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Avian integument is thin, elastic, and loosely attached to the body, giving birds the freedom of movement needed for flight. Its epidermis is both keratinized and lipogenic, and the skin as a whole acts as a sebaceous secretory organ. The skin is covered by feathers over most of the body, but many birds show colored bare skin or integumentary outgrowths on the head and neck. Heavily cornified epidermis covers the beak, claws, spurs, and the scales on the legs and feet. These structures (except the back of the leg and underside of the foot) contain beta-keratin like that in reptilian scales. Most birds have sebaceous secretory glands at the base of the tail and in the ear canals. Feathers are the most numerous, elaborate, and diverse of avian integumentary derivatives. Their diversity is due to the possibilities inherent in their basic plan of a shaft with two orders of branches and the use of modified beta-keratin as a strong, light, and plastic building material. The evolution of feathers in birds has been accompanied by the development of complex systems for producing colors and patterns, the innovations of feather arrangement and follicles with their musculature and innervation, and the process and control of molting.
Historical-narrative evolutionary explanations for the origin and further evolution of avian feathers involve two steps. The first phase reconstructs a series of probable morphological changes from a reptilian scale to the primitive feather. The second deduces possible functions and biological roles of the features and feasible selective demands on these features at all stages in its evolution. The best explanation for the evolutionary origin of feathers would be one consistent with historical-narrative evolutionary explanations for the origin and further evolution of other features in the history of birds. Feathers of Recent birds have a number of functions and biological roles, and it is difficult to ascertain which of these functions and roles were involved in feather origin. Two major rival published theories are based on the roles of feathers in insulating the body against heat loss and in providing an aerodynamic surface for flight. However, because of the lack of knowledge about the roles and ecological relationships of protofeathers and of the most primitive feathers, it is not possible to test strongly either of these theories, or others as proposed in this symposium, against objective empirical observations to determine which is falsified or is the most probable. Finally it is argued that test of historical-narrative evolutionary explanations, including classifications and phylogenies, is generally difficult to impossible because of the lack of the necessary objective empirical observations.
A number of hypotheses have been suggested for the origin of birds and feathers. Although distributions of functional complexes have frequently been used to test phylogenetic hypotheses, analysis of the origin of feathers remains hampered by the incomplete fossil record of these unmineralized structures. It is also complicated by approaches that confuse the origins of birds, feathers, and flight without first demonstrating that these relate to the same historical event. Functional speculation regarding the origin of feathers usually focuses on three possible alternatives: (1) flight; (2) thermal insulation; or (3) display. Recent fossil finds of Late Cretaceous feathered dinosaurs in China have demonstrated that feathers appear to have originated in taxa that retained a significant number of primitive nonavian features. Current evidence strongly suggests that birds are theropod dinosaurs, and that the most primitive known feathers are found on non-flying animals. This further suggests that feathers did not evolve as flight structures. Thermoregulatory, display, and biomechanical support functions remain possible explanations for the origin of feathers. As the earliest function of feathers was probably not for aerial locomotion, it may be speculated that the transitional animals represented by the Chinese fossils possessed skin with the tensile properties of reptiles and combined it with the apomorphic characteristics of feathers.
The origin of birds has been discussed since the discovery and description of Archaeopteryx in Bavaria in 1861. By 1868, Thomas Henry Huxley realized its significance as a connecting form, which illustrated how birds might have evolved from dinosaurs. A century later John Ostrom articulated a convincing modern case for the origin of birds from theropod dinosaurs. Recent cladistic analyses of theropod, bird and bird-like fossils seem to confirm this scenario of bird origins. The purpose of this paper is to examine both the philosophic principles and the practice of cladistic analysis upon which the dinosaur-bird link is currently based. Cladistics is based on a Popperian philosophy that emphasizes the hypothetical nature of all knowledge. Such a philosophy seems more suitable for analyzing idealized characters unrooted in time or space rather than real objects. A philosophy of critical realism seems more congenial for analysis of evolutionary biological individuals having a real history. Cladistics uses parsimony as a first principle, which may be rejected on the grounds that nature is prodigal in every regard. Parsimony based on morphology suffices only when there are no other data sets to consider. Cladistics systematically excludes data from stratigraphy, embryology, ecology, and biogeography that could otherwise be employed to bring maximum evolutionary coherence to biological data. Darwin would have convinced no one if he had been so restrictive in his theory of evolution. The current cladistic analysis of bird origins posits a series of outgroups to birds that postdate the earliest bird by up to 80 million years. This diverts attention from the search for real bird ancestors. A more coherent analysis would concentrate the search for real avian ancestors in the Late Jurassic.
