Adaptation, constraint, and compromise in avian postnatal development

Unexpected Performance in Developing Birds

Birds are well known for their ability to fly, and flight-capable adult birds have many anatomical specializations for meeting the demands of aerial locomotion. Juvenile birds in altricial species typically acquire these specializations close to fledging and leave the nest with some flight capability. In contrast, juveniles in most precocial species begin navigating their environment with rudimentary anatomies and may not develop full-sized wings or musculoskeletal apparatuses for several months. This manuscript explores how juvenile birds achieve high levels of locomotor performance in the absence of flight specializations, by synthesizing work on two groups of precocial birds with very different developmental strategies. Galliforms like the Chukar Partridge (Alectoris chukar) have early wing development and are capable of flight within weeks. Compared with adults, juvenile chukars have less aerodynamically effective feathers and smaller muscles but compensate through anatomical, kinematic, and behavioral mechanisms. In contrast, waterfowl have delayed wing development and initially rely on leg-based locomotion. In Mallards (Anas platyrhynchos) and their domesticated derivatives, leg investment and performance peak early in ontogeny, but then decline when wings develop. Chukar and mallard juveniles thus rely on different mechanisms for negotiating their surroundings in the absence of flight specializations. In conjunction with work in other animals, these patterns indicate that juveniles with developing locomotor apparatuses can achieve surprisingly high levels of locomotor performance through a variety of compensatory mechanisms.

Allometric wing growth links parental care to pterosaur giantism

Pterosaurs evolved a broad range of body sizes, from small-bodied early forms with wingspans of mostly 1-2 m to the last-surviving giants with sizes of small airplanes. Since all pterosaurs began life as small hatchlings, giant forms must have attained large adult sizes through new growth strategies, which remain largely unknown. Here we assess wing ontogeny and performance in the giant Pteranodon and the smaller-bodied anurognathids Rhamphorhynchus, Pterodactylus and Sinopterus. We show that most smaller-bodied pterosaurs shared negative allometry or isometry in the proximal elements of the fore- and hindlimbs, which were critical elements for powering both flight and terrestrial locomotion, whereas these show positive allometry in Pteranodon. Such divergent growth allometry typically signals different strategies in the precocial-altricial spectrum, suggesting more altricial development in Pteranodon. Using a biophysical model of powered and gliding flight, we test and reject the hypothesis that an aerodynamically superior wing planform could have enabled Pteranodon to attain its larger body size. We therefore propose that a shift from a plesiomorphic precocial state towards a derived state of enhanced parental care may have relaxed the constraints of small body sizes and allowed the evolution of derived flight anatomies critical for the flying giants.

Ecological and life-history drivers of avian skull evolution

One of the most famous examples of adaptive radiation is that of the Galápagos finches, where skull morphology, particularly the beak, varies with feeding ecology. Yet increasingly studies are questioning the strength of this correlation between feeding ecology and morphology in relation to the entire neornithine radiation, suggesting that other factors also significantly affect skull evolution. Here, we broaden this debate to assess the influence of a range of ecological and life-history factors, specifically habitat density, migration, and developmental mode, in shaping avian skull evolution. Using 3D geometric morphometric data to robustly quantify skull shape for 354 extant species spanning avian diversity, we fitted flexible phylogenetic regressions and estimated evolutionary rates for each of these factors across the full data set. The results support a highly significant relationship between skull shape and both habitat density and migration, but not developmental mode. We further found heterogenous rates of evolution between different character states within habitat density, migration, and developmental mode, with rapid skull evolution in species that occupy dense habitats, are migratory, or are precocial. These patterns demonstrate that diverse factors affect the tempo and mode of avian phenotypic evolution and that skull evolution in birds is not simply a reflection of feeding ecology.

Energy density of mesopelagic fishes from the Atlantic Ocean

Mesopelagic fishes play a central role in marine food webs linking primary consumers to top predators. In this study, measures of direct calorimetry were used to analyse the energy density (ED) of 34 mesopelagic species collected at 12 stations in the equatorial and tropical Atlantic. Mean ED ranged from 2.7 kJ g^-1 wet weight (WW) for the lanterfish Lampanyctus nobilis to 8.7 kJ g^-1 WW for the lanterfish Benthosema glaciale. This study includes species of the orders Myctophiformes and Stomiiformes and represents migrants, non-migrants and partial-migrants species. The majority of the species were grouped into the medium-energy quality category (ED from 4 to 6 kJ g^-1 ); Myctophiformes showed higher energetic values than the Stomiiformes. For the different species, the ED values were discussed in relation to spawning period and energy allocation strategies for reproduction and growth and feeding and migratory behaviour, as well as the ecoregion of study. These values will be useful for future assessment of energetic transfer between trophic levels and energetic modelling of Atlantic ecosystems.

