Dirty Browsers – Determining a menu for North America’s fossil camels

Even with the young politician Jefferson Davis behind their adoption by the military, camels were a hard sell to the U.S. government. Along with other military men, Davis was convinced that camels could replace horses as the standard beasts of burden used by cavalry on the ever-expanding western frontier, but most congressmen and senators balked at the idea. When Davis tried to formally get a military appropriation for fifty camels (along with Arab trainers and other supplies) in 1851, the senators present for his speech thought the idea of cannon-carrying camels to be too frivolous to merit serious consideration, and the following year a similar request was similarly shot down.

But Davis was not to be deterred. The defeats the camel appropriation requests had suffered raised the idea’s profile, and when Davis took office as the U.S. Secretary of War in 1853 he renewed his push to see American soldiers riding camels across the arid western scrub. Once again, Davis’ attempt for an 1853 appropriation failed, but with some support from two midwestern senators, Davis was finally able to secure the $30,000 needed for the experiment. The “Camel Corps” project was made into law in March of 1855.

With the bureaucratic matters in order, Davis sent military officers out to the desert bordering the eastern edge of the Mediterranean Sea to gather information and procure dromedary camels to bring back to the United States. By the summer of 1856, a small herd had been offloaded and was being observed in Texas – even the local people, at first skeptical that camels could carry much weight at all, were impressed with the abilities of these animals to bear large burdens. Further tests confirmed that camels were adept at going over and through terrain which was impassible to horses and mules, and it seemed that Davis’ had been right about the utility of the camels.

Despite the promise the early observations held, western military outposts were not as enthusiastic about receiving the camels. The quartermasters preferred horses, and the soldiers disliked the Arab camel caretakers even more than the camels themselves. Nevertheless, those soldiers who did use camels found them to perform as well – if not better – than horses when traveling through the desert, and the experiment continued even after Davis left office in 1857. Then came the Civil War. When the Confederacy, led by Davis, broke away from the federal government in 1861 the camel experiment was effectively halted. Many of the animals were sold off, some were set loose to go feral, and others remained at their military outposts, causing a fuss among local people who thought them to be ugly, smelly nuisances (at least until Confederate troops captured some of these outposts and let the camels run off into the desert).

On its surface, the importation of dromedary camels to the United States would seem to be a government-facilitated invasion of a foreign species. Camels are animals of northern Africa and Asia, not North America. Yet, through the perspective of geologic time, the introduction of camels to the United States is no stranger than the importation of horses to the continent by European explorers. Much like horses, camels evolved in North America before being entirely extirpated from it; the introduction of modern species to the American west was something of a homecoming for a lineage which had been absent from it for more than 10,000 years.

Although the exotic camels brought to the United States acclimated well to life in the west, not all fossil camels lived in dry, scrubby habitats. Over the course of 45 million years many different genera of camels occupied an array of habitats, from closed forests to open grasslands. One way to appreciate this diversity – both of camels and the ecosystems they inhabited – is to look at the distinctive patterns of scratches and pits left by plant food on their teeth. As communicated in a new Palaeogeography, Palaeoclimatology, Palaeoecology paper, this is precisely what scientists Gina Semprebon and Florent Rivals have done.

The word “camel” is typically attributed to two species of mammal – the dromedary and Bactrian camels – but the extant camelids encompass a wider variety of creatures, including the llamas, alpacas, vicuñas and guanacos of South America. Despite their present range, though, for the first 36 million years of their evolution camelids were restricted to North America, with their heyday occurring around 16 million years ago during the Miocene (a time when many different large mammals, including horses and predatory whales, were also undergoing evolutionary radiations). By six million years ago, the llama and camel lineages had split and camelids had spread to other continents, and the remaining North American lineages became extinct at the end of the Pleistocene along with the giant ground sloths, mammoths, saber-toothed cats, and other megafauna. Although there are no endemic camelids left in North America, camelids persisted on this continent longer than any other, and so the fossil record of North American camelids provides a rich source of information about their paleobiology.