Developmental anatomical data are insufficient to discuss plausible intermediates between an ancestral, scaled, reptilian skin and appendage-bearing, avian skin. We also review adult tissue replacement and ubiquitous mechanisms underlying skin morphogenesis. Combining developmental data sensu lato with consideration of necessary biological roles permits evaluation of major form/function trends in skin evolution. New data on feathers reveal retention of the sauropsid synapomorphy of vertical alteration of α- and β-keratogenesis. By identifying roles that were obligatorily maintained throughout evolution, we demonstrate constraints on hypothetical skin morphologies in preavian taxa. We analyze feather origins as a problem of emergence of complex form via modulations of morphogenesis. While existing data do not permit presentation of sequential, hypothetical, intermediates culminating in a plumage, the analysis: (1) implies that a protofeather and its follicle are most easily derived from isolated, flattened, elongate, reptilian scales; (2) explains diversification of feather morphs from a contour-like “basic” feather and the similarity between feather and hair follicles; and thus (3) reveals several developmental constraints on structures proposed as antecedent to avian feathers, whether hypothetical constructs or palaeontological interpretations. Although these conclusions do not depend on any previous scenario, they are consistent with Regal's (1975) model and the limited, fossil evidence, especially that of the “basal archosaur” Longisquama.
The integuments of extant vertebrates display a variety of epidermal appendages whose patterns, morphology and terminal differentiation (epidermal keratins) depend upon interactions between ectodermal (epidermis) and mesodermal (dermis) tissues. In reptiles and birds, appendage morphogenesis precedes terminal differentiation. Studies have demonstrated that appendage morphogenesis influences the expression of the appendage specific keratin genes. However, little is known about the nature of the structural genes expressed by the epidermal appendages of reptiles. How pattern formation and/or appendage morphogenesis influence terminal differentiation of reptilian appendages is not known.
The epidermal appendages of reptiles and birds are characterized by the presence of both alpha (α) and beta (β) type keratin proteins. Studies have focused on the genes of avian β keratins because they are the major structural proteins of feathers. The occurrence of β keratin proteins in the scales and claws of both birds and reptiles and their immunological cross-reactivity suggest that the genes for reptilian β keratins may be homologous with those of birds. In bird appendages, the β keratins are the products of a large family of homologous genes. Specific members of this gene family are expressed during the development of each appendage. Recent sequence analyses of feather β keratins, from different orders of birds, demonstrate that there is more diversity at the DNA level than was implied by earlier protein sequencing studies.
Immunological techniques show that the same antibodies that react with the epidermal β keratins of the chicken (Gallus domesticus) react with the epidermal β keratins of American alligators (Alligator mississippiensis). Furthermore, a peptide sequence (20 amino acids) from an alligator claw β keratin is similar to a highly conserved region of avian claw, scale, feather, and feather-like β keratins. These observations suggest that the β keratin genes of avian epidermal appendages have homologues in the American alligator. Understanding the origin and evolution of the β keratin gene families in reptiles and birds will undoubtedly add to our understanding of the evolution of skin appendages such as scales and feathers.
The avian epidermis is composed of unique sebokeratinocytes that elaborate and secrete sebum-like lipids as they cornify. In addition to the lipid droplets, the avian epidermis elaborates, but rarely secretes, lipid—enriched organelles, the multigranular bodies. The multigranular bodies are analogous to the lamellar bodies of mammals (Menon et al., 1991), the secretion of which results in formation of occlusive lipid bilayers characteristic of mammalian stratum corneum and providing the permeability barrier. However, in contrast to mammals, the avian multigranular bodies form the reserve barrier mechanism. In the basal state, when multigranular bodies are not secreted, the avian cutaneous barrier is deficient, but allows evaporative cooling for thermoregulation. However, under conditions of water deficit, multigranular body secretion allows for rapid facultative waterproofing, as shown in zebra finches (Taenyopygia guttata). In certain glabrous regions of the skin, such as the maxillary rictus, interdigital web, and combs and wattles in the domestic fowl, there is a high degree of epidermal lipid secretion. Also specialized feathers such as powder downs elaborate lipid rich material, which can be classified as secretion. Additionally, an inverse relation between epidermal lipogenesis and the degree of feathering has been demonstrated, as in temporarily bare areas (e.g., brood patches) and following permanent feather loss from the head accompanying attainment of maturity in certain ibises and storks. In the latter, the neo-apteria often hold large reserves of carotenoids dissolved in the lipid droplets, possibly related to an altered gradient of retinoids influencing feather morphogenesis. Unusual secondary functions of epidermal lipids include cosmetic coloration (e.g., in the Japanese Crested Ibis) and chemical defense (e.g., in the Pitohui).
