Imagine a rhinoceros. For the sake of argument, let's say it's a white rhinoceros. Don't worry if you can't envision every little anatomical flourish in your mind. We're going to modify this beast a bit.

First thing's first - lose the horn. We have no use for it. Next, lengthen the neck a bit. Not too much - we're not turning this rhino into a giraffe - but enough so that the neck is slightly more than half the length of its back. Now for the legs. Stretch them out so that the rhino's belly is a little higher off the ground. Scale the thing up until it stands about 18 feet at the shoulder and weighs about 17 tons, and you're done.

What you have just made is a dead ringer for Paraceratherium, the largest land mammal of all time. This immense rhino - part of an entirely-extinct group of hornless rhinos called hyracodonts - lived in Asia between 37 and 23 million years ago, but even this giant would have been dwarfed by the dragons of earlier epochs. Futalognkosaurus, a sauropod dinosaur that was apparently elsewhere when the good names were being given out, stretched over 100 feet long from its tiny head to the tip of its tail, and it probably weighed in excess of 75 tons. The largest dinosaurs may have been even bigger still. Estimates based on the long-lost bones of the sauropod Amphicoelias reconstruct this dinosaur as being over 131 feet long and weighing in at over 122 tons. Compared to these giants, Paraceratherium seems rather puny.

Thankfully for the pride of Paraceratherium, a wide gulf in time separated it from the Mesozoic titans. The giant rhino arrived on the scene about 28 million years after the last of the non-avian dinosaurs became extinct, but its ancestors and evolutionary cousins lived alongside dinosaurs for 135 million years. During the long tenure of the dinosaurs, mammals did not get very large at all. The three-foot long Repenomamus - a 130 million year old mammal that ate baby dinosaurs - was as big as they came in those days. The ecological dominance of the dinosaurs kept mammals small, but, once the dinosaurs disappeared, what prevented them from attaining enormous size? Why have there never been elephants as big as Apatosaurus or rhinos equal in stature to Brachiosaurus?

Giants are difficult to make. Remember all those classic b-movies featuring giant insects, like THEM!, The Deadly Mantis, and Earth vs. the Spider? Mercifully for us, arthropods of city-destroying size could never exist. They would not be able to obtain enough oxygen to survive, and, without major modifications to their bodies, would collapse under their own weight. (There were giant arthropods over 300 million years ago, but even they were not of horror-movie size. Unless you're deathly afraid of huge millipedes, anyway.) The same goes for vertebrates. The elephant-sized, bloodthirsty bunnies from Night of the Lepus would require changes in proportion to cope with the different stresses and strains that come with increased body size. There are constraints that limit how big organisms can get and what shape they can take as they get bigger.

The operation of these unwritten rules of anatomical architecture can be seen in the evolution of the largest land mammals. In a paper published last year in Science, researchers Jessica Theodor, Mark Uhen, and collaborators tracked changes in mammal body size after the end-Cretaceous mass extinction. In a world denuded of dinosaurs, mammals underwent a major evolutionary radiation, including increases in size among multiple lineages. In fact, every single group of mammals appears to have reached their respective maximum body sizes by about 40 million years ago. There have been no major increases in size among land-dwelling mammals since that time.

The Science study focused on how these mammals - such as Paraceratherium and the extinct, anchor-tusked elephant Deinotherium - got so big in the first place. The title of World's Largest Mammal shifted between different lineages present on different continents, and the ecological role of "giant herbivore" was filled by various species over time. Why mammals didn't get any bigger is a trickier question to answer. The amount of living space available to large mammals might provide a partial solution. Populations of large herbivorous animals require a massive amount of forage distributed over a large area. The larger the available space for populations of a species to range over, the larger they can become, and limited land area caps potential body size.

If this is correct, however, dinosaurs would have been limited in the same ways, and they still got bigger. Something else must have been at work. In interviews about the new research, some of the scientists involved proposed that a difference in metabolism might have accounted for the disparity. "Mammals have higher metabolic rates [than dinosaurs]," Theodor told the New York Times, "and we think they are kind of capped at a lower size." Similar statements appeared in the Salt Lake Tribune and a CBC report. The implication is that if dinosaurs had slow metabolisms and required less energy to survive, then they would have been able to grow to gigantic sizes impossible for mammals that burned their ingested plant fuel at a quicker rate.

