While the new Science paper on Triassic dinosauromorphs was definitely the “big news” of last week (and the new PLoS paper on skimming pterosaurs [or not] seems to be this week’s), another interesting paper on dinosaurs came out in Biology Letters that didn’t receive quite as much attention. The research, “Growth patterns in brooding dinosaurs reveals the timing of sexual maturity in non-avian dinosaurs and genesis of the avian condition,” non-avian dinosaur nesting sites provide some rather interesting insights into how dinosaurs grew and (perhaps) mated.
As the authors note in the abstract, the closest living relatives of dinosaurs can be found amongst crocodilians and birds (descendants of dinosaurs). While the growth rate of crocodiles slows down as they approach sexual maturity (crocodiles grow throughout their lives), birds grow to their adult size well before they start mating, giving the researchers some baselines to compare with dinosaurian growth rates. Using seven specimens (Oviraptor philoceratops; Oviraptor sp.; 2 X Citipati osmolskae, “one deinonychosaur (Troodontidae nov. sp.)” from China and Mongolia found in nesting positions, as well as Deinonychus antirrhopus and Troodon formosus found in association with eggs) the authors assumed that each of these dinosaurs was a parent (a point I’ll get back to later), although the sex of the dinosaurs could not be determined. The age of the dinosaurs, however, was determined by looking at annual growth lines in the bone as well as overall body size comparisons, allowing the researchers to get a fairly good idea as to the “stage of life” each of the dinosaurs may have been in.
After looking at the ages of the dinosaurs and determining their growth rates (as can be seen in a colorful graph in the paper, which is open access) the researchers found that the dinosaurs seemed to have distinctly non-avian growth rates and reproduction. Rather than quick growth leading to adult size, followed by reproductive maturity sometime afterwards, the dinosaurs studied seemed to show that they were brooding on nests (and therefore assumed to be mating) before they had reached full adult size. This trend is closer to that of crocodilians, where growth slows down at the time of reproductive maturity, but is not the “hurry up and wait” model that extant birds employ. Being that the dinosaurs studied are regarded to be among the closest to birds (although not ancestral to birds), it is a bit surprising to see this difference. The authors explain, however, that the difference in growth rates may not have been something inherited from dinosaurs, especially since some of the earliest known birds took at least a year to reach maturity while extant birds may do so in a matter of weeks (but this all depends on the size of the bird and other factors that it would be foolish to generalize).
I have to wonder, however, were all the dinosaurs on the nests actually parents? It would seem to be a relatively straightforward find being that the parents of potential offspring would sit on the nest, but the Pied Kingfisher (Ceryle rudis) keeps pecking at my brain when I consider this topic. For those who have not heard the story, the Pied Kingfishers employ an interesting nest-care strategy; breeding pairs will sometimes accept the assistance of a “helper,” especially when resources like food are scarce. Helpers may be related (“primary helpers” that are always tolerated) or unrelated (“secondary helpers”) to the nesting pair (Reyer, 1984), although it appears that all the helpers are males. This means that secondary males, as in the Reyer study, may be accepted or rejected as they would otherwise be in competition with the breeding male, while relatives are more generously tolerated.
The main activity of the helpers, however, is catching food to bring to the nest, thus enhancing the breeding success of the parents. While there are many bird species that have “helpers” (over 150 according to Reyer, 1979), I don’t know of any that help the parents incubate the eggs or sit on the nest. In fact, I probably wouldn’t expect to see such a behavior being that eggs are the primary investment and I wouldn’t imagine a mother giving up her care of the eggs to a male relative (and especially not an unrelated male), but I am not going to say that such a system would be impossible given my ignorance on this topic. Still, I have to wonder if non-breeding dinosaurs assisted nesting parents in any way, either by gathering food or (like I said, highly improbable) related non-reproductively-mature young sitting on the nest. This is certainly not parsimonious and there is simply no way that I can think of to tell, but I guess I continued through this post (even though I’m fairly sure that I’m wrong) to illustrate my thought process. I saw a study, thought of a related piece of information, got an idea, and tried to follow through by looking up other papers to help determine whether there are modern analogs that might give me some hint as to whether I could be right. It appears that I’m not, but I’m definitely better for the time spent, and hopefully this definitely adds to my still in-progress post about the evolution of precocial vs altricial young.
[Anne-Marie also offers her thoughts on the dino-sex paper on her blog]