Expectations of Body Size

Large animals are much more charismatic than small ones. It is no coincidence that the majority of the animals listed as endangered and in need of help by the World Wildlife Fund are large mammals. Lately it has become fashionable to sneer at their popularity. Yet it is unfair to single out this hard-working group for criticism, for the hard fact remains that the large mammals—the last of the megamammals— are indeed endangered. Assuming that this group will show some of the highest extinction rates of the modern fauna, we might expect that future evolution will produce many new species of large-bodied animals.

Can we expect any new large animals? Recent scientific studies of the size distributions of mammals and their history of evolution suggest that large species might not appear after all. Mammalogists have long noted an interesting aspect of mammalian size distributions. There are over 4,700 species of mammals on Earth today, and their range in body size is impressive: the smallest (e.g., the tiniest shrews, such as the genus Microsorex) have an adult weight of about 2.5 grams, whereas the largest (Balaenoptera musculus, the blue whale) weighs about 1.6 X 10' grams—a difference of twenty orders of magnitude. And there are all size (weight) classes in between. Yet if the size distribution of mammals for each major continent on Earth is graphed, it is immediately apparent that each distribution is skewed to the right of the graph: there is a greater number of large mammal species than would be expected if the sizes were randomly distributed. However, this trend does not occur on small continents such as Australia, on large islands such as Madagascar, or on smaller islands. On smaller landmasses it turns out that the distribution of mammalian sizes is relatively symmetrical. Moreover, the smaller the landmass, the smaller the size of its largest mammals, and the larger the size of its smallest mammals. On small land areas the two tails of the distribution curve disappear. Finally, when small landmasses are completely isolated from"other land areas, an extraordinary thing happens: large species evolve into dwarfs, and small species develop gigantism. But these are relative processes: an elephant evolving down to half size (the size of a horse) is still a large mammal (if a small elephant), whereas a mouse that doubles in size may be large for a mouse, but it is still a very small mammal. Can these observations be used to predict the future body sizes of newly evolving mammals (or any other type of animals, which presumably are affected in similar fashion)?

They can. Earlier we saw that global trade and travel are effectively recreating a supercontinent, bringing about the homogenization of the fauna that typified such large, single continents of the past. However, we have also seen that barbed wire, canals, roads, and freeways are subdividing the continents into smaller habitats. This trend is shaping the fauna according to the rules of island biogeography. Thus, we see the world being transformed, in an evolutionary sense, into an environment favoring low diversity, as well as the dwarfing of large species and the enlarging of small ones, with extinction occurring among the largest and smallest. The Age of Megamammals is well and truly over, with the last few wild mega-mammals now consigned to parks and zoos. As long as humanity survives at large population numbers, it will not return.

What might this new world look like? Let us invoke H. G. Wells's vehicle again for a fanciful, if dyspeptic, flight:

The Time Machine came to a stop. Ten million years had passed in the blink of an eye. The Time Voyager stepped from his machine and surveyed his surroundings. He was on the edge of a giant flat plain. Small fires dotted the broad expanse, sending thin blue columns of smoke into the cloudy and humid sky. The sun was setting, looking no different from the sun of his own time. Not for the first time, he wondered whether the machine had somehow malfunctioned.

Walking away from the Time Machine, he took better stock of his surroundings. He seemed to be in a gigantic garbage dump of some sort. Untold numbers of flies filled the air, their buzzing a constant hum of background Muzak. Roads crisscrossed the plain at regular intervals, but no vehicles could be seen. He looked more carefully at the refuse-strewn plain he was striding across, and was startled to see chaotic movement among the material swirling around him in the hot wind. At first he thought that thousands of huge insects were moving in the litter. But on closer inspection he discovered that while there were indeed numerous cockroach-sized insects afoot, many of the scurrying, tiny forms were mammals, a few as large as cats but most rat-, mouse-, or even shrew-sized.

He sat down on his haunches, motionless, and watched as more and more curious small mammals began to emerge. Clearly, many of the species he could now see were unfamiliar to him. Although their bodies looked like those of the rodents of his time, their heads were distinctly different. It was clear that many distinct species were present, some with long tapered heads, others with thin ribbonlike tongues, others with blunt heads and large knoblike teeth, still others with huge batlike eyes. Some had fur in a variety of camouflage patterns, while others were hairless. Some were heavily armored with armadillo-like scales. Some had front legs exquisitely adapted for digging; others had long needlelike claws extending from their toes. The small forms wormed among the garbage, some using their impossibly long tongues to probe into the piled refuse, while others broke open some of the many scattered bones to root out the marrow. While he watched, one of the small mammals was suddenly lifted into the air by a flicking rope of some sort, and then he saw the body being carried into a large, waiting mouth. A huge snake lay coiled not far away. Its tongue was like that of a frog, capable of flicking outward and grasping its prey. He saw another large snake moving on short legs like those of a centipede, and yet another moving its head in and out of the piled garbage, looking for small prey housed within.

Watching this menagerie of the small, he tried to compile a list of species new to him, losing count after tallying more than forty. It was not that everything was alien, for these creatures were surely descended from the mammals and reptiles of his own time, but they were just as clearly evolved, forming entirely new groups of species. And still more and more animals began to appear in the gathering twilight. The biggest animal that he saw was the size of a pig, and seemed indeed to be some sort of bizarre swine. But this "pig," if that is what it was, seemed well adapted for pushing through the piled garbage in search of food. It had a small trunk instead of a nose, which allowed it to root efficiently through the piles of rotting offal. Numerous small ratlike creatures hung from its sides, like remoras clinging to a shark. He thought at first that they might be babies, but they were clearly parasites of some sort, looking like hairy lampreys with greedy sucking mouths. Or perhaps vampires would be a better description. The rats, it seemed, had evolved. '

He shuddered in revulsion at this bestiary. All seemed exquisitely adapted to these piles of garbage; in fact, all seemed adapted exclusively for life in this setting. In the distance he saw a copse of trees, and decided to leave the gigantic dump for a more "natural" setting, not realizing how natural garbage dumps were in this world. He began striding through the garbage, heading for the distant patch of green. Suddenly, shadows below and a cacophonous, raucous cawing from above announced a flight of birds over his head. They were crows, but bigger than those from his own time, and with brilliant plumage. He ignored them, but was jolted from behind with a sharp, piercing pain. Swearing, he put his hand to the back of his head, and found it covered with blood. He looked up to see another of the large crows diving at his head. He ducked just in time, seeing a large, eagle-like beak and great talons with a long, knifelike barb extending from one of the large feet. He began running back toward the center of the garbage, seeking shelter of some sort, but the crows, more than a dozen strong, attacked viciously. They let him run in terror back toward the trees, and as he got closer, he saw why: more than a hundred sat perched in the first row of trees, watching as their compatriots herded this particularly stupid human toward the waiting, hungry flock. The lions of the world now had wings.

The most ubiquitous mammalian resource of the future: the human body


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