PERSPECTIVES EVOLUTION The male-only nest care system of some birds may have its evolutionary origins in theropod Who’s Your Daddy? dinosaur behavior. Richard O. Prum he recognition that birds are theropod related to birds—Troodon, Oviraptor, and rex as well as in basal birds (7) indicate that dinosaurs has redefined the science of Citipati—that the individuals caring for those female Troodon, Oviraptor, and Citipati should Tornithology as extant dinosaur biol- clutches of eggs were males. Because the also exhibit medullary bone. Varricchio et al. ogy (1). The placement of birds in a detailed basal lineage of living birds, or the earliest show that the Troodon, Oviraptor, and Citipati evolutionary context has led to exciting dis- branch in the avian phylogenetic tree, also has individuals fossilized at nests lacked medullary coveries about the commonalities birds share predominantly male-only nest care, their dis- bone, independently supporting the conclusion with their dinosaur ancestors, including covery may have uncovered the dinosaurian that they were males. feathers (2) and possibly flight (3). Insights origins of the breeding biology of living birds. Over 90% of living birds have biparental have been gained into both the origins Varricchio et al. use two lines of evidence to care. A small group of species have exclu- of avian biology and the natural history of support their revolutionary conclusion. First, sively female care, but less than 100 species of some of the most charismatic dinosaurs—the they compared clutch volumes to adult body 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 on December 19, 2008 www.sciencemag.org Parental care in theropod dinosaurs. Fossil evidence has shown that theropod vided the care, similar to living birds at the base of the avian phylogeny, includ- dinosaurs such as Oviraptor cared for their young (left)(6). Based on a new ing ostriches (Struthio camelus) (middle) and highland tinamous (Nothocercus analysis, Varricchio et al. (4) hypothesize that it was male theropods who pro- bonapartei) (right). meat-eating, bipedal theropods. The divi- sizes for a sample of living archosaurs—the may or may not be within a defended territory. dends continue with the report by Varricchio group of reptiles that includes crocodilians, The birds with the most consistent pattern of et al. on page 1826 of this issue (4), in which birds, and other dinosaurs. They found that male nest care are the basal lineage of living Downloaded from the authors show that fatherhood in thero- Troodon, Oviraptor, and Citipati have larger birds, called Paleognathes, which include the pods was about more than just looking clutch volumes for their body sizes than most flightless ostrich, emu, cassowary, kiwi, and macho and gnashing teeth. of the more than 400 extant species of birds and rhea, and the flying neotropical tinamous (9) Biological views of dinosaur parenting crocodilians examined, but that their clutch vol- (see the figure, right panel). have evolved a lot over the past century. In umes closely match the expected values for Some behavioral ecologists have hypoth- 1924, Osborn named a Cretaceous theropod birds with exclusively male parental care. esized that male-only parental care was the Oviraptor, or “egg seizer,” because it had been Clutch volumes can evolve to be larger in original breeding system of living birds (8), fossilized “in the very act of robbing [a] species without maternal care, because females and the basal phylogenetic position of dinosaur egg nest” (5). In 1995, new speci- may have more resources to devote to eggs if Paleognathes has been used to support this mens showed that Oviraptor was not stealing they provide no care and because a “clutch” hypothesis (10). In the absence of any data those eggs, but caring for them and possibly may be composed of eggs from multiple on parental care in extinct dinosaurs, how- even brooding them (see the figure, left panel) females. Second, Varricchio et al. took advan- ever, phylogenetic systematists have argued (6). Now, Varricchio et al. (4) present com- tage of a distinctive feature of avian reproduc- that female-only parental care, found both in pelling evidence from three theropods closely tive physiology to determine the sex of the birds and crocodilians, was the primitive dinosaurs from their bones. Many female birds breeding system of birds (11). Most re- lay down a distinctive layer of spongy, cently, Wesolowski (12) argued against the Department of Ecology and Evolutionary Biology and medullary bone inside their long bones during theropod origin of avian breeding behavior Peabody Museum of Natural History, Post Office Box 208105, Yale University, New Haven, CT 06520, USA. reproduction (7). Recent observations of while reiterating the male-care-first hypoth- 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 PATRICIA