Thursday, 7 August 2014

Eating for Your Size, or the Secret of the Dinosaurs

     Shrews and sauropods will both be germane to this discussion, but I shall frame it by asking: how did the dinosaurs get so big, and why has nothing else ever grown so huge?
     Well, first of all, is the second half of that question correct? Yes and no. The sauropods were certainly the most gigantic animals ever to walk the earth. These were the familiar four-poster, hump-backed dinosaurs with ridiculously small heads and incredibly long necks and tails. And they were BIG - an order of magnitude bigger than any other dinosaur. Even the smallest topped the ton, and the largest, Argentinosaurus may have reached 90 tonnes. Few of the other dinosaurs, including those two-legged terrors we've all come to love, would have gone beyond 8½ tonnes, the weight of a very heavy elephant, and some were as small as chickens.
     To put this in perspective, a big giraffe will weigh about a tonne and stand 5.5 metres [18 feet] high. Now imagine a hornless rhinoceros standing that high at the shoulder, with a neck stretching out to allow it to browse 8 metres [26 feet] above the ground. That was Paraceratherium, which shook the ground of Asia until 23 million years ago. The very biggest specimens may have reached 12 metres [39 feet] in length and weighed in at up to 20 tonnes: far bigger than any non-sauropod dinosaur. It was the largest land mammal which has ever existed, probably the largest which could ever exist.
     Nevertheless, one has to admit that, when considering the total mass of flesh on the ground at any one time, the age of dinosaurs comes up trumps. Why?
     Well, for a start, there were marginally fewer changes in climate over that period than during the age of mammals, so giants could continue to grow before they were cut down by changed circumstances. Also, the Mesozoic was generally hotter than it is now. However, the major reason can be expressed in four points.
  • Every animal needs to eat. Some of the food goes to energy, some to maintenance, some to growth (when young), and some to wastage, because it cannot be digested.
  • Big animals require more food in absolute terms.
  • Small animals require more food in relative terms.
  • Warm-blooded animals require more food than cold-blooded ones.
     The first point should be so blatantly obvious it need not be mentioned, but it is amazing how often it is overlooked. In the first Alien movie we saw the little monster burst out of its victim's chest and go scurrying into the bowels of the spaceship. When we next saw it, an undisclosed time later, it was big enough to tear a human being limb from limb. What, for heaven's sake, had it been eating all that time? Was it living on stale air?
     The second point is also pretty obvious. To grow big requires time, and giant animals always have long gestation periods, and take many years to reach maturity. Many of the smaller rodents, on the other hand, can reproduce when only a few months or a few weeks old, meaning that in good years they can spread out across the land until their populations finally collapse due to starvation: the famous plaques of lemmings, voles, or mice. One never hears of plagues of elephants or rhinoceroses.
     However, larger animals do have one advantage: their bodies are more compact. Due to the square cube rule, an ounce of man requires more skin to wrap around it then an ounce of elephant, and an ounce of mouse even more. Of course, it also means a large animals's lungs and bowels must be more convoluted in order to provide more surface for the absorption of air and food respectively.
     Nevertheless, it is through the skin that heat and water are lost and gained, putting the small animals at a sizable disadvantage. Even in our own species, a child is more likely to suffer from heat stress, dehydration or hypothermia than its parent. Furthermore, an Eskimo, who needs to conserve heat, is short and squat, with only two thirds of the skin surface of the tally, lanky, but equally heavy natives of northern Australia and the headquarters of the Nile. All other things being equal, species and races are bigger and rounder in cold climates, while in the tropics legs, tails, and ears grow long and spindly.
     At the bottom of the size scale the issues are brutal: no matter how well insulated you are, you are going to lose heat and water. The only solution is to replace it - and that means eating and drinking on a magnificent scale. Popular science articles will tell you that if a human being ate as much as a spider he would consume a sheep for breakfast and an ox for both lunch and dinner. I can always remember my lecturer telling us about a small shrew which had to eat a meal every fifteen minutes. "How does it manage to sleep?" I asked. "Very fitfully," he replied. (In practice, it would go into torpor ie drop its temperature to almost the same level as the environment. At night it would close up shop, so to speak. Every sleep would become a mini hibernation.)
