Winter World_ The Ingenuity of Animal Survival - Bernd Heinrich [46]
Golden-crowned kinglet, close to life-size.
At the time, my naked body weight was 155 pounds. Fully clothed (in a light synthetic L. L. Bean jacket and boots) I came to 166 pounds. Six of these pounds were attributable to the boots alone. The bird had added 7.4 percent to her body weight for insulation, which was about twice the percentage that I had added to insulate myself. However, more than half (55 percent) of the 11 pounds of insulation I wore were designated for my feet. The kinglet, by comparison, used none of its insulation for that purpose.
My little plucked kinglet reminded me of a scaled-down dinosaur. That’s not coincidental, as the affinities between some dinosaurs and birds have long been noted. What could not so easily be compared was body temperature, which is, unfortunately, a long-presumed defining difference between birds and reptiles. Since birds are increasingly considered evolutionary descendants of ancient reptiles, it has seemed logical, in evolutionary biology circles, to presume that their ability to regulate body temperature is also a recent, more highly evolved trait. This I dispute. Regulation of a high body temperature is no new thing for us presumably superior warm-blooded vertebrates. It is routine in all large flying insects, ancient animals whose line predates the dinosaurs. Recent discoveries in insects prove that regulation of a high body temperature with the aid of internally generated heat by shivering, which is maintained within the body with the aid of insulation, is not as newly evolved a capacity as was commonly thought. The most parsimonious conclusion, given their size and flight capacity, is that some dragonflies or dragonfly-like insects were endotherms—able to store heat—at least 300 million years ago. And at the present time many other insects of a great taxonomic diversity still regulate body temperature, using internally generated heat, insulation, counter-and alternate-current mechanisms of heat retention and loss, evaporative cooling, basking, and alternating of activity patterns. Others don’t regulate their body temperature at all. Whether or not any one does depends almost trivially on body size and lifestyle. What this suggests then is that thermoregulation can be altered through evolution to suit specific circumstances with little respect to ancient ancestors, as such. Insects also tell us that in small animals, endothermy (the process by which internally generated heat is stored to maintain body temperature) is associated with insulation. In many bees and some other insects, the insulation is created from hairlike projections. In moths it is a thick pile created from modified scales instead, and in dragonflies it is a layer of air sacs surrounding the body. In mammals it is fur, and in birds, of course, it is what we call feathers, structures that hold the key to bird body temperature regulation and associated lifestyle. But of course feathers are of special importance, because some feathers also serve another, totally different function—they are the birds’ passport to flight.
THERE ARE LONG DEBATES in the scientific literature about endothermy in dinosaurs and about the evolution of bird flight. The original rationales were constrained by a multiplicity of simplistic presumptions: Dinosaurs are “reptiles” and present-day reptiles are not insulated and therefore dinosaurs were presumed to be cold-blooded. It was a shock, when in 1861, just two years after Charles Darwin published On the Origin of Species by Natural Selection, workmen in a limestone quarry in southern Bavaria discovered a fossil that was clearly a little dinosaur complete with teeth and a long tail, and yet was also a bird, because it showed imprints of feathers exquisitely preserved in the 150-million-year-old Cretaceous limestone. Archaeopteryx, or ancient bird, as it was dubbed, was and still is the oldest bird fossil ever found.