A selective regime favoring a streamlining of body contours and surfaces is proposed as having been instrumental in driving the morphological and functional transformations of an unfeathered reptilian integument into a feather-bearing avian one. This hypothesis is consistent with a new, structurally and functionally coherent analysis of the microanatomy of the avian feather-bearing integument as a complex, integrated organ system that includes an intricate, hydraulic skeleto-muscular apparatus of the feathers, a dermo-subcutaneous muscle system of the integument, and a subcutaneous hydraulic skeletal system formed by fat bodies. Key elements of the evidence supporting the new hypothesis are (1) the presence of depressor feather muscles that are not needed as antagonists for the erector feather muscles, but can counteract external forces, such as air currents; (2) the fact that the highly intricate feather-bearing integument represents a machinery to move feathers or to stabilize them against external forces; (3) the crucial role of the coat of feathers in streamlining the body contours and surfaces of birds; (4) the aerodynamic role of feathers as pressure and turbulence sensors and as controllable temporary turbulators; and (5) the critical role that a streamlined body plays in avian flight and is likely to have played in the evolutionary transformations from ecologically and locomotorily versatile quadrupedal reptiles to volant bipedal birds without passing through parachuting or gliding stages. These transformations are likely to have occurred more than once. The ancestral birds were probably small, arboreal, hopping, and using flap-bounding, or intermittent bounding, flight.
The plumage of birds provides a critical thermal buffer between the animal and its environment. Rates of energy expenditure are strongly influenced by the thermal properties of the environment or the microclimates the animal occupies. Current data suggest that the addition of solar radiation is equivalent to three to four-fold changes in wind speed and that solar heat gain can be extremely sensitive to changes in wind speed.
Dry heat transfer through the plumage occurs by three avenues 1) conduction and free convection through air 2) conduction along the solid elements of the plumage and 3) radiation. Overall, about 95% of the total heat flow is evenly divided between the first two avenues. Radiative heat transfer accounts for only about 5% of total heat flow. Plumage color, as well as the microstructure and micro-optical properties of plumage elements, when combined with environmental properties (e.g., wind speed), determine the radiative heat loads that birds acquire from solar radiation. Although plumage color or reflectivity determines the fraction of incident solar radiation that is absorbed by the plumage and generates heat, the fraction of this heat that contributes to the thermal load on the animal can vary greatly. In a fibrous coat such as a plumage, there is some variable penetration into the coat, with absorption over a range of coat depths. Factors such as feather micro-optics and structure are critical determinants of radiation penetration into avian coats. Significant differences in solar heat loads can also result from behavioral adjustments in plumage thickness.
Conventional wisdom notwithstanding, fur and feathers are unlikely to have arisen in direct association with elevated metabolic rates in early mammals, birds, or their ancestors. A complete insulative fur coat probably appeared first in the earliest mammals long after mammalian ancestors (therapsids) had attained mammalian, or near-mammalian, metabolic rates. The evolution of feathers was unlinked to the evolution of modern avian metabolic rates since early, fully flighted birds (i.e., Archaeopteryx) retained an ectothermic metabolic status. Recent claims of “feathered dinosaurs” should be regarded with caution.
This paper describes how climate variation in time and space can constrain community structure on a global scale. We explore body size scaling and the energetic consequences in terms of absorbed mass and energy and expended mass and energy. We explain how morphology, specific physiological properties, and temperature dependent behaviors are key variables that link individual energetics to population dynamics and community structure.
This paper describes an integrated basic principles model for mammal energetics and extends the model to bird energetics. The model additions include molar balance models for the lungs and gut. The gut model couples food ingested to respiratory gas exchanges and evaporative water loss from the respiratory system. We incorporate a novel thermoregulatory model that yields metabolic calculations as a function of temperature. The calculations mimic empirical data without regression. We explore the differences in the quality of insulation between hair and feathers with our porous media model for insulation.
For mammals ranging in size from mice to elephants we show that calculated metabolic costs are in agreement with experimental data. We also demonstrate how we can do the same for birds ranging in size from hummingbirds to ostriches. We show the impact of changing posture and changing air temperatures on energetic costs for birds and mammals. We demonstrate how optimal body size that maximizes the potential for growth and reproduction changes with changing climatic conditions and with diet quality. Climate and diet may play important roles in constraining community structure (collection of functional types of different body sizes) at local and global scales. Thus, multiple functional types may coexist in a locality in part because of the temporal and spatial variation in climate and seasonal food variation. We illustrate how the models can be applied in a conservation and biodiversity context to a rare and endangered species of parrot, the Orange-bellied Parrot of Australia and Tasmania.
