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rhinoceros giants

by Donald Prothero, May 15 2013

When I started my graduate career at the American Museum of Natural History in New York in 1976, I soon realized that I had stumbled upon an incredible opportunity. In addition to the world-famous fossil halls that have amazed generations of visitors, there are at least a hundred times as many fossils stored in research collections for study by qualified scientists. This is where the real work of paleontology takes place: specialists dedicated to the study of one group of organisms spending weeks to months to years examining every fossil in the collection, trying to reconstruct their anatomy, determine their relationships, and decipher what is the correct taxonomic name for any group of specimens. Without this fundamental work determining which species are valid, and when and where they lived, all other work in paleontology (especially computer models which are based on counting taxa studied by others and compiling them into databases) is “garbage in, garbage out.”

The American Museum is particularly important for such research, because it has the original collections of pioneering paleontologist Edward Drinker Cope, collected from the 1870s and 1880s, plus the huge numbers of fossils accumulated by its legendary paleontologists from 1895-1935 (Henry Fairfield Osborn, William Diller Matthew, Walter Granger, and others), as well as later collections obtained by the most brilliant paleontologist of the twentieth century, George Gaylord Simpson. The collections of dinosaurs, other reptiles, birds, amphibians, fish, and other vertebrates huge, but they are all outstripped by the gigantic collection of (mainly North American) fossil mammals. In the 1920s, the millionaire Childs Frick (son of the robber baron Henry Clay Frick, Andrew Carnegie’s partner) became interested in the origin of the mammals he used to shoot on big-game hunts. Starting about 1930 and for the next 35 years he used his wealth to pay for field crews to work year-round in the important fossil beds of the western United States, making giant collections from key localities and finding many more localities. Consequently, where we used to have just isolated teeth and jaws and maybe a skull of most mammals, the Frick Collection usually has many complete skulls or skeletons. This allows a paleontologist to see the complete anatomy of a particular mammal, examine variability within a population, and determine a much more informed and modern classification of names that had been based on isolated scraps of teeth described a century ago. Thus, most of the major groups of North American fossil mammals have to be completely restudied using the huge Frick Collection before we can make any conclusions about how many species existed, and when and where they lived.

To give you a sense of the size of the collection, there is a separate wing to house just the fossil mammals, built in an interior courtyard so it is invisible to the public. The fundraising and construction was started after Frick died in 1965, and not completed until shortly before I arrived in 1976. The Frick Wing has 10 floors altogether: an entire floor of rhinos, an entire floor of camels, an entire floor of horses, an entire floor of mastodonts and mammoths, three more floors of other groups of mammals, and the top three floors are the prep lab, the offices, classrooms, library, teaching collections, and other essential spaces. The horse floor is largely studied and published, but almost nothing has been done on the camel floor or the mastodont floor. When I arrived in 1976, the Museum’s curatorial assistant Dr. Earl Manning took me under his wing and introduced me to the study of North American rhinos, which had been neglected since the 1920s. At first we worked on projects together, but after he left in 1980, I continued working on the rhinos for another 25 years, finally publishing my comprehensive book-length monograph on them in 2005. Where once rhinos were a mess of invalid species, outdated names, mistaken identifications, and uncertain relationships, now they are one of the best-documented groups of North American mammals. Using my book, you can identify any bone of any North America rhino to genus and species.

In addition to the North American rhinos, I spent a lot of time looking at the gigantic specimens of the huge hornless Asian rhinos known as indricotheres. The American Museum has the best collection of them outside Beijing and Moscow, acquired by the legendary Central Asiatic Expeditions to Mongolia in the 1920s. After my 2005 North American rhino book, I thought it might be fun to write about these amazing creatures, which weighed as much as 20 metric tonnes, larger than the largest elephant or mammoth. When I had a sabbatical in late 2011, I finally had a chance to sit down and write a book about them, and the book has just appeared.

The first thing to realize is that much about what you see about extinct animals on TV documentaries is artistic guesswork, not based on any hard evidence. In the case of indricotheres, we have only the bones, and only partial skeletons at that. There is no direct information about the color of the animal, skin texture, what it ate, how it walked or how it behaved or sounded. All of this information, so often a crucial part of the CG animations that now dominate most documentaries about prehistoric life, are entirely conjectural and cannot be determined directly from the bones. The usual approach is to model indricotheres on the basis of living rhinos, with thick gray hairless skins with numerous folds, although we have no skin impressions or mummified specimens to test this idea, one way or another. The behavior and colors and sounds of the animals in these CG animations (such as in the documentary “Walking with Prehistoric Beasts”) are completely imaginary, and have no basis in any real-world data. Although most scientists are aware of this, a surprising number of people who watch these TV shows are stunned when they find out that so much of the show is pure fiction for entertainment, rather than science. The only real science in these shows is the interviews of expert paleontologists, and the pictures of bones and fossil localities.