A multi-tissue view on telomere dynamics and postnatal growth

Trade-offs between growth and self-maintenance are common in nature, such that early-life effects on growth can generate lasting consequences on survival and longevity. Telomeres-putative biomarkers of self-maintenance-may link early growth with these later phenotypic effects, but evidence for growth-telomere trade-offs is mixed. Null or even positive relationships between growth and telomeres may be driven by heterogeneity in resource availability or invariable allocation towards telomere maintenance within a population. We used nestling tree swallows (Tachycineta bicolor) to assess the directionality and timing of relationships between growth and telomere length in several tissues. We focused on two important phases of growth: first, the peak of postnatal growth occurring around 6 days old when nestlings grow by ~33% in a single day, and subsequently, the later phase of growth occurring as body mass plateaus near adult size at 12 days old. We quantified telomere attrition in blood during postnatal growth, as well as telomere length in the blood, brain, adrenals, and liver at 12 days old. Growth was unrelated to telomere length in the liver and telomere dynamics in blood. However, brain telomere length was positively correlated with peak growth, and adrenal telomere length was positively related to later growth, particularly for chicks that had experienced a temporary stressor. These observations suggest that variation in resource availability may mask trade-offs, generating positive correlations between growth and telomere length at the population level. They also provide insights into complex relationships between growth and self-maintenance that can be revealed by looking in multiple tissues.

Larger guts and faster growth in mice selected for high basal metabolic rate

Postnatal growth in birds and mammals is the time of highest vulnerability and relatively high energy demands and therefore shapes the organisms' future outcomes. Several different factors might impose limitations on growth in juveniles, one of them being the efficiency of the digestive process and size of the gastrointestinal tract. We tested the gut size-growth rate relationship using a unique experimental model-mice from a selection experiment designed to produce two lines with divergent levels of basal metabolic rate (BMR): the high BMR (H-BMR) and low BMR (L-BMR) line types. These lines differ with respect to not only BMR, but also correlated traits-internal organ size and food intake. Applying a cross-fostering design and a thermoregulatory burden imposed by shaving the mothers, we demonstrated that the mass of intestine strongly affected the growth rate, with the H-BMR pups having larger intestines and growing faster, and with reduced growth rate of pups of both lines nursed by shaved L-BMR mothers. Our study also provides a functional link between high growth rate of neonates and high BMR of adults, partly reflecting metabolic costs of maintenance of their guts.

Disentangling the avian altricial-precocial spectrum: Quantitative assessment of developmental mode, phylogenetic signal, and dimensionality

The altricial-precocial spectrum describes patterns of variation in avian developmental mode that greatly influence avian life histories. Appraising a given species' position on this spectrum is therefore fundamental to understanding patterns of avian life history evolution. However, evaluating avian developmental mode remains a relatively subjective task reliant on untested assumptions, including the notion that developmental strategies are distributed along a single dimension of statistical variation. Here, we present a quantitative multivariate framework that objectively discriminates among meaningfully different modes of avian development. We gathered information on seven hatchling and post-hatching traits for up to 4000 extant bird species, and find that most traits related to developmental mode show high phylogenetic signal and little intraclade variation, allowing unknown values to be reliably interpolated. Principal component analyses (PCAs) of these traits illustrate that most variation in hatchling state can be quantified along one dimension of trait space. However, our PCAs also reveal an important second dimension explaining variation in post-hatching behavior, enabling factors related to hatchling state and post-hatching behavior to be disentangled. In order to facilitate future macroevolutionary studies of variation in avian developmental strategies, as well as explorations of covariation between developmental mode and other aspects of avian biology, we present PC scores for 9993 extant avian species.

A histological survey of avian post-natal skeletal ontogeny

Bone histology of crown-group birds is a research topic of great interest, permitting insight into the evolution of remarkably high growth rates in this clade and variation across the altricial-precocial spectrum. In this study, we describe microanatomical characteristics of the humerus and femur in partial growth series from 14 crown group birds representing ten major clades (Struthioniformes, Galliformes, Apodiformes, Columbiformes, Charadriiformes, Accipitriformes, Strigiformes, Psittaciformes, Falconiformes, and Passeriformes). Our goals were to: (1) describe the microanatomy of each individual; (2) make inter-and intra-taxonomic comparisons; (3) assess patterns that correspond with developmental mode; and (4) to further parse out phylogenetic, developmental, and functional constraints on avian osteological development. Across taxa, the femoral and humeral tissue of neonates can be broadly characterized as highly-vascularized, disorganized woven bone with great variation in cortical thickness (inter-and intrataxonomically, within an individual specimen, and within a single section). The tissue of precocial chicks is relatively more mature at hatching than in altricial, but other categories along the developmental spectrum were less easy to distinguish, thus we were unable to identify a definitive histological proxy for developmental mode. We did not find evidence to support hypotheses that precocial chicks exclusively have thicker cortices and more mature bone in the femur than the humerus at time of hatching; instead, this is a characteristic of nearly all taxa (regardless of developmental mode), suggesting deep evolutionary origins and the effects of developmental channeling. Bone tissue in adults exhibited unexpected variation, corresponding to differences in body size. Large-bodied birds have cortices of fibrolamellar bone, but organization of tissue increases and vascularity decreases with diminishing body size. The outer circumferential layer (OCL) also appears at earlier growth stages in small-bodied taxa. Thus, while the OCL is indicative of a cessation of appositional growth it is not always indicative of cortical maturity (that is, maximum organization of bony tissue for a given taxon). Small size is achieved by truncating the period of fast growth; manipulation of the timing of offset of bone growth is therefore an important factor in changing growth trajectories to alter adult body size.