It is likely that feathers evolved from a conical shaped tubercle rather than a plate-like structure. Although the morphology of the presumably most primitive feather is unknown, minimal conditions for its production include the cellular capacity to synthesize feather proteins (=ϕ-keratin) which provides the molecular phenotype, and a follicular mechanism for production and assembly of molecular and gross structure. Once the minimal structural element, presumably recognizable as a barb, existed, a variety of phenotypes followed rapidly. A tubercular growth center of appropriate size could produce a simple barb-like element, with cortex and medulla. This might be recognized externally as a bristle, but need never existed as a separate morphological unit. Rather, if individual placodes gave rise to multiple barb ridges that fused proximally, a structure resembling natal down would have resulted. Subsequent differentiation is controlled by the follicular symmetry, and the feather shape is regulated by barb length. Barb length is directly related to growth period. As feathers appear to grow at roughly similar, size independent rates, shape is determined by individual barb growth periods. The simple fusion of individual proto-barbs would produce a morphology identifiable as natal down. Although this might be the simplest feather structure, others could emerge quickly, perhaps simultaneously, a consequence of the same redundant processing. Once the machinery existed, broad phenotypic plasticity was possible. I constructed a feather phylogram based on these conditions, the fossil record, and ontogeny. I organized the subsequent changes in morphology by perceived complexity. The changes are simply individual responses to similar processes that might be time (when in ontogeny) and space (where on body) dependent.
Theropod (carnivorous) dinosaurs spanned a range from chicken-sized to elephant-sized animals. The primary mode of locomotion in these dinosaurs was fairly conservative: Theropods were erect, digitigrade, striding bipeds. Even so, during theropod evolution there were changes in the hip, tail, and hindlimb that undoubtedly affected the way these dinosaurs walked and ran, a trend that reached its extreme in the evolution of birds. Some derived non-avian theropods developed hindlimb proportions that suggest a greater degree of cursoriality than in more primitive groups. Despite this, fossilized trackways provide no evidence for changes in stride lengths of early as opposed to later non-avian theropods. However, these dinosaurs did take relatively longer strides—at least compared with footprint length—than bipedal ornithischian dinosaurs or ground birds. Judging from trackway evidence, non-avian theropods usually walked, and seldom used faster gaits. The largest theropods were probably not as fleet as their smaller relatives.
Most current phylogenetic hypotheses based upon cladistic methodology assert that birds are the direct descendants of derived maniraptoran theropod dinosaurs, and that the origin of avian flight necessarily developed within a terrestrial context (i.e., from the “ground up”). Most theoretical aerodynamic and energetic models or chronologically appropriate fossil data do not support these hypotheses for the evolution of powered flight. The more traditional model for the origin of flight derives birds from among small arboreal early Mesozoic archosaurs (“thecodonts”). According to this model, protoavian ancestors developed flight in the trees via a series of intermediate stages, such as leaping, parachuting, gliding, and flapping. This model benefits from the assemblage of living and extinct arboreal vertebrates that engage in analogous non-powered aerial activities using elevation as a source of gravitational energy. Recent reports of “feathered theropods” notwithstanding, the evolution of birds from any known group of maniraptoran theropods remains equivocal.
The evolution of birds and feathers are examined in terms of the aerodynamic constraints imposed by the arboreal and cursorial models of flight evolution. The cursorial origin of flight is associated with the putative coelurosaurian ancestry of birds. As presently known, coelurosaurs have a center of mass located in the pelvic region and an elongated pubis that is ventrally or anteriorly directed. Both of these characteristics make it difficult to postulate an origin of flight that would involve a gliding phase because the abdomen cannot be flattened into an aerodynamic shape. Moreover, the cursorial model must counteract gravity using the hindlimb and, thus, selection for the power requirement for lift-off would not focus on the forelimb. Therefore, if the hypothesis proposing a coelurosaurian ancestry of birds is to remain viable, it must be via an as yet undiscovered taxon that is compatible with the morphological and aerodynamic constraints imposed by flight evolution.
The arboreal model, currently centers around non-dinosaurian taxa and is more parsimonious in that early archosaurs have short pubes that do not preclude an aerodynamic body profile. Moreover, the arboreal proavis uses gravity to create the airflow over the body surfaces and is, thus, energy efficient. Consideration of the initial aerodynamic roles of feathers and feather design are consistent with a precursory gliding phase. Whether avian ancestry lies among coelurosaur theropods or earlier archosaurs, we must remain mindful of the complex aerodynamic dictates of gliding and powered flight and avoid formalistic approaches that co-opt sister taxa, with their known body form, as functional ancestors.
The oldest known feathers from the Late Jurassic are already modern in form and microscopic detail. Because these oldest examples are assignable to an extinct branch (Sauriurae) of the basal avian dichotomy, their features must have been established at a significantly earlier date. The skin of a wide variety of dinosaurs is now known and is unlikely to represent a predecessor to a feather bearing integument. Examples of feathered dinosaurs result from erroneous identification of internal structures as part of the skin covering, and from the confusion of flightless birds from the Early Cretaceous of China with dinosaurs.