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Tetrapod Zoology

Tetrapod Zoology

Amphibians, reptiles, birds and mammals - living and extinct

The Pronghorn Antelope: Designed by Committee

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Adult male Pronghorn, photographed in Oregon. Image in public domain.

So much for posting more on ratites – alas, I just haven’t had time to finish the next article. Inspired by an article recently published by my friend and homeboy Brian Switek, I thought it time to republish this 2010 article. Enjoy.

The Pronghorn or Pronghorn antelope* Antilocapra americana is a strikingly unique artiodactyl, endemic to western North America. Historically, it ranged from southern Manitoba and Washington in the north to northern Mexico in the south, and to western Iowa in the east. Between 40 and 50 million Pronghorns were alive in 1850; excessive hunting had reduced this number to 13000 by 1920. Subsequent conservation efforts have resulted in modest recovery, there currently being between half a million and one million Pronghorns.

* Also known as the Cabrit, Prong Buck, Speedgoat (my favourite) or just Antelope.

Pronghorn female and juvenile; image by Jack Dykinga, in public domain.

Growing to about 1.5 m in total length, and with a shoulder height of approximately 1 m, the Pronghorn is a cursorial browser-grazer of plains, scrub-steppes and deserts. It’s renowned for its phenomenal running speed. Most sources (e.g., Nowak 1999, Kitchen & Maher 2001) state that it’s been clocked at 86 km/h (55 mph): a top speed of 100 km/h (62 mph) is given by some and there’s even one claim of 113 km/h (70 mph). Astonishingly, it can run 11 km in 10 minutes, giving an average running speed of 65 km/h (40 mph) (Lindstedt et al. 1991). The top speeds credited to Cheetah Acinonyx jubatus are 96-101 km/h (60-63 mph), though it’s easy to find claims of 114-120 km/h (70-75 mph). While the highest claimed running speeds credited to Pronghorn are likely to be too high, there’s no doubting the incredible stamina of these animals: their average speed exceeds that of any other terrestrial animal, including the cheetah.

A Pronghorn might be off the ground for distances of up to 8 m when running at full-tilt, and its hooves are heavily cushioned with cartilage. The question of why the Pronghorn is so fast when no modern American predator is anywhere near as speedy has often been asked and the favoured answer is predictable.

Pronghorn vs car, who will win? Illustration from Casa Editrice AMZ's Animal Life in North America.

The Pronghorn is an anatomical oddball and is among those animals sometimes described as having been ‘designed by committee’ (camels and therizinosaurs are among the others). As depicted in the self-explanatory cartoon below, it’s been said that its feet (which are fully didactyl, lacking dew hooves) recall those of giraffes, that the erectile hair patches on its rump resemble those of Antidorcas (the Springbok), and so on.

Neat diagram from one of the Orbis World of Wildlife volumes, written by Felix Rodriguez de la Fuente.

A key pronghorn character is the presence of paired, supraorbitally positioned frontal horns that are never shed. While the bony core is unforked, its keratinous covering is forked (in males) and is shed annually.

Another beautiful male Pronghorn (hey, sorry females, you're nice too). Image by Mongo, in public domain.

In males (like the one shown here), the horns are longer than the ears and have obvious anterior prongs. The horns of females are typically shorter than the ears and are usually simple spikes without the prongs. As usual for horned mammals, however, lots of variation has been reported. Females with especially large horns, and females with no horns at all, are on record. Males also differ from females in having black patches on the face and neck, and in running with the nose tilted down towards the ground (as opposed to running with the head’s long-axis held parallel to the horizon: it would be interesting to know why this sexual dimorphism occurs). There’s loads of other weird stuff worth commenting on: intrauterine siblicide, shock-moulting, their reluctance to leap over obstacles, and their attraction to unfamiliar objects.

Five subspecies have been named: the large, widespread, ‘typical’ A. a. americana, the critically endangered A. a. peninsularis of Baja California (less than 200 persist), the endangered A. a. sonoriensis of north-west Mexico and southern Arizona, A. a. mexicana of Mexico and the south-western USA, and A. a. oregona of Oregon. The validity of some of these subspecies has been questioned, but some – like A. a. sonoriensis – have stood up to scrutiny (Paradiso & Nowak 1971).

As usual, introductions and relocations made by people have messed up the original distributions of these subspecies. The transplantation of thousands of Pronghorns into New Mexico between 1936 and 1957, for example, means that any original subspecies boundaries there have become blurred (O’Gara 1978). People have also moved Pronghorns (possibly belonging to A. a. oregona) from Oregon to Washington, and animals from southern Arizona (where the endangered A. a. sonoriensis occurs) have had their numbers boosted by introduction from northern Arizona. For some stupid reason, 40 Montanan Pronghorns were introduced to Lanai in the Hawaiian Islands in 1959 (there’s a very sad story here; it ends with the last members of the herd dying out by the mid 1970s).

Holotype horns of A. anteflexa (at right), named by Gray (1856), compared to 'normal' horns of a normal A. americana.

Captive Pronghorn with unusual horns. Image by Chris Valle, licensed under Creative Commons Attribution-Share Alike 3.0 Unported, 2.5 Generic, 2.0 Generic and 1.0 Generic license.

Little known is that a second species of Pronghorn – A. anteflexa – was proposed by Gray (1856) for a pair of horns (see above) shown to him by the Earl of Derby (their provenance was unknown). These horns were particularly long and the apex of each horn curved strongly forwards. However, horns of this sort seem to crop up as oddities within normal A. americana populations: very similar horns are seen in the captive individual shown below (from Los Angeles Zoo; photo by Chris Valle).

As is so often the case, the living Pronghorn is merely the surviving remnant of a much more diverse group, and some extinct pronghorns were spectacular and very unusual compared to Antilocapra (though.. don’t get me wrong, others were boring and mundane).

For more on pecoran artiodactyls at Tet Zoo, see…

Refs – -

Gray, J. E. 1856. Notice on the horns of an unrecorded species of pronghorn (Antilocapra), in the collection of the Derby Museum, Liverpool. The Annals & Magazine of Natural History 17 (2nd series), 424-426.

Kitchen, D. & Maher, C. R. 2001. Pronghorn. In Macdonald, D. (ed) The New Encyclopedia of Mammals. Oxford University Press, pp. 528-529.

Lindstedt, S. L., Hokanson, J. F., Wells, D. J., Swain, S. D., Hoppeler, H. & Navarro, V. 1991. Running energetics in the pronghorn antelope. Nature 353, 748-750.

Nowak, R. M. 1999. Walker’s Mammals of the World, Sixth Edition, Volume II. The Johns Hopkins University Press (Baltimore and London).

O’Gara, B. W. 1978. Antilocapra americana. Mammalian Species 90, 1-7.

Paradiso, J. L. & Nowak, R. M. 1971. Taxonomic status of the Sonoran pronghorn. Journal of Mammalogy 52, 855-858.

The views expressed are those of the author and are not necessarily those of Scientific American.

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