ADVERTISEMENT
  About the SA Blog Network













Tetrapod Zoology

Tetrapod Zoology


Amphibians, reptiles, birds and mammals - living and extinct
Tetrapod Zoology Home

Pronghorn, designed by committee

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


Email   PrintPrint



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 americanaMammalian Species 90, 1-7.

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

Darren Naish About the Author: Darren Naish is a science writer, technical editor and palaeozoologist (affiliated with the University of Southampton, UK). He mostly works on Cretaceous dinosaurs and pterosaurs but has an avid interest in all things tetrapod. His publications can be downloaded at darrennaish.wordpress.com. He has been blogging at Tetrapod Zoology since 2006. Check out the Tet Zoo podcast at tetzoo.com! Follow on Twitter @TetZoo.

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





Rights & Permissions

Comments 23 Comments

Add Comment
  1. 1. Halbred 6:10 pm 03/25/2014

    This is kind of off-topic, but it’s something I’ve wondered for a long time:

    Aren’t antlers more costly than horns? Pronghorns and other horned ruminants don’t have to re-grow their horns every year. What is the advantage (if there is one) to having antlers over permanent horns? Also, did antlered animals evolve from horned animals (or vice versa)?

    Link to this
  2. 2. Tayo Bethel 7:02 pm 03/25/2014

    where do antilocaprids fit in the artiodactyl radiation?I’ve seen suggestions that they are giraffoids.
    Just a bit of speculation on horns and antlers–anyone can correct me if I’m wrong. Artiodactyls of several lineages seem prone to evolving headgear of various shapes and functions. As far as I know, antlers are restricted to the Cervoidea–dear and the like, Giraffoids evolved hornlike structures as well. Bovids evolved yet another form ofheadgear which are consideredtrue horns–permanent, hollow structures.
    As for the costs of producing horns versus antlers,antlers are probably more costly to produce. Those ungulates which grow permanet horns have to grow only one set, which they retain through the rest of their lives. Antlers, on the other hand, have to be regrown every year and are dependent on proper nutrition on a yearly basis for maximum development. Poor nutrition this year? Small antlers,probably reduced attractiveness and definitely reduced fighting ability. Soin my opinion antlers are probably more costly than horns at leastin the short term.

    Link to this
  3. 3. souhjiro 7:41 pm 03/25/2014

    perhaps antlers evolved initially only as sexual selection ornaments, because primitive cervids rely more on enlarged canines to fight? Not sure about primitive giraffi(oi)ds.

    Link to this
  4. 4. irenedelse 3:34 am 03/26/2014

    Hmm. If antlers are more costly in the long run, it makes them more susceptible to serve as indicators of an animal’s overall fitness, in a sexual selection scenario… Or so it would seem?

    Link to this
  5. 5. Jerzy v. 3.0. 5:35 am 03/26/2014

    Antlers, once grown, require no calories (as they are dead) and can be shed during non-breeding season. So antlers may be advantageous over horns in the all-year time budget. I think, by percentage of body weight, largest antlers (reindeer, elk, giant deer) grow much bigger than the largest horns.

    I guess antlers are also more resistant, being a pure bone.

    It may be interesting to check the annual calorie budget and mechanical properties of antlers and horns?

    Link to this
  6. 6. Jerzy v. 3.0. 5:50 am 03/26/2014

    @3,4
    The ‘ornament’ theory has little merit in ungulates. There is little indication that female ungulates judge size of horns/antlers of males, unlike eg. females of some colorful birds. Horn/antlers are mostly for male dominance, with the common runaway selection towards the weapon as long as possible, being able to reach the rival when the horns/antlers are locked. Females accept the winner, may promote male-male competition, but there is little indication that they actively judge tools.

    BTW, is evolutionary ecology, in any chance, often not scientific? Based on the assumption that today female preferences or ecological constraints have been valid for millenia or millions of years?

    Link to this
  7. 7. naishd 5:55 am 03/26/2014

    Thanks for comments so far. Where do pronghorns fit within phylogeny? There’s a very brief, old Tet Zoo article on that here, with giraffids + pronghorns shown forming a clade. Most molecular studies have supported this hypothesis.