Although most of the stuff you see in CG animations of prehistoric beasts (including the indricotheres) is mostly guesswork, there are living analogues that can give us some guidance about indricothere biology. The best models might be elephants, which approach indricotheres in body size. There are certain constraints about life at such large body size for elephants that must also apply to the indricotheres:

Thermoregulation: Elephants have a huge body volume and mass compared to their surface area (remember, volume increases as a cube while area only increases as a square). As the debate about hot-blooded dinosaurs back in the 1980s revealed, such huge animals with an endothermic physiology (that is, they generate their own body heat from metabolism) have a severe problem getting rid of excess body heat, especially if they live in warm climates. Living elephants have huge ears as radiators to shed excess body heat from their bloodstream, and it is reasonable to infer that indricotheres did too. African elephants and rhinos and hippos spend much of their daytime resting in the shade or wallowing in waterholes and mud puddles to cool down, and so must have the indricotheres. Elephants, rhinos and hippos feed and move mainly at night, as indricotheres must have done. Elephants and rhinos both have largely naked skin since hair holds in body heat, which is why such elephant-like naked gray skin seems appropriate for indricotheres. Large-bodied endothermic mammals are in a constant battle to dump body heat and avoid overheating.

Digestion: There are certain other constraints for large-bodied herbivores as well. All herbivores eat large amounts of cellulose in their diets, which is a relatively indigestible carbohydrate. Most plant eaters must use some kind of specialized gut bacterium in their digestive tract to break down the cellulose and release the nutrients. Such a breakdown requires fermentation, and takes time to absorb the nutrients from the fermentation process into the lining of the intestines. There are two basic types of herbivore digestion: foregut fermenters and hindgut fermenters.

The only living foregut fermenters are the ruminant artiodactyls (camels, cattle, sheep, goats, antelopes, deer, and pronghorns), which do this by “ruminating” using a four-chambered stomach. The first chamber, the rumen, is a digestive vat full of bacteria, so that when they swallow a bite of partially chewed plant material, it goes immediately into the rumen where it begins bacterial breakdown. Later, when they are resting, ruminants regurgitate some of the contents of their rumen back into their mouths, where they can “chew their cud” and break the material down further, before swallowing it again. By the time the food reaches the lining of their intestines, it is highly broken down into nutrients and easily absorbed. Thus, ruminants use nearly every bit of their food efficiently, and can survive on relatively small amounts of good-quality vegetation. But if they eat too much high-quality vegetation, they can become bloated and their rumen can swell and even rupture and kill them with all the gas released from the rapid bacterial fermentation.

The remaining herbivorous mammals are hindgut fermenters. These include the perissodactyls (odd-toed hoofed mammals, today including the horses, tapirs and rhinos), the elephants, the non-ruminant artiodactyls (pigs, peccaries, and hippos), and other herbivores such as rabbits and some primates. Instead of a highly specialized foregut with a rumen, they have the normal mammalian digestive tract, with an esophagus, acid-filled stomach, and then intestines for absorption. Most have a pouch off the intestine called a caecum that is the primary location of bacterial fermentation.  Lacking a rumen, the hindgut fermenters pass the mostly undigested cellulose through the digestive tract until it reaches the caecum, but bacterial fermentation only just starts in the caecum before the food goes through the remaining intestines and is then excreted. Consequently, they get relatively little nutrition out of each bite of fodder, and must eat much larger volumes of mostly low-quality food (especially grasses) to get enough to live on. Most hindgut fermenters, like horses, rhinos, and elephants, are by necessity be high-volume low-efficiency eaters, and eat huge volumes of material just to survive, since they are so poor at extracting the nutrients. When you see the feces of these animals (like the “road apples” of horses), they are typically full of undigested plant matter compared to the “cowpies” of a ruminant, or the tiny pellets of a deer or pronghorn. Rabbits are a special case. If you have ever kept rabbits in a hutch, you will notice that they eat their own feces. This gives them a chance to run the food through their gut a second time after the bacterial fermentation has had time to work, and get more nutrition this way.

For these reasons, there are certain things we can say with confidence about indricothere feeding dynamics. Because they were not ruminant artiodactyls, they had to be hindgut fermenters, like horses, other rhinos, and elephants, so they must have consumed and processed huge amounts of food in a day, just as elephants do now. Their feces would have been full of undigested plant material, just like those of a horse or an elephant. Like almost all large herbivores, they must have had a big part of their abdomen occupied by their large digestive tract, giving them a large bulging “gut” like that of an elephant. The fermentation in their gut, by the way, creates additional body heat, which exacerbates the problems they have of producing excess body heat to begin with.

