PerspectiveEvolution

Who's Your Daddy?

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Science  19 Dec 2008:
Vol. 322, Issue 5909, pp. 1799-1800
DOI: 10.1126/science.1168808

The recognition that birds are theropod dinosaurs has redefined the science of ornithology as extant dinosaur biology (1). The placement of birds in a detailed evolutionary context has led to exciting discoveries about the commonalities birds share with their dinosaur ancestors, including feathers (2) and possibly flight (3). Insights have been gained into both the origins of avian biology and the natural history of some of the most charismatic dinosaurs—the meat-eating, bipedal theropods. The dividends continue with the report by Varricchio et al. on page 1826 of this issue (4), in which the authors show that fatherhood in theropods was about more than just looking macho and gnashing teeth.

Biological views of dinosaur parenting have evolved a lot over the past century. In 1924, Osborn named a Cretaceous theropod Oviraptor, or “egg seizer,” because it had been fossilized “in the very act of robbing [a] dinosaur egg nest” (5). In 1995, new specimens showed that Oviraptor was not stealing those eggs, but caring for them and possibly even brooding them (see the figure, left panel) (6). Now, Varricchio et al. (4) present compelling evidence from three theropods closely related to birds—Troodon, Oviraptor, and Citipati—that the individuals caring for those clutches of eggs were males. Because the basal lineage of living birds, or the earliest branch in the avian phylogenetic tree, also has predominantly male-only nest care, their discovery may have uncovered the dinosaurian origins of the breeding biology of living birds.

Parental care in theropod dinosaurs.

Fossil evidence has shown that theropod dinosaurs such as Oviraptor cared for their young (left) (6). Based on a new analysis, Varricchio et al. (4) hypothesize that it was male theropods who provided the care, similar to living birds at the base of the avian phylogeny, including ostriches (Struthio camelus) (middle) and highland tinamous (Nothocercus bonapartei) (right).

CREDIT: (LEFT PANEL) DENNIS FINNIN AND MICK ELLISON/AMERICAN MUSEUM OF NATURAL HISTORY. (MIDDLE PANEL) KEVIN SCHAFER/CORBIS. (RIGHT PANEL) MICHAEL AND PATRICIA FOGDEN/CORBIS

Varricchio et al. use two lines of evidence to support their revolutionary conclusion. First, they compared clutch volumes to adult body sizes for a sample of living archosaurs—the group of reptiles that includes crocodilians, birds, and other dinosaurs. They found that Troodon, Oviraptor, and Citipati have larger clutch volumes for their body sizes than most of the more than 400 extant species of birds and crocodilians examined, but that their clutch volumes closely match the expected values for birds with exclusively male parental care. Clutch volumes can evolve to be larger in species without maternal care, because females may have more resources to devote to eggs if they provide no care and because a “clutch” may be composed of eggs from multiple females. Second, Varricchio et al. took advantage of a distinctive feature of avian reproductive physiology to determine the sex of the dinosaurs from their bones. Many female birds lay down a distinctive layer of spongy, medullary bone inside their long bones during reproduction (7). Recent observations of medullary bone in the theropod Tyrannosaurus rex as well as in basal birds (7) indicate that female Troodon,Oviraptor, and Citipati should also exhibit medullary bone. Varricchio et al. show that the Troodon, Oviraptor, and Citipati individuals fossilized at nests lacked medullary bone, independently supporting the conclusion that they were males.

Over 90% of living birds have biparental care. A small group of species have exclusively female care, but less than 100 species of living birds have exclusively male parental care (8). In these species, males build the nest, incubate the eggs, and raise the young, whereas females mate with multiple males and lay their eggs in multiple nests, which may or may not be within a defended territory. The birds with the most consistent pattern of male nest care are the basal lineage of living birds, called Paleognathes, which include the flightless ostrich, emu, cassowary, kiwi, and rhea, and the flying neotropical tinamous (9) (see the figure, right panel).

Some behavioral ecologists have hypothesized that male-only parental care was the original breeding system of living birds (8), and the basal phylogenetic position of Paleognathes has been used to support this hypothesis (10). In the absence of any data on parental care in extinct dinosaurs, however, phylogenetic systematists have argued that female-only parental care, found both in birds and crocodilians, was the primitive breeding system of birds (11). Most recently, Wesolowski (12) argued against the theropod origin of avian breeding behavior while reiterating the male-care-first hypothesis. In a result that is sure to surprise both camps, Varricchio et al. show that the theropod origin of avian breeding behavior is consistent with male parental care as the primitive breeding system of birds, thus resolving the conflict between ecological and phylogenetic ornithologists.

These researchers have an excellent track record of establishing details of extinct theropod biology that have changed our views of the origins of avian biology. In 1997, Varricchio et al. (13) proposed that Troodon laid their eggs two at a time and that the uniquely avian behavior of laying the clutch of eggs over a series of days evolved in theropods before the origin of birds or flight. This bold idea was dramatically confirmed in 2005 with a discovery of an oviraptoran fossil with a pair of shelled eggs inside her pelvis (14).

According to the new hypothesis (4), the parental behaviors of living Paleognathes (like the cassowary) and extinct theropods (like Oviraptor) are homologous, and their breeding systems remained unchanged since their common ancestry. But could male parental care have evolved independently in Paleognathes and Cretaceous dinosaurs? There are many lineages between the Paleognathes and the oviraptorans and troodontids (including the huge, flightless, colonial diver Hesperornis, the pigeon-sized Confuciusornis with elongate ornamental tail feathers, and the archetypal Archaeopteryx). Many of these creatures seem so similar in ecology to various modern birds with biparental care that it is tempting to think that their breeding biology should also be similar. However, Varricchio et al.'s hypothesis may be supported by the observation that the male-only parental care system has resisted evolutionary change. Most Paleognathes have retained this breeding system, despite substantial ecological radiation, since before the K/T boundary over 65 million years ago (15, 16). Thus, there may be substantial constraints to evolving female incubation if her female ancestors have not done so for tens of millions of years before her.

In the absence of a coherent hypothesis for the origin of birds during the greater part of the 20th century, most evolutionary explanations of avian biology focused on how unique birds are (1). Scientists are now identifying the dinosaurian origins of many of the formerly unique features of birds. Are there limits to ornithological revelations that the theropod origin of birds will yield? It seems not. Focused research and lucky paleontological discoveries may someday uncover the theropod origin of bird song, avian respiration, and more.

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