pig (Sus scrofa), red deer (Cervus elaphus), and camel (Camelus bactrianus). U = ulna, R = radius, c = cuneiform, l = lunar, s = scaphoid, u = unciform, m = magnum, td = trapezoid. In the sheep and the camel, the long compound bone, supporting the two main (or only) toes is the cannon bone.
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Infraclass: Eutheria
Superorder: Laurasiatheria
Order: Artiodactyla
Owen, 1848

The even-toed ungulates (order Artiodactyla) are ungulates (hoofed animals) whose weight is borne approximately equally by the third and fourth toes, rather than mostly or entirely by the third as in odd-toed ungulates (perissodactyls), such as horses.

The name Artiodactyla comes from (Greek: ἄρτιος (ártios), "even", and δάκτυλος (dáktylos), "finger/toe"), so the name "even-toed" is a translation of the description.[1] This group includes pigs, peccaries, hippopotamuses, camels, llamas, chevrotains (mouse deer), deer, giraffes, pronghorn, antelopes, goat-antelopes (which include sheep, goats and others), and cattle. The group excludes whales (Cetacea), although DNA sequence data indicate they share a common ancestor, making the group paraphyletic. The phylogenetically accurate group is called Cetartiodactyla (from Cetacea + Artiodactyla).[2]

Of the roughly 220 artiodactyl species, many are of great dietary, economic, and cultural importance to humans.

A further distinguishing feature of the group is the shape of the astragalus (talus), a bone in the ankle joint, which has a double-pulley structure. This gives the foot greater flexibility.[3]

Evolutionary history

Cladogram showing relations within Artiodactyla[4]

As with many extant mammal groups, even-toed ungulates first appeared during the early Eocene (about 54 million years ago). In form, they were rather like today's chevrotains: small, short-legged creatures that ate leaves and the soft parts of plants. By the late Eocene (46 million years ago), the three modern suborders had already developed: Suina (the pig group); Tylopoda (the camel group); Ruminantia (the goat and cattle group, and the Cetruminantia, which still includes the extant hippos and both whale groups). Nevertheless, artiodactyls were far from dominant at that time; the odd-toed ungulates (ancestors of today's horses and rhinoceroses) were much more successful and far more numerous. Even-toed ungulates survived in niche roles, usually occupying marginal habitats, and presumably at that time they developed their complex digestive systems, which allowed them to survive on lower-grade food.

The appearance of grasses during the Eocene, and their subsequent spread during the Miocene (about 20 million years ago), allowed a major change; grasses are very difficult to digest, and the even-toed ungulates, with their highly developed stomachs, were better able to adapt to this coarse, low-nutrient diet, and soon replaced the odd-toed ungulates as the dominant terrestrial herbivores. Now-extinct Artiodactyla that developed during the Miocene include the genera Ampelomeryx, Tauromeryx, and Triceromeryx.


This classification is based on Spaulding et al., 2009[5] and the extant families recognised by Mammal Species of the World published in 2005.[6] Currently, the cetaceans and even-toed ungulates have been placed in Cetartiodactyla as sister groups, although DNA analysis has shown cetaceans evolved from within Artiodactyla. The most recent theory into the origins of Hippopotamidae suggests that hippos and whales shared a common semiaquatic ancestor that branched off from other artiodactyls around 60 million years ago.[7][8] This hypothesized ancestral group likely split into two branches around 54 million years ago.[9] One branch would evolve into cetaceans, possibly beginning with the protowhale Pakicetus from 52 million years ago, with other early whale ancestors collectively known as Archaeoceti, which eventually underwent aquatic adaptation into the completely aquatic cetaceans.[10]

Anatomy, physiology, and morphology

Giraffes necking (Giraffa camelopardalis) in Ithala Game Reserve, northern KwaZulu Natal, South Africa

The even-toed ungulates stand on an even number of toes; the group's three suborders differ in other characteristics. Suina (pigs and peccaries) have retained four toes of fairly equal size, have simpler molars, short legs, and often have enlarged canine teeth that form tusks. Camelids and Ruminantia tend to be longer-legged, to walk on only the two central toes (though the outer two may survive as rarely used dew claws) and to have more complex cheek teeth that are well-suited to grinding up tough grasses.

Diet and feeding

The ancestors of the even-toed ungulates were omnivores that preferred plant material; now, even-toed ungulates are generally herbivorous, although species in the suborder Suina are, like their primitive ancestors, omnivores. Larger stomachs and longer intestines have evolved because plant material is more difficult to digest than meat.[12]

[14] Microbial fermentation with the formation of high volatile fatty acid levels has been observed in the fore stomach; it has been proposed that their complex fore stomach is a means to slow digestive passage and increase digestive efficiency.[14] Hippopotamuses have three-chambered stomachs and do not ruminate. They consume around 68 kg of grass and other plant matter each night. They may cover large distances (up to 20 miles) to obtain their food, which they digest with the help of microbes that produce cellulase. Their closest living relatives, the whales, are obligate carnivores.