AND MICHAEL PANEL) (RIGHT SCHAFER/CORBIS. KEVIN PANEL) (MIDDLE HISTORY. NATURAL OF MUSEUM ELLISON/AMERICAN MICK AND FINNIN DENNIS PANEL) (LEFT CREDIT: E-mail: [email protected] medullary bone in the theropod Tyrannosaurus esis. In a result that is sure to surprise both www.sciencemag.org SCIENCE VOL 322 19 DECEMBER 2008 1799 Published by AAAS PERSPECTIVES camps, Varricchio et al. show that the thero- Paleognathes and the oviraptorans and limits to ornithological revelations that the pod origin of avian breeding behavior is troodontids (including the huge, flightless, theropod origin of birds will yield? It seems consistent with male parental care as the colonial diver Hesperornis, the pigeon-sized not. Focused research and lucky paleonto- primitive breeding system of birds, thus Confuciusornis with elongate ornamental tail logical discoveries may someday uncover the resolving the conflict between ecological feathers, and the archetypal Archaeopteryx). theropod origin of bird song, avian respira- and phylogenetic ornithologists. Many of these creatures seem so similar tion, and more. These researchers have an excellent track in ecology to various modern birds with record of establishing details of extinct thero- biparental care that it is tempting to think that References 1. R. O. Prum, Auk 119, 1 (2002). pod biology that have changed our views of their breeding biology should also be similar. 2. R. O. Prum, A. H. Brush, Quart. Rev. Biol. 77, 261 (2002). the origins of avian biology. In 1997, However, Varricchio et al.’s hypothesis may 3. X. Xu et al., Nature 421, 335 (2003). Varricchio et al. (13) proposed that Troodon be supported by the observation that the 4. D. J. Varricchio et al., Science 322, 1826 (2008). 5. H. F. Osborn, Am. Mus. Nov. 144, 1 (1924). laid their eggs two at a time and that the male-only parental care system has resisted 6. M. A. Norell, J. M. Clark, L. Chiappe, D. Dashzeveg, uniquely avian behavior of laying the clutch of evolutionary change. Most Paleognathes Nature 378, 774 (1995). eggs over a series of days evolved in theropods have retained this breeding system, despite 7. M. H. Schweitzer, J. L. Wittemeyer, J. R. Horner, Science 308, 1456 (2005). before the origin of birds or flight. This bold substantial ecological radiation, since before 8. J. D. Ligon, The Evolution of Avian Breeding Systems idea was dramatically confirmed in 2005 with the K/T boundary over 65 million years ago (Oxford Univ. Press, Oxford, 1999). a discovery of an oviraptoran fossil with a pair (15, 16). Thus, there may be substantial con- 9. S. J. J. F. Davies, Ratites and Tinamous (Oxford Univ. Press, Oxford, 2001). of shelled eggs inside her pelvis (14). straints to evolving female incubation if her 10. T. Wesolowski, Am. Nat. 143, 39 (1994). According to the new hypothesis (4), the female ancestors have not done so for tens of 11. B. S. Tullberg, M. Ah-King, H. Temrin, Philos. Trans. R. parental behaviors of living Paleognathes millions of years before her. Soc. B 357, 251 (2002). 12. T. Wesolowski, Behav. Ecol. 15, 520 (2004). (like the cassowary) and extinct theropods In the absence of a coherent hypothesis 13. D. J. Varricchio, F. Jackson, J. J. Borkowski, J. R. Horner, (like Oviraptor) are homologous, and their for the origin of birds during the greater part Nature 385, 247 (1997). breeding systems remained unchanged since of the 20th century, most evolutionary expla- 14. T. Sato, Y. Cheng, X. Wu, D. K. Zelenitsky, Y. Hsiao, Science 308, 375 (2005). their common ancestry. But could male nations of avian biology focused on how 15. J. Clarke et al., Nature 433, 305 (2005). parental care have evolved independently in unique birds are (1). Scientists are now iden- 16. J. W. Brown et al., BMC Biol. 6, 6 (2008). Paleognathes and Cretaceous dinosaurs? tifying the dinosaurian origins of many of the on December 19, 2008 There are many lineages between the formerly unique features of birds. Are there 10.1126/science.1168808 COMPUTER SCIENCE The use of robots to care for the young and the old, and as autonomous agents on the The Ethical Frontiers of Robotics battlefield, raises ethical issues. www.sciencemag.org Noel Sharkey obots have been used in laboratories ments that allowed robots to become mobile, semiautonomous function are ideal for visual and factories for many years, but their interactive machines. Artificial intelligence and auditory monitoring; radio-frequency Ruses are changing fast. Since the turn has not met its early promise of truly intelli- identification tags provide alerts when chil- of the century, sales of professional and per- gent machines, but researchers in the emerg- dren move out of range. The robots can be sonal service robots have risen sharply and are ing field of human-robot interaction have controlled by mobile phone or from a window estimated to total ~5.5 million in 2008.