Nestling Physiology Is Independent of Somatic Development in a Common Raptor, the American Kestrel (Falco sparverius)

AbstractAlthough many studies have documented the developmental trajectory of somatic traits in birds, few measure physiological traits, and even fewer document individual variation in developmental trajectory across ecological context. Hematological traits underlying aerobic capacity can be predictive of nestling survival, fledgling flight ability, and ultimately recruitment. This study aimed to assess individual variation in the developmental trajectory of two physiological traits that underlie aerobic capacity, hematocrit and hemoglobin concentration, in relation to somatic development and ecological context. Our study species, the American kestrel (Falco sparverius), is sexually dimorphic and therefore likely to show sexual variation in developmental trajectory and nestling maturity. We used lay date, year, brood size, nestling sex ratio, and parental nest visit rate to assess ecological context. Although somatic traits showed similar trajectories across nestlings, developmental trajectory for hematocrit and hemoglobin concentration showed individual variation not previously documented. This individual variation in developmental change, or trajectory, for physiological traits could not be explained by somatic development, sex, parental nest visit rate, lay date, year, brood size, or nestling sex ratio. However, we did find higher final hemoglobin concentration in 2018 and in nests with earlier lay dates. These findings demonstrate the importance of assessing physiological traits that capture aspects of individual quality distinct from somatic traits. Future studies are needed to understand the causes of individual variation in developmental trajectory, which cannot be explained by the ecological variables presented here, and the potential fitness consequences of this variation.

Plumage development and environmental factors influence surface temperature gradients and heat loss in wandering albatross chicks

Young birds in cold environments face a range of age-specific thermal challenges. Studying the thermal biology of young birds throughout ontogeny may further our understanding of how such challenges are met. We investigated how age and environmental parameters influenced surface temperature gradients across various body regions of wandering albatross (Diomedea exulans) chicks on Bird Island, South Georgia. This study was carried out over a 200 d period during the austral winter, from the end of the brood-guard period until fledging, bridging a gap in knowledge of surface temperature variation and heat loss in developing birds with a long nestling stage in severe climatic conditions. We found that variation in surface temperature gradients (i.e. the difference between surface and environmental temperature) was strongly influenced by chick age effects for insulated body regions (trunk), with an increase in the surface temperature gradient that followed the progression of plumage development, from the second set of down (mesoptiles), to final chick feathers (teleoptiles). Environmental conditions (primarily wind speed and relative humidity) had a stronger influence on the gradients in uninsulated areas (eye, bill) than insulated regions, which we interpret as a reflection of the relative degree of homeothermy exhibited by chicks of a given age. Based on biophysical modelling, total heat loss of chicks was estimated to increase linearly with age. However, mass specific heat loss decreased during the early stages of growth and then subsequently increased. This was attributed to age-related changes in feather growth and activity that increased surface temperature and, hence, metabolic heat loss. These results provide a foundation for further work on the effects of environmental stressors on developing chicks, which are key to understanding the physiological responses of animals to changes in climate in polar regions.

Development syndromes in New World temperate and tropical songbirds

We studied avian development in 49 to 153 species of temperate and tropical New World passerine birds to determine how growth rates, and incubation and nestling periods, varied in relation to other life-history traits. We collected growth data and generated unbiased mass and tarsus growth rate estimates (mass n = 92 species, tarsus n = 49 species), and measured incubation period (n = 151) and nestling period (n = 153), which we analyzed with respect to region, egg mass, adult mass, clutch size, parental care type, nest type, daily nest predation rate (DMR), and nest height. We investigated covariation of life-history and natural-history attributes with the four development traits after controlling for phylogeny. Species in our lowland tropical sample grew 20% (incubation period), 25% (mass growth rate), and 26% (tarsus growth rate) more slowly than in our temperate sample. Nestling period did not vary with respect to latitude, which suggests that tropical songbirds fledge in a less well-developed state than temperate species. Suboscine species typically exhibited slower embryonic and post-embryonic growth than oscine passerines regardless of their breeding region. This pattern of slow development in tropical species could reflect phylogenetic effects based on unknown physiological attributes. Time-dependent nest mortality was unrelated to nestling mass growth rate, tarsus growth rate, and incubation period, but was significantly associated with nestling period. This suggests that nest predation, the predominant cause of nest loss in songbirds, does not exert strong selection on physiologically constrained traits, such as embryonic and post-embryonic growth, among our samples of temperate and lowland tropical songbird species. Nestling period, which is evolutionarily more labile than growth rate, was significantly shorter in birds exposed to higher rates of nest loss and nesting at lower heights, among other traits. Differences in life-history variation across latitudes provide insight into how unique ecological characteristics of each region influence physiological processes of passerines, and thus, how they can shape the evolution of life histories. While development traits clearly vary with respect to latitude, trait distributions overlap broadly. Life-history and natural history associations differ for each development trait, which suggests that unique selective pressures or constraints influence the evolution of each trait.

