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.


The skeleton of Citipati, preserved on top of a nest (Specimen IGM 100/979). From Clark et al. 1999.

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.)


Reconstruction of a brooding Citipati (Specimen IGM 100/979), with the missing skeletal elements restored in white. From Clark et al. 1999.

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

More like this

For a long time feathered dinosaurs just looked weird to me. Seeing fuzzy Deinonychus or some other dromeosaur with a splash of plumage never looked quite right and I didn't understand why in the course of a few years predatory dinosaurs went from being scaly to being covered in down. Most of the…
tags: evolution, behavioral ecology, parental care, egg incubation, dinosaurs, birds The Oviraptorid dinosaur, Citipati osmolskae, on a nest of eggs that was unearthed in the Gobi desert of Mongolia by the American Museum of Natural History. Image: Mick Ellison, American Museum of Natural…
Dr. Jack Horner is one of the most recognized paleontologists working in the field today, and is presently the Ameya Preserve Curator of Paleontology and Montana State University Regents' Professor of Paleontology. He has authored numerous books, papers, and popular articles, and during his career…
One of the few things that everybody knows about dromaeosaurs - the sickle-clawed maniraptoran theropods best represented by Velociraptor from Mongolia and Deinonychus from Montana - is that they possessed a peculiar tail. Super-long zygapophyses and chevrons formed a bizarre, inter-twined array…

I heard about this paper a few days ago and balked. But then the media got ahold of it and it got worse. You cover all my points of contention with it (well done!), so I have nothing more to add.

I'm curious as to the number of dinosaurs that sported feathers or protofeathers. Than maniraptoran theropods had feathers is widely accepted but I still see tyrannosaurs and other large theropods depicted as scaly bipedal lizards. Wouldn't they also have had some sort of covering? How far back in the line did feathers evolve?

My guess is that protofeathers are basal for coelurosaurs. Feathers are not known for any theropods more primitive than Sinosauropteryx, and in fact one non-maniraptoran tetanuran (Juravenator) does NOT have feathers. But you're right, jck: tyrannosaurs were coelurosaurs, but I imagine that they shed their feathers once they reached a certain size. Chicks and juveniles were probably covered in downy stuff, though.

The "modern synthetic theory of evolution" is credited to the paleontologist George G. Simpson, the geneticist Theodosius Dobzansky, and the systematist Ernst Mayr. I had the honor of meeting Mayr and Dobzansky, and heard Simpson speak on mammal phylogeny, but did not meet him. Dobzansky was, by the by, a practicing Christian.

By Jim Thomerson (not verified) on 02 May 2008 #permalink

Zach is correct: at present, protofeathers seem to have appeared by the common ancestor of tyrannosauroids, compsognathids, and maniraptorans.

Pinnate feathers (feathers with a shaft and branches from that shaft) are known for oviraptorosaurs, deinonychosaurs, and avialians (birds, broadly defined). These three clades are also the only ones for which we have fossil evidence of brooding, and (possibly not coincidentally) ones where the glenoid (shoulder joint) is oriented such that the arms could be extended laterally (see the picture of the Citipati above.)

At present we do not have identified basal coelurosaur nests, but maniraptoran nests typically have eggs arranged with a space in the middle where the parent sat. Should we find nests for compsognathids, ornithomimosaurs, or small tyrannosauroids and the eggs all fill in the space of the middle of the nest, my speculation would be that they were not brooders; if there were that space for mom (& dad?) in the middle, then the case is stronger for brooding in protofeathered theropods.

Zach & Thomas- thanks for the info. One more question, Zach. Why would tyrannosaurs have shed their feathers once they got bigger?

Well, consider two things:

1) Feathers insulate. Once an animal gets to the size of a full grown T.rex (40-odd feet), the bugger might have a tough time cooling off, what with its body covered in a layer of insulating down. Of course, I'm not real sure that anybody has actually tested this assumption, but big mammals lose their hair for a reason.

2) Birds (and probably non-avian feathered theropods) preen their feathers regularly to keep dirt out and to coat the feathers with oil. Oil helps the feathers adhere to each-other and form a continuous surface, which helps to keep the feathers undamaged and clean. Waterbirds preen, but they also ruffle their feathers and splash themselves with water to much the same effect. Feathers themselves are waterproof--in part because of their structure, but also thanks to all that oil.

At any rate, I can't see an animal as huge as a T.rex preening itself. Its feathers, without preening, would degrade to the point where it would be better off without feathers. T.rex had neither the neck length or flexibility to preen itself.

Science Daily also goofed up on the evolutionary relationships of birds and dinosaurs. this is what they concluded in their press release:

"The ability to produce brown fat evolved in a common ancestor of birds and mammals, but the ability to generate heat was lost in the group that gave rise to birds and lizards after it separated from the mammalian lineage (the researchers found the lizard genome similarly lacks a UCP1 gene). This strongly implies that dinosaurs, which diverged from birds even later than lizards, also lacked brown fat."

implying birds are ancestral to dinosaurs.

Still, I always picture tyrannosaurids with sprays of brightly-colored feathers on the head. All those bumps & bosses must have had some function; why not supporting display feathers?

By Nathan Myers (not verified) on 02 May 2008 #permalink

Don't forget to allow for the sheer size of the largest tyrannosaurids - even if they had had a full covering of feathers, I don't think they would have had obvious plumes. Tyrannosaurus rex had a skull about a metre and a half long, so any decent plumes would have been about a metre. That's a very long and heavy feather.

If tyrannosaurs had protofeathers rather than pinnate feathers would they function more like fur? I can see where insulation would be a problem for a big animal in a hot climate but I wonder if a tyrannosaur might be more like a giraffe than an elephant with regard to mass vs. surface area. Also, some tyrannosaurids have been found pretty far north. Would that climate have been more temperate, at least in the winter?

I'm not sure what a "protoplume" would look like but maybe some of that covering would be ornamental and have been retained after the infant covering was lost. A Tyrannosaurus with boring gray skin would still be an impressive sight, but one with iridescent plumage would give Zach's dragons a run for the money!

So we can hypothesize about yet another function (relating to brooding) of a piece of the "mousetrap" (which Behe &al. tell us can only function as a whole, complete unit of never-changing function).

JCK and Christopher,

Don't forget: when tyrannosaurOIDs inherited their protofeathers, they were rather small (1-3 m long) coelurosaurs. Gigantic tyrannosauroids (Dryptosaurus, Appalachiosaurus, Tyrannosauridae) were a Late Cretaceous phenomena.

As to the condition (protofeathered? and if so, where?) in advanced tyrannosauroids: we have no real idea. At least some parts of the body had very small scales (smaller than those found in comparable-sized hadrosaurids and ceratopsids), but we have little idea of where exactly those scale patches go. Someday, perhaps, but not yet...

There is a good paper in Feathered Dragons by Thomas Hopp and Mark J. Orsen titled "Dinosaur Brooding Behavior and the Origin of Flight Feathers." It has both photos of modern birds and good reconstructions, and includes Citipati. Seems to make a good case!