In order to ascertain the dietary habits of the extinct North American camelids, Semprebon and Rivals looked at three different aspects of their molars: the height of their teeth (the higher the tooth crowns, the rougher the diet), mesowear (wear on tooth cusps caused by long-term feeding patterns by an individual), and microwear (pits and scratches made by food during the time shortly before the death of the animal). Together these three different kinds of data outline not only the dietary preferences of individual animals, but also shifts in dietary patterns over time, which are themselves signals of the kinds of environments inhabited by camelids at different points in earth history.

After surveying tooth characteristics in a range of camelid taxa – from the small, early genus Poebrotherium and the giraffe-like, Miocene form Aepycamelus to the recently-extinct Camelops – Semprebon and Rivals found that, in general, fossil camels had much tougher diets than their living counterparts. In terms of mesowear, specifically, fossil camelids showed higher degrees of long-term wear on their teeth from the Eocene through the early Miocene. At this point there was a brief reversal in these trends, but after the mid-Miocene fossil camelids again showed increasing amounts of wear on their molars until the mid-Pleistocene, when there was another drop. Changes in tooth crown height roughly tracked this pattern – tooth crowns became higher during times when there were elevated degrees of mesowear on camelid teeth – indicating that for much of their evolutionary history camelids consumed tough, abrasive foods with reversals occurring in the mid-Miocene and from the mid-Pleistocene to recent time. In terms of tooth crown height and mesowear, living camels and llamas are more like early camelids than most of their fossil relatives.

Given the patterns in tooth height and mesowear seen in other herbivorous mammals, it might be expected that many fossil camelids primarily fed on tough grasses. This does not appear to be the case. According to Semprebon and Rivals, most fossil camelids were actually browsers, but they foods they selected were tougher on their teeth. A prime example is the early Miocene genus Stenomylus. This long-limbed, antelope-like camelid had high-crowned teeth and appears to have been adapted to open, grassy habitats, but the patterns of microwear on its teeth are comparable to those of living herbivores which primarily browse on leaves. The reason for this seeming contradiction – a browser with the body of a grazer – may be that Stenomylus was a “dirty browser”, or ate soft plants covered in a significant amount of grit. If this was the case, then the high-crowned teeth of camelids were not adaptations to tough grasses, but to softer foods coated in bits of hard extraneous matter which could quickly wear down teeth.

Stenomylus was not an isolated oddball. Some camelids – such as the giraffe-like Aepycamelus – browsed on “cleaner” plant foods high off the ground, and camelid Megatylopus was a grazer, but, when looked at from a wider perspective, Semprebon and Rivals found that camelids moved into open habitats relatively early in their evolution and have primarily been dirty browsers in terms of diet. As far as teeth are concerned, the grit-covered foods camelids consumed caused their teeth to converge in form with those of grazers. This similarity has misled some paleontologists in the past. In his massive 1913 treatise A History of the Land Mammals of the Western Hemisphere, paleontologist W.B Scott wrote “The mode of evolution displayed by the camels does not differ in any significant respect from that seen in the horses,” and included an illustration showing how camels, too, evolved in a straightforward fashion from small browsers to large grazers. Now – thanks to the new study by Semprebon and Rivals and previous work on horses by paleontologists such as Bruce MacFadden – we know that neither camelids nor horses evolved in such a linear fashion, and camelids in particular have undergone some dental reversals as the available plant foods have changed during the past 20 million years.

So, although Jefferson Davis did bring camelids back to North America, the imported dromedaries were not equivalent to most of the camelids which lived on the continent during prehistory. The relatively narrow swath of modern camelid diversity does not contain an exact proxy for the species which have been lost from North America, and the feral camels which were loosed into the west interacted with the local flora and fauna in ways different from their prehistoric cousins. Seen in the context of their fossil relatives, though, modern camelids seem even stranger than they already are – they are evolutionary mosaics which the body of a grazer meets the diet of a browser.

Gina M. Semprebon and Florent Rivals (2010). Trends in the paleodietary habits of fossil camels from the Tertiary and Quaternary of
North America Palaeogeography, Palaeoclimatology, Palaeoecology, 295, 131-145 DOI: 10.1016/j.palaeo.2010.05.033