Mark Twain reputedly said it wasn’t what he didn’t know that bothered him, but what he knew that wasn’t true. A recent contribution on dinosaur physiology in Nature, by palaeontologist Meike Köhler and her colleagues in Spain and Norway, shows something we “knew” that wasn’t true.
The study challenges the hypothesis that dinosaurs could have been cold-blooded reptiles.
Mammals, being “warm-blooded”, generally grow at a constant rate. This growth is recorded in their bones in a backhanded way, as an absence of LAGs (Lines of Arrested Growth).
LAGs are discrete surfaces, seen microscopically as lines in cut surfaces, within bone where the tissue has periodically stopped growing, usually for a season – winter or dry season.
LAGs are found, for example, in bones of modern lizards. They were taken to indicate a low and variable body temperature (being “cold-blooded”) and an associated low, variable metabolic rate. This, in the “off season” when resources were scarce, led to a temporary cessation of bone growth.
But Köhler and her colleagues found that a suite of 41 large, placental mammals – antelope, goats, and deer – from tropical African to northern Europe, produce LAGs.
Since the early 20th century, there have been claims that dinosaurs, like modern mammals, were warm-blooded. Not until the late 1960s were these claims taken seriously with the realisation that birds were living dinosaurs – and birds are “warm-blooded”.
With dinosaurs being extinct – except for birds, which obviously are not typical of the extinct dinosaurs – the problem arose of how to improve on just speculating about their physiology.
It’s reasoned that a constant warm body temperature was linked to an elevated metabolic rate, that in turn permitted rapid continuous growth in the young.
This growth could be seen in bone microstructure – bone without LAGs.
LAGs are formed during periods of low metabolism, taken to correspond to periods of low environmental temperature or other seasonal stress. Bones from lab reptiles, where the temperatures through their lives were known, substantiated this notion.
Bones of large placental mammals were thought to be free of LAGs, because of the higher metabolic rates and constant body temperatures in (most) mammals.
Supporters of the warm-blooded dinosaur notion cited certain evidence in support of their view, such as the well-defined joints of limb bones of theropods such as Deinonychus, that implied an active life. They also cited just about everything else they could think of – which was not always convincingly relevant.
Critics of this view pointed to dinosaurian features that were like those found in modern lizards and absent in modern birds and mammals – features they took to imply a less metabolically-expensive life. One of those features was LAGs, thought to be usually absent in mammals.
But Köhler and colleagues’ work shows LAGs are present, and in mammals from the tropics to the Arctic. So their occurrence in dinosaurs doesn’t preclude endothermy, and one line of evidence against the warm-blooded dinosaur notion has been removed.
But this doesn’t mean we can assume that all, or any, dinosaurs were warm-blooded. After all, it isn’t even clear that all mammals are warm-blooded in the same sense. Some, such as echnidas, seem to have variable body temperatures, but others, humans for example, usually don’t.
The problem, obviously, is that dinosaurs are extinct. If they weren’t, we could measure their temperatures and metabolic rates.
Metabolic rate, what is really of interest here because it enables an active lifestyle, generates the “warmth” of warm-blooded animals. For extinct creatures metabolic rate has to be inferred from other indicators.
But being warm-blooded is not a simple, single property, but a suite of properties. Just because these properties all occur together in living animals, doesn’t mean that we can assume this association is necessary. Maybe some of these features once conferred benefits unrelated to “warm-bloodedness”.
Feathers in birds are now linked with warm-bloodedness, but may have arisen for their value in showy displays, and not for insulation. Albert Einstein reputedly said that science should be made as simple as possible, but no simpler.
Too often both proponents and opponents of warm-blooded dinosaurs have tried to make this notion simpler than possible, as Köhler and her colleagues have shown. And we can expect more such demonstrations to come.