But pinning metabolism as the culprit that kept mammals down would be premature. This is because we still don't know what sort of metabolism sauropod dinosaurs had. By dint of being reptiles, sauropods have traditionally been thought of as occupying a sweet spot between having a low metabolic rate and body temperatures influenced by that of the surrounding environment. Large sauropods were so huge that they would have easily retained heat, the argument goes, allowing them to maintain high body temperatures without actually having a mammal-like metabolism. (The greater the internal volume of a dinosaur, the longer it would take for them to warm up or cool down.) This would have allowed sauropods to be active without having to eat as much for their body size, and they would be in less danger of overheating. Nice and neat, but multiple lines of evidence - particularly signs of rapid growth in the bone microstructure of these dinosaurs - have led paleontologists to reassess this view. The sauropods may have had mammal-like metabolisms after all, and the secret to this arrangement may have been a trait they shared in common with their distant, living cousins.

The front 2/3 of the sauropod skeleton - from the neck through the abdomen - contains numerous pockets and indentations. This is absolutely stunning natural architecture, and it also indicates the presence of air sacs like those seen in modern birds. Modified from simpler versions present in the last common ancestor of the sauropod and theropod dinosaur lineages, way back down the evolutionary tree, these air sacs would have been beneficial to the giant dinosaurs in several ways. In addition to allowing the dinosaurs to breathe much more efficiently than a mammal of comparable size, they may have also acted as an internal cooling system that would prevent the large sauropods from overheating. These physiological boons may have allowed the sauropods to have more active metabolisms than previously thought.

From a structural point of view, though, the air sacs significantly lightened the skeletons of these dinosaurs without sacrificing strength. The intrusions of the air sacs reduced the density of sauropod skeletons and gave them enough of an engineering edge to push the limits of body size. Mammals lacked this advantage, as well as the physiological benefits the air sacs brought with them, and this may explain why the biggest mammals of all time were still small compared to the largest dinosaurs.

Air sacs are not the entire picture, though. As argued in a recent, comprehensive review by paleontologist Martin Sander and colleagues, one of the key, early features of the sauropod dinosaurs was a long neck. Rather than extensively chewing food as mammals do - something that requires a big, heavy head and therefore limits possible neck length - sauropods just horked down plant food and processed it inside their digestive systems. By vacuuming up plant food, the heads of sauropods remained small and their necks were able to become longer and longer to reach plants that could not be plucked by other herbivores. In turn, this affected the evolution of air sacs within the body and opened up evolutionary options not available to large mammals. The basic construction of dinosaurs made all the difference.

Will there ever be mammals that rival Diplodocus or even Argentinosaurus in size? It would be foolish for me to give a definite "Yes" or "No" answer, but it doesn't seem likely. The immense size of the biggest sauropods was made possible by a suite of evolutionary quirks not seen among mammals. If air sacs are the key to huge size, then it is little wonder that mammals never grew as large. But who knows what might happen as evolutionary pressures continue to change life on this planet? Perhaps, during some distant time, truly gigantic mammals will shake the earth, but all we can do is imagine what they might be like.

Top Image: White rhinos (Ceratotherium simum). From Flickr user Turkinator.

References:

R. McNeill Alexander (1998). All-time giants: the largest animals and their problems Palaeontology, 41 (6), 1231-1245

Osborn, H.F. (1923) "The Extinct Giant Rhinoceros Baluchitherium." Natural History.

Sander, P., Christian, A., Clauss, M., Fechner, R., Gee, C., Griebeler, E., Gunga, H., Hummel, J., Mallison, H., Perry, S., Preuschoft, H., Rauhut, O., Remes, K., Tütken, T., Wings, O., & Witzel, U. (2011). Biology of the sauropod dinosaurs: the evolution of gigantism Biological Reviews, 86 (1), 117-155 DOI: 10.1111/j.1469-185X.2010.00137.x

Smith, F., Boyer, A., Brown, J., Costa, D., Dayan, T., Ernest, S., Evans, A., Fortelius, M., Gittleman, J., Hamilton, M., Harding, L., Lintulaakso, K., Lyons, S., McCain, C., Okie, J., Saarinen, J., Sibly, R., Stephens, P., Theodor, J., & Uhen, M. (2010). The Evolution of Maximum Body Size of Terrestrial Mammals Science, 330 (6008), 1216-1219 DOI: 10.1126/science.1194830

Wedel, M. (2009). Evidence for bird-like air sacs in saurischian dinosaurs Journal of Experimental Zoology Part A: Ecological Genetics and Physiology, 311A (8), 611-628 DOI: 10.1002/jez.513