Locomotion and Home Range: As an animal increases in body size, the stresses on their limb bones increases even more because of the power of three expansion of volume and the corresponding mass increase. Models of the dynamics of large dinosaurs show that they could not have run very fast, or their limbs would break. Modern elephants also cannot run very fast compared to true specialized runners like antelopes, horses or cheetahs. Their maximum speed in an all-out charge clocked at only 18 mph (29 kph), but their normal walking speed is about 6-12 mph (10-19 kph). Remember, they have an advantage in their speed because they have much longer limbs and strides than any other animal. Given that indricotheres were just slightly larger than modern elephants, we can predict that they too would have not been fast runners, but ambled along at a moderate pace like that of an elephant.

However, African elephants are capable to moving enormous distances (typically 20 miles or 32 km) in the course of a day, migrating from one food source to another. To support their food needs of about 300 pounds (140 kg) of food they consume in the 16 hours of each day they eat, elephants need huge home ranges of 300-600 square miles (750-1500 square km). Consequently, huge home ranges and long migrations would be expected of indricotheres as well, especially if they lived in a harsh desert scrub setting with scarce food sources that were easily wiped out. A similar model has been proposed for the large sauropod dinosaurs, which lived in a scrubby, semi-arid habitat in the Late Jurassic time (Morrison Formation), and probably roamed in small herds from one patch of trees to another.

Predators and Life Habits: Certain other ecological parameters are also dictated by the giant body sizes of elephants and indricotheres. Once they reach a large enough body size, healthy modern elephants have no natural predators—not even lions or tigers are foolish enough to tackle them. (This has all changed now with human poaching, which has nearly wiped out elephants in the wild). Only the babies and young calves are vulnerable to predators, and in elephant herds, there is a strong matriarchal hierarchy so that every calf is closely protected not only by its mother, but also by its sisters, grandmother, aunts, great-aunts, and other close female relatives. All non-human predation of elephants in the wild occurs when predators catch vulnerable calves.  Almost a quarter of the calves born to Asian elephants are lost to tigers before they reach their first birthday. If indricotheres maintained small herds in the elephant mode, such freedom from predation except for the young would also be true. However, in the Bugti beds there are gigantic crocodiles (Crocodylus bugtiensis) that are 10-11 m (33-36 feet) long! These would have been large enough to attack almost any indricothere that might be at the edge of the river to drink. Indeed, many of the specimens from the Bugti beds have crocodile tooth marks on them.

There is also a well-known relationship between the gestation period, size of the litter, and body size. Elephants have the longest gestation period of any land creature (22-24 months, or about two years). The females do not reach sexual maturity until they are ten years old, and may produce a single calf every three to four years, the slowest reproductive rate of any mammal. Such could be expected of indricotheres as well, since growing to such large body sizes, and having such large calves, is very similar to the constraints on elephant reproduction. Like elephants, indricotheres would be expected to grow quickly at first, then grow relatively slowly once they reached maturity.

There is also a strong relationship between body size, metabolic rate, and blood pressure. An elephant has a relatively slow metabolic rate. Its heart beats only 30 times per minute, while humans have a pulse of 60 beats per minute, and hamsters have a pulse rate of over 450 beats per minute! The indricothere heart would have had a pulse rate close to that of an elephant, but probably a bit higher. This is because it must have also been able to exert a blood pressure close to the 300 mm Hg that giraffes produce (humans typically have a blood pressure of 120) to be able to lift its head so far above the ground without fainting.

Finally, there is also a well-known scaling of longevity with body size, with larger animals (and their slower heart rates) living longer. A rodent typically lives no longer than 3-5 years, a cat or a goat about 15 years, pig or monkey about 20-25 years, and a cow or giraffe about 25-30 years. Elephants typically live 35-50 years in the wild (at least they did until recent years, when poaching has nearly wiped them out), and the record is 71 years. Similar lifespans could be expected of indricotheres as well.

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5 Responses to “rhinoceros giants”

  1. Trimegistus says:

    All this from bones. Cuvier would have approved of this article.

  2. Nick says:

    This is exactly the kind of article I come here to read. You explain what most people would consider a dry, technical subject wonderfully. Thanks for all you do for science and rationality.

  3. Alektorophile says:

    Great article. Indricotheres and fellow prehistoric mammals are a wonderfully interesting subject, one I incidentally first learned to really appreciate watching the BBC’s “Walking with…” programs with my nephews some years ago, largely imaginary animations notwithstanding.

    What I also quite like about your article and book, is the very interesting background information you give on research and collections at the AMNH, truly one of the great museums of the world. Thank you.

  4. Mike G says:

    I don’t know if I would have expected an article like this on a skeptic blog website, but I sure did enjoy it!. Very interesting. Thanks.

  5. Gavin P says:

    This article gives a fascinating insight into a group of (extinct) animals that I had never even heard of before. It is beginning to dawn on me that even the knowledge of an “intelligent layman” is an infinitesimal speck in our boundless ocean of ignorance.