Rumination occurs in the microbes (bacteria, protozoa, and fungi) produce cellulase, which is needed to break down the cellulose found in plant material. Without this mutual symbiosis, ruminants would find plant material indigestible.[13]

Habitat and distribution

Even-toed ungulates are found on every continent but Antarctica; they were introduced to Australia and New Zealand by humans.[15]

Relationship with humans

The even-toed ungulates are of more economic and cultural benefit than any other group of mammals.[12] Clear evidence exists of Great Rift Valley.[12] Cro-Magnons relied heavily on reindeer for food, skins, tools, and weapons; with dropping temperatures and increased reindeer numbers at the end of the Pleistocene, they became the prey of choice. By around 12,500 years ago, reindeer remains accounted for 94% of bones and teeth found in a cave above the Céou River.[16]

Today, cattle are the basis of a multibillion dollar industry worldwide. The international trade in beef for 2000 was over $30 billion and represented only 23% of world beef production.[17]

Jewish biblical laws of kashrut define a cloven hoof as one of two key requirements for an animal to be capable of consideration for kosher consumption.


Humans have hunted many species of artiodactyls without regulation. This has caused half of the even-toed ungulates to be near extinction, especially in areas with decreased economic development. Conservation efforts to increase local population growths have been undertaken. Some have been so effective, population control has been enforced. The even-toed ungulate has experienced habitat loss in addition to climate change. Climate change has forced many species to move poleward. An example would be moose, which are heat intolerant, whose southernmost populations have declined sharply in response to increased temperatures.

There are 168 artiodactyl species on the IUCN Red List of Threatened Species. Seven are listed as extinct, two as “extinct in the wild”, 26 as “endangered”, one as “near threatened”, and 73 as “lower risk”. Information is lacking for the 13 other species.[18]


  1. ^ American Heritage Dictionary of the English Language, 3rd edition, 1992, p. 105
  2. ^ Montgelard C, Catzeflis FM, Douzery E (1 May 1997). "Phylogenetic relationships of artiodactyls and cetaceans as deduced from the comparison of cytochrome b and 12S rRNA mitochondrial sequences.".  
  3. ^ Savage, R. J. G. & Long, M. R. (1986). Mammal Evolution: an illustrated guide. New York: Facts on File. p. 208.  
  4. ^ Spaulding, Michelle; O'Leary, Maureen A.; Gatesy, John; Farke, Andrew Allen (2009). "Relationships of Cetacea (Artiodactyla) Among Mammals: Increased Taxon Sampling Alters Interpretations of Key Fossils and Character Evolution". PLoS ONE 4 (9): e7062.  
  5. ^ a b Spaulding, M; O'Leary, MA; Gatesy, J (2009). Farke, Andrew Allen, ed. "Relationships of Cetacea (Artiodactyla) Among Mammals: Increased Taxon Sampling Alters Interpretations of Key Fossils and Character Evolution". PLoS ONE 4 (9): e7062.  
  6. ^ Wilson, D. E. & Reeder, D. M., ed. (2005). Mammal Species of the World (3rd ed.). Johns Hopkins University Press. pp. 111–184.  
  7. ^ "Scientists find missing link between the dolphin, whale and its closest relative, the hippo". Science News Daily. 2005-01-25. Retrieved 2007-06-18. 
  8. ^ Gatesy, J. (1 May 1997). "More DNA support for a Cetacea/Hippopotamidae clade: the blood-clotting protein gene gamma-fibrinogen".  
  9. ^ Ursing, B. M.; Arnason, U. (1998). "Analyses of mitochondrial genomes strongly support a hippopotamus-whale clade".  
  10. ^ Boisserie, Jean-Renaud; Lihoreau, F. & Brunet, M. (February 2005). "The position of Hippopotamidae within Cetartiodactyla".  
  11. ^
  12. ^ a b c d "Artiodactyl". Encyclopædia Britannica Online.  
  13. ^ a b Janis, C. & Jarman, P. (1984). Macdonald, D., ed. The Encyclopedia of Mammals. New York: Facts on File. pp. 498–499.  
  14. ^ a b Shively, C. L. et al. (1985). "Some Aspects of the Nutritional Biology of the Collared Peccary". The Journal of Wildlife Management 49 (3): 729–732.  
  15. ^ Pough, F. W., Janis, C. M. & Heiser, J. B. (2005) [1979]. "Major Lineages of Mammals". Vertebrate Life (7th ed.). Pearson. p. 539.  
  16. ^ "Bones From French Cave Show Neanderthals, Cro-Magnon Hunted Same Prey". ScienceDaily. 2003. Retrieved 2008-10-17. 
  17. ^ Clay, J. (2004). World Agriculture and the Environment: A Commodity-by-Commodity Guide to Impacts and Practices. Washington, D.C., USA: Island Press.  
  18. ^ "Artiodactyla". Encyclopedia of Life. Retrieved 15 November 2014. 

External links

  • Ungulate Taxonomy: A new perspective from Groves and Grubb (2011)
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