Were dinosaurs ectotherms or fast-metabolizing endotherms whose activities were unconstrained by temperature? To date, some of the strongest evidence for endothermy comes from the rapid growth rates derived from the analysis of fossil bones. However, these studies are constrained by a lack of comparative data and an appropriate energetic framework. Here we compile data on ontogenetic growth for extant and fossil vertebrates, including all major dinosaur clades. Using a metabolic scaling approach, we find that growth and metabolic rates follow theoretical predictions across clades, although some groups deviate. Moreover, when the effects of size and temperature are considered, dinosaur metabolic rates were intermediate to those of endotherms and ectotherms and closest to those of extant mesotherms. Our results suggest that the modern dichotomy of endothermic versus ectothermic is overly simplistic.

Birds are prolific colonists of islands, where they readily evolve distinct forms. Identifying predictable, directional patterns of evolutionary change in island birds, however, has proved challenging. The "island rule" predicts that island species evolve toward intermediate sizes, but its general applicability to birds is questionable. However, convergent evolution has clearly occurred in the island bird lineages that have undergone transitions to secondary flightlessness, a process involving drastic reduction of the flight muscles and enlargement of the hindlimbs. Here, we investigated whether volant island bird populations tend to change shape in a way that converges subtly on the flightless form. We found that island bird species have evolved smaller flight muscles than their continental relatives. Furthermore, in 366 populations of Caribbean and Pacific birds, smaller flight muscles and longer legs evolved in response to increasing insularity and, strikingly, the scarcity of avian and mammalian predators. On smaller islands with fewer predators, birds exhibited shifts in investment from forelimbs to hindlimbs that were qualitatively similar to anatomical rearrangements observed in flightless birds. These findings suggest that island bird populations tend to evolve on a trajectory toward flightlessness, even if most remain volant. This pattern was consistent across nine families and four orders that vary in lifestyle, foraging behavior, flight style, and body size. These predictable shifts in avian morphology may reduce the physical capacity for escape via flight and diminish the potential for small-island taxa to diversify via dispersal.B irds on islands helped to inspire the theory of evolution by natural selection (1, 2), and they continue to illuminate its mechanisms (e.g., ref.3). Some studies have reported that the bodies and bills of island birds systematically shift in size, reflecting evolution toward a generalist niche in species-poor communities (4-8). The tendency for island taxa to converge toward intermediate body size after colonizing islands is known as the island rule (4), but this ecogeographic rule has proven to be an inconsistent predictor of evolutionary trends in island bird populations (9-12). Detailed studies of island radiations have revealed idiosyncratic patterns of body size and bill size evolution among species, with morphological changes attributable to taxon-specific changes in foraging ecology (e.g., ref. 12). This inconsistency raises the question as to whether there are predictable evolutionary trends that apply generally to island birds.The most striking evolutionary trend among island birds is the loss of flight. Transitions to flightlessness are rapid and irreversible (13,14), with each instance involving the substantial reallocation of mass from the forelimbs to the hindlimbs and near elimination of costly flight muscles (15-18). More than 1,000 independent lineages of island birds have lost flight, including rails, parrots, pigeons, owls, waterfowl, and passerines (13-1...

Theoretical and empirical studies of life history aim to account for resource allocation to the different components of fitness: survival, growth, and reproduction. The pioneering evolutionary ecologist David Lack [(1968) Ecological Adaptations for Breeding in Birds (Methuen and Co., London)] suggested that reproductive output in birds reflects adaptation to environmental factors such as availability of food and risk of predation, but subsequent studies have not always supported Lack's interpretation. Here using a dataset for 980 bird species (Dataset S1), a phylogeny, and an explicit measure of reproductive productivity, we test predictions for how mass-specific productivity varies with body size, phylogeny, and lifestyle traits. We find that productivity varies negatively with body size and energetic demands of parental care and positively with extrinsic mortality. Specifically: (i) altricial species are 50% less productive than precocial species; (ii) species with female-only care of offspring are about 20% less productive than species with other methods of parental care; (iii) nonmigrants are 14% less productive than migrants; (iv) frugivores and nectarivores are about 20% less productive than those eating other foods; and (v) pelagic foragers are 40% less productive than those feeding in other habitats. A strong signal of phylogeny suggests that syndromes of similar life-history traits tend to be conservative within clades but also to have evolved independently in different clades. Our results generally support both Lack's pioneering studies and subsequent research on avian life history. D avid Lack's classic account of ecological adaptations for breeding in birds (1) influenced generations of evolutionary ecologists who have had the benefit of more data and better methods of analysis (e.g., refs. 2-11). Subsequent studies, however, have not always supported Lack's conclusions about the primary importance of factors that affect birds directly, such as predation risk and food availability, and indirectly, such as seasonality (2,6,12,13).Subsequent to Lack's classic study, life-history theory explored adaptive resolutions of trade-offs in allocation of limited time, energy, and material resources to survival, growth, and reproduction (e.g., refs. 14-16). Here we use a larger dataset and improved phylogenetic analyses to document how reproductive output varies with body size and other aspects of avian lifestyle, such as parental care, diet, foraging mode, and migratory status. As our measure of reproductive output, we use the rate of production of reproductive biomass, termed productivity hereafter, and calculate for each species as (egg mass) × (number of eggs per clutch) × (clutch frequency, i.e., number of successful clutches per year) all divided by body mass. Egg production is fueled by metabolism, so the mass-specific rate of productivity should scale negatively with body size, roughly similarly to mass-specific metabolic rate (17). Furthermore, environmental limits on energy supply and physiological an...

The tendency for flying organisms to possess small genomes has been interpreted as evidence of natural selection acting on the physical size of the genome. Nonetheless, the flight-genome link and its mechanistic basis have yet to be well established by comparative studies within a volant clade. Is there a particular functional aspect of flight such as brisk metabolism, lift production or maneuverability that impinges on the physical genome? We measured genome sizes, wing dimensions and heart, flight muscle and body masses from a phylogenetically diverse set of bird species. In phylogenetically controlled analyses, we found that genome size was negatively correlated with relative flight muscle size and heart index (i.e. ratio of heart to body mass), but positively correlated with body mass and wing loading. The proportional masses of the flight muscles and heart were the most important parameters explaining variation in genome size in multivariate models. Hence, the metabolic intensity of powered flight appears to have driven genome size reduction in birds.