Dinosaurs not effective brooders?

In 1944, G.G. Simpson published his important (but often overlooked) book Tempo and Mode in Evolution, one of the key works of the Modern Synthesis that attempted to explain the relevance of paleontology to evolutionary studies. In the Introduction, Simpson includes this passage about the lack of communication between geneticists and paleontologists;

The attempted synthesis of paleontology and genetics, an essential part of the present study, may be particularly surprising and possibly hazardous. Not long ago paleontologists felt that a geneticist was a person who shut himself in a room, pulled down the shades, watched small flies disporting themselves in milk bottles, and thought that he was studying nature. A pursuit so removed from the realities of life, they said, had no significance for the true biologist. On the other hand, the geneticists said that paleontology had no further contributions to make to biology, that its only point had been the completed demonstration of the truth of evolution, and that it was a subject too purely descriptive to merit the name "science." The paleontologist, they believed, is like a man who undertakes to study the principles of the internal combustion engine by standing on a street corner and watching the motor cars whiz by.

Now paleontologists and geneticists are learning tolerance for each other, if not understanding. As a paleontologist, I confess to inadequate knowledge of genetics, and I have not met a geneticist who has demonstrated much grasp of my subject; but at least we have come to realize that we do have problems in common and to hope that difficulties encountered in each separate type of research may be resolved or alleviated by the discoveries of the other.

More than 60 years after this passage appeared the lack of understanding between the two scientific disciplines remains. I admit that when it comes to genetics I know very little, and readily identify with Simpson's confession of "inadequate knowledge." Likewise, those in the field of genetics often know little about paleontology, and even though studies of development are starting to bridge the gap (i.e. see Your Inner Fish or the technical volume Major Transitions in Vertebrate Evolution), there's still something of a communication breakdown.

Such is the case with one particular statement made in a recent paper published in BMC Biology about the lack of "brown fat" in living birds and reptiles. Brown fat is one of two types of adipose tissue (the other being "white fat") found in the body, being most abundant in hibernating animals and infant mammals. Given that brown fat is primarily an adaptation to generate body heat when things get chilly it prevalence in animals that need to maintain high body temperatures is not surprising. Birds, on the other hand, seem to have the ability to create brown fat but lack a gene called UCP1 that controls the heat-generating function of brown fat. UCP1 is also absent from lizards, which led the authors of the paper to suggest that the gene may have been present in the last common ancestor of living birds and mammals (which would have been so old that it would not have been recognizable as belonging to either group). This would mean that UCP1 was absent in the last common ancestor of living lizards and birds (which, again, would have resembled neither). Add to this the fact that UCP1 is present in some fish and amphibians and it appears that UCP1 was used for something else originally but was ultimately lost among the amniotes that would give rise to lizards and archosaurs.

Now keep in mind that birds evolved from maniraptoran theropods; the results suggest that such dinosaurs would have lacked UCP1 as well. If true, this suggests that dinosaurs did not have brown fat or the ability to use it to generate heat, although the authors of the paper note that birds and turtles have brown-fat-like tissues that seem to be sources of energy that can be quickly mobilized for muscle use. Still, while I can't think of any way in which fossil evidence could be mustered to support or refute the hypothesis that dinosaurs lacked mammal-like brown fat, the implications of the proposed absence has led to something of a mess in the paper and in media reports.

ScienceDaily, which is little more than a press-release aggregator, says "A new paper contains the surprising implication that the same lack of heat-generating tissue may have contributed to the extinction of dinosaurs." The paper makes no such claim, and the extinction of dinosaurs is not covered at all by the paper nor the popular article. Where this claim came from, I have no idea.

More troublesome, however, is a statement made in the conclusion of the paper about the brooding habits of dinosaurs. After noting that dinosaurs would have required feathers, "non-shivering thermogenesis mediated by expression of avUCP in muscles," or some other way to generate heat to keep eggs warm without shivering to do so, the authors write;

No information is available concerning avUCP in dinosaurs, but it is not likely that these large featherless animals were effective brooders. The lack of thermogenic BAT may
thus have been more of a liability to the developing embryos of oviparous dinosaurs than
to those of birds.

This statement contains several errors. First, not all dinosaurs were large and featherless. Such may be the classic image, but "large and featherless" is not an inherent property of what makes a dinosaur. If we're talking about ornithischians like Maiasaura or large saurischians like Apatosaurus, I could see their point, but it is a big mistake to consider dinosaurs to be gargantuan, elephant-skinned behemoths from a Charles R. Knight painting. How did these creatures care for their eggs? In the case of the hadrosaur Maiasaura, eggs were laid in a somewhat circular pattern inside a nest (the nest itself in a larger nesting colony), and vegetation was placed amongst the eggs. As the vegetation rotted it would have produced heat which would have helped keep the eggs warm. This is very similar to what living crocodylians like the American alligator (Alligator mississippiensis) do today, although other large dinosaurs may have used different strategies to regulate the temperature of their nests.

What is most troubling, however, is that the spate of recent discoveries of small, feathered dinosaurs goes practically unmentioned (it is only the birds that are attributed feathers!), as well as the discovery of theropods in brooding positions over nests. As far as I am aware there are at least four known specimens of the oviraptorid Citipati in brooding positions, one of the most famous being IGM 100/979 from the Ukhaa Tolgod locality in Mongolia. Described in 1999 (Clark et al. 1999), the fossil preserves Citipati in brooding position over a large nest of eggs. (I have always found it interesting, though, that the head, neck, tail, and much of the dorsal part of the animal is missing, looking as if it someone took a chainsaw at hip height and cut through the entire skeleton from front to back. This is probably due to post-preservation effects like erosion as the remainder of the skeleton is articulated and intact.)

When looking at the remains of this particular oviraptorid, it is important to remember that it probably was covered in feathers. If it did indeed have the long secondary feathers on its arms, the position the skeleton is in makes sense as the dinosaur would have been covering the nest with its feathers (which, of course, would help regulate heat). Unfortunately fossils of Citipati bearing feathers have not yet been discovered and it is not known whether they had long feathers on their arms, but for now the evolutionary association of Citipati with feathered maniraptors like Caudipteryx give us no reason to think that it would have been without at least some feathery covering. (The announcement of an osteological character in Velociraptor that denotes the presence of secondary feathers may allow paleontologists to determine whether theropods had secondary feathers even if the feathers themselves are not preserved.)

I do not wish to slam the authors of a paper that presents an important hypothesis about the loss of a tissue important to thermoregulation, but it is difficult for me to understand how dinosaurs could be characterized as large, featherless animals that probably did not take up brooding positions when there is concrete fossil evidence to the contrary. The section I've criticized makes up a small part of the paper, yes, but it points to the continuing disconnect between geneticists and paleontologists. If the authors of the paper had brought in a paleontologist familiar with feathers & brooding behavior the mistakes could have been avoided and the paper made stronger as a result. The nesting habits of many dinosaurs is still unknown and how they regulated the heat of their broods (if they put in any such effort at all) is even more difficult to get out, but the fossil evidence that can provide us with some answers (or at least allow us to formulate better questions) should not be ignored.

Clark, J.W.; Norell, M.; Chiappe, L.M. (1999) "An oviraptorid skeleton from the late Cretaceous of Ukhaa Tolgod, Mongolia, preserved in an avianlike brooding position over an oviraptorid nest." American Museum Novitates, No. 3265, pp. 1-36

Mezentseva, N.V.; Kumaratilake, J.S.; Newman, S.A. (2008) "The brown adipocyte differentiation pathway in birds: an evolutionary road not taken." BMC Biology, Vol. 6 (17), pp. 1-37