     Shrews and spiders eat meat, a high quality diet. Vegetation has less nutritional value, so a really tiny herbivore is in a bind. Anyone who has watched a caterpillar on a leaf will know that it is simply an elongated eating machine. A herbivore as small as a dik-dik - a miniature antelope weighing in at 3 - 6 kg [7-16 lb] - tends to seek high quality forage.
    An elephant may weigh 1,000 times as much as a dik-dik, but its food requirements are only 180 times as great. Nevertheless, a factor of 180 is not to be sneezed at. So most of the megaherbivores - those weighing more than a tonne - feed on enormous quantities of poor quality, woody material whose digestibility is very low but whose availability is excellent. Thus we have the irony that very large animals spend almost the whole day feeding - just like the very small. They also live a much more sedate lifestyle, not only because they have few enemies, but because they must conserve energy. A giant can put on short bursts of energy - witness a charging elephant - but by and large they move quietly and unhurriedly. It is medium sized animals, such as ourselves, which have time for leisure.
     Another strategy of giants in to feed lower down the food chain. The really huge animals are herbivores rather than carnivores. In the sea, where the food chain is much longer, the largest whales and the largest sharks feed on plankton rather than fish.
     No contrast could be greater than that between the dignified plodding of an elephant, the friskiness of a gazelle, and the nervous scampering of a mouse. The even tinier shrew is an even greater bundle of frenzied energy, forever hurrying down its well worn pathways in the grass, constantly squeaking its ultrasonic contact calls, forever on the alert for prey or predators, as if its life depended on being in perpetual motion - as indeed it does. Small animals exist on a higher, faster plane than the rest of us, more active, more ravenous, and more dangerous. Their fires burn more fiercely and burn out more quickly. A year or two is all they can expect to last.
     Now we come to the last point. Warm- and cold-bloodedness are really misleading terms, because both groups function at similar temperatures. The difference is that one group has to bask in the sun to reach the appropriate temperature, while the other relies on a complex system of circulation, digestion, and locomotion in order to raise their internal temperature, plus insulation in the form of fur or feathers to keep it in. The official terms are "ectothermy" (external heat) and "endothermy" (internal heat). Because we ourselves are endotherms, we assume that endotherms are superior, and in general terms, they are. However, they suffer one major drawback: they requires much more food in order to stoke their internal fires. An endotherm needs five to ten times as much food as an exotherm of the same weight.
     You can see now why a tiny shrew needs to have a meal every fifteen minutes, and why it has to shut down its fires while asleep. Very big mammals face a quite different problem, especially in the tropics: how to lose heat. That is why elephants are hairless, and have big ears. Combined with the need to be constantly eating, that puts a maximum limit to the size of any mammal.
     Just the same, from the point of view of an exotherm, massive bulk is a form of insulation. It takes a long time for heat to leak out from the body core, but by the same token, a long time is required to heat up. A crocodile has to sunbathe for several hours in order to reach a workable temperature, and it has been calculated that there are not enough hours in the day for a dinosaur to be able to do it. So how did they manage?
     For the past forty years it has been evident that dinosaurs were "warm-blooded", at least to some extent. No doubt it varied with the species and the period. But the most likely scenario is that they had an intermediate metabolism ie they were "luke-warm-blooded". Essentially, they were probably active at a slightly lower body temperature than ours, and achieved it by means of a combination of physiology, and the the insulating effects of size. The fact that their era was normally hotter than ours no doubt helped, although there were dinosaurs thriving during the long nights of the Arctic and Antarctic.
     An intermediate metabolism would require a lot less food than a mammal of comparable size, although more than an ordinary reptile. The great sauropods would have munched their way through the Mesozoic like herds of elephants. Dinosaurs the size of elephants, such as Triceratops, would have eaten like a bison. A big carnivore, such as Tyrannosaurus, would have needed as much meat as a lion. A dead Triceratops  would have lasted one for months, or until it went putrid. It is more likely they hunted in packs and shared it.
    Of course, once the asteroid struck, they all became extinct. The world was inherited by the mammals, which cannot grow any larger than Paraceratherium, and even it was an outlier. The really gigantic monsters will never be seen again.

The next chapter is Geological and Historical Time.
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