    As for the evolution of antlers: yes, there’s little doubt that they are highly costly, and in fact several (albeit not many) studies have looked at the energetics involved — when growing antlers, and thus putting on 60g or so of calcium and phosphorus A DAY, a big deer’s energy expenditure might rise by something like 20%. This means that deer are constrained to regions where they have ready access to rich calcium and phosphorus sources – at least, if they want to grow healthy antlers (and sometimes they can’t). How do these sorts of figures match up to the growth of horns in other artiodactyls? In many artiodactyls, 75% or so of horn growth is finished by the time they’re 4 years old, so for most of their adult life (10-20 years for most species), comparatively little is being spent on horn growth. It seems that antlers are costly, far more costly than horns. This seemingly makes big-antlered deer especially prone to extinction (see papers by Moen et al. and others on Megaloceros), and provides support for the view that antler evolution was driven by runaway sexual selection.

    In writing this text I mostly had in mind data from…

    Festa-Bianchet, M., Coltman, D. W., Turelli, L. & Jorgenson, J. T. 2004. Relative allocation to horn and body growth in bighorn rams varies with resource availability. Behavioral Ecology 15, 305-312.

    Moen, R. & Pastor, J. 1998. A model to predict nutritional requirements for antler growth in moose. Alces 34, 59-74.

    - ., Pastor, J. & Cohen, Y. 1999. Antler growth and extinction of Irish elk. Evolutionary Ecology Research 1, 235-249.

    Link to this
  8. 8. naishd 6:15 am 03/26/2014

    Jerzy’s message came in while I was writing. Jerzy says “The ‘ornament’ theory has little merit in ungulates. There is little indication that female ungulates judge size of horns/antlers of males” (comment # 6). Oh really? Then you might want to read…

    Bro-Jørgensen, J. 2011. Intra- and intersexual conflicts and cooperation in the evolution of mating strategies: lessons learnt from ungulates. Evolutionary Biology 38, 28-41.

    Folstad, I., Arneberg, P. & Karter, A. J. 1996. Antlers and parasites. Oecologia 105, 556-558.

    Malo, A. F., Roldan, E. R. S., Garde, J., Soler, A. J. & Gomendio, M. 2005. Antlers honestly advertise sperm production and quality. Proceedings of the Royal Society B 272, 149-157.

    Vanpé, C., Gaillard, J.-M., Kjellander, P., Mysterud, A., Magnien, P., Delorme, D., Van Laere, G., Klein, F., Liberg, O. & Hewison, A. J. M. 2007. Antler Size Provides an Honest Signal of Male Phenotypic Quality in Roe Deer. The American Naturalist 169, 481-493.

    Things are complex, with several factors probably contributing to horn and antler evolution (including intrasexual competition, andromimicry and predator defence), but these studies and others do indicate that female choice is an important driver of horn and antler evolution, their form signalling genetic quality and hence linking to mating success.

    Link to this
  9. 9. Jerzy v. 3.0. 12:03 pm 03/26/2014

    Hi Darren,

    I stand corrected on the energy cost of antlers. Although male deer substantially reduce the cost by growing antlers when the food is superabundant.

    About female preference to antlers – nothing in the first paper you cite suggests that females choose males directly on the basis of antler/horn size. Quite the opposite. Factors are male ability to maintain good vigor, good territory or keep other males from even trying to harass females.

    Link to this
  10. 10. naishd 1:01 pm 03/26/2014

    Jerzy: thanks for the comment. The first paper listed above (Bro-Jørgensen 2011) discusses the idea that female choice is related to territory-holding, and hence body size and acoustics in males. But… territory-holding, in turn, is linked to weapon size since this is a key feature contributing to the outcome of male-male combat. In other words, female choice may still be selecting for big/elaborate-horned males.

    As usual, the mistake here may be the implied assumption that we can make sweeping judgements across large groups of species. I suspect that female choice might be driving horn/antler size in some species more than others, with horn/antler size being less significant as goes mating success in some species.

    Link to this
  11. 11. Halbred 5:06 pm 03/26/2014

    One other factor I’ll mention, specifically about moose (since they’re everywhere in Anchorage): antlers increase in size every year up to a certain age, after which point they begin to not only decrease in size but also tend to become asymmetrical or have oddly-angled or curving tines. I wonder if this is also true in other cervids?

    Link to this
  12. 12. irenedelse 8:26 pm 03/26/2014

    Darren, Jerzy: thanks for the info. It’s interesting that andromimicry is cited as a factor in the evolution of horns and antlers in some species. Like the gnu wildebeest IIRC this article! Not the same situation in most cervids, though, with their important sexual dimorphism, especially in antlers.

    Link to this
  13. 13. irenedelse 8:28 pm 03/26/2014

    Antlers or the lack thereof, I should add, of course.

