The Extinction of the Hundsheim Rhino - Being a Generalist Isn't All It's Cracked Up to Be

In any given environment, it might be expected that a generalized or unspecialized species might be less prone to extinction than one which depends upon a narrow temperature range, a peculiar kind of food, or other aspect of natural history which is key to its survival. An herbivorous mammal which can subsist on a variety of grasses, leaves, and other plant foods, for example, may be more likely to survive an ecological disruption than a species tied to foliage which might die back during a climate shift or other ecological changeover. In times of trouble, generalists often survive, but the tale of the extinct Hundsheim rhinoceros - Stephanorhinus hundsheimensis - grates against these conventions.

Between about 1.4 million and 600,000 years ago the Hundsheim rhinoceros ranged over much of Europe and Northern Asia, or what is known as the Palaearctic. It was a rhinoceros adept at chewing. While it lacked incisors, its premolar teeth took on the shape of molars, forming a long grinding surface with which to mash up the variety of plant foods it plucked from ground level up to low-hanging branches. It was a mixed feeder which lacked specializations exclusively associated with grazers or browsers, and this flexibility led paleontologists Ralf-Dietrich Kahlke and Thomas Kaiser to wonder if Hundsheim rhino diets varied from place to place. They have just published their results in Quaternary Science Reviews.

In order to determine what was on Hundsheim rhino menus across populations, Kahlke and Kaiser looked at the teeth of individuals from Su¨Ãenborn and Voigtstedt - two distinct sites in central Germany representing different prehistoric environments. What they were looking for were patterns of mesowear. While microscopic scratches and pits on teeth - called "microwear" - often represent what an animal ate near the time of its death, patterns of mesowear are signals of long-term feedings habits represented by the height and sharpness of cusps on the upper molar teeth (in this case, molars 1-3). Since, in mammals, adult teeth only grow once and are never repaired, the pattern of tooth wear can be a signal of whether the animal was regularly eating abrasive grasses with bits of grit stuck to them (which would wear down teeth quickly), soft leaves (often leaving cusps sharp), or a mixture of both. Compared with mesowear data from living mammals with known feeding habits, this approach can help pin down the diets of long-extinct mammals.

When Kahlke and Kaiser plotted the data, the two populations fell among different clusters. The Su¨Ãenborn specimens fell within a varied cluster or mixed feeders and grazers, and their wear patterns mostly closely matched that seen on the living reedbuck Redunca redunca. The Voigtstedt fossils, by contrast, fell in among the browsers, including the living Sumatran rhino (Dicerorhinus sumatrensis), the giraffe, and mule deer. There was clearly a dietary disparity between the two populations, indicating that the Hundsheim rhino was capable of subsisting on a variety of plant foods. The differences in diet make sense given what the two different habitats were like. According to Kahlke and Kaiser, the Su¨Ãenborn population inhabited a cool, open environment inhabited by grazing mammals, while the Voigtstedt animals lived in warmer, more humid forest habitats.

The Hundsheim rhinoceros was a flexible feeder. Regardless of whether the Su¨Ãenborn population ate more grasses due to the greater availability of those plant foods or was forced to eat more grass due to competition for browse with deer and the steppe mammoth, the fact that these animals were grazers reflects that the Hundsheim rhino was a capable dietary generalist. Why, then, did it become extinct around 500,000 years ago?

For almost the entirety of its existence the Hundsheim rhinoceros was the only rhino present in Europe and western Asia. Its ability to process a range of plant food allowed it to spread far and wide even as other species, such as some of the mammoths they lived alongside, became more specialized. Hundsheim rhino populations began to die back when other rhino species moved into the same habitats during the Middle Pleistocene. Among them was the forest rhino Stephanorhinus
kirchbergensis, a species which would have been a tough competitor for the same high-quality browse as the Hundsheim rhino. At the other end of the feeding spectrum, the grazing species Stephanorhinus hemitoechus moved into open habitats during cold, dry periods around 600,000 years ago, putting the squeeze on the Hundsheim rhino in the grasslands.

As hypothesized by Kahlke and Kaiser, the introduction of these specialized rhinos may have pushed the Hundsheim rhinoceros out of both wooded and open habitats. When the Hundsheim rhinoceros had no competition from other rhinos it could get by just fine, especially in forested areas in which no other mammals were feeding at the same level, but the more specialized rhinos may have out-competed it in both settings. If pressure was being put on the Hundsheim rhinoceros by only one specialized species it might have become adapted to the other type of habitat, but it may not have been able to withstand competition on two fronts. Sometimes, it seems, being a generalist can be a significant liability.

Kahlke, R., & Kaiser, T. (2010). Generalism as a subsistence strategy: advantages and limitations of the highly flexible feeding traits of Pleistocene Stephanorhinus hundsheimensis (Rhinocerotidae, Mammalia)â Quaternary Science Reviews DOI: 10.1016/j.quascirev.2009.12.012