    Link to this
  14. 14. irenedelse 4:59 am 03/27/2014

    Ugh. Penultimate sentence in comment #12 is incoherent. Don’t know what I was thinking about. It should read: “Like in the gnu (or wildebeest) article, IIRC.”
    Sorry.

    Link to this
  15. 15. Jerzy v. 3.0. 5:39 am 03/27/2014

    Hi Darren,

    I was commenting on the concept of ‘ornaments’ and female choice based on these ornaments. This is not true for any ungulate I know.

    To illustrate mixing between male-male competition, female choice based on male-male competition and female choice based on ornaments: ‘quality of armor of medieval knights was strongly correlated with knight success among damsels, so we think that armor was produced mostly to be shown to and evaluated by damsels’.

    @11
    It is also true for eg. red deer.

    Link to this
  16. 16. naishd 7:20 am 03/27/2014

    Jerzy (comment # 15) — I say again, nothing here is simple and we have to be careful about making broad-brush assertions that seem to apply to all horned or antler-bearing mammals. I think you’re right that the horns/antlers are not used as ‘display ornaments’ in all species concerned (that is, their evolution has not been driven by female choice)… as noted above, male-male display, andromimicy and so on may be important in some species… but, actually, female choice may well be driving horn/antler evolution in some others. Malo et al. (2005), cited above, found Red deer antler form to be an honest indication of fitness, and thus concluded that their evolution may well be driven by female mating choice. Vanpé et al. (2007) (cited above) concluded much the same for Roe deer, stating that “we suggest that in territorial species such as the roe deer, antler size may be a major cue in male display and female mate choice, acting as a reliable and honest signal of individual male phenotypic quality”.

    So, I say again: there is indeed data indicating that horn and/or antler form is indeed driven by female choice in some horned mammal species. By the way, the idea that horn/antler form is driven by male-male competition looks to be true in some taxa (sheep, for example), but it’s one of those things that’s often an untested assumption.

    Link to this
  17. 17. blakemarkwell 1:37 pm 03/27/2014

    Nice article. Admittedly, I was looking forward to hearing your opinion regarding the conjecture surrounding the predational influence of Miracinonyx on the speed of Antilocarpa. It’s a cool just-so story; maybe Brian Switek’s take on this topic converges with your own?

    Link to this
  18. 18. DavidMarjanovic 8:08 am 03/28/2014

    its feet (which are fully didactyl, lacking dew hooves) recall those of giraffes

    Or entelodonts. *shudder*

    intrauterine siblicide

    Wait. What? How does that even work?

    Link to this
  19. 19. irenedelse 12:12 pm 03/28/2014

    @ David #18:
    “Intrauterine siblicide”: that would be the absorption of one embryo by another during development, I guess? Leading to chimerism, and sometimes teratologic offspring. (It would also make a great name for a band…)

    Link to this
  20. 20. naishd 12:23 pm 03/28/2014

    Intrauterine siblicide: believe it or not, pronghorn embryos kill siblings while in the womb, spearing neighbouring embryos with a hard cap of dead tissue (O’Gara 1969). This is an often-mentioned aspect of the biology of this species. However, it isn’t anything like the active siblicide that occurs in some sharks – I’ve always imagined it as a sort of passive consequence of crowding in the uterus.

    Ref – -

    O’Gara, B. W. 1969. Unique aspects of reproduction in the female pronghorn [Antilocapra americana Ord). American Journal of Anatomy 125, 217-231.

    Link to this
  21. 21. irenedelse 2:26 pm 03/28/2014

    @ Darren:
    Wow, actual siblicide in the womb. In tetrapods. I knew about similar things in insects but, wow.

    Link to this
  22. 22. DavidMarjanovic 7:46 pm 03/31/2014

    spearing neighbouring embryos with a hard cap of dead tissue

    what

    In tetrapods.

    Oh, Salamandra atra does that – but shark-style: they eat each other. I’ve seen a photo in a paper that looked like the tail of one was growing into the mouth of another.

    Link to this
  23. 23. irenedelse 2:51 am 04/4/2014

    Ok, murderous embryos, nearly as fast as cheetahs, horns like can-openers… Pronghorns are pretty bad*ss… Nobody ever tried to use them in war? ;-)

    Ahem. More seriously, I’m curious too about that “hard cap of dead tissue”. Is it a precursor to horns or hooves or something that disappears later in development?

    Link to this

Add a Comment
You must sign in or register as a ScientificAmerican.com member to submit a comment.

More from Scientific American

Scientific American Back To School

Back to School Sale!

12 Digital Issues + 4 Years of Archive Access just $19.99

Order Now >

X

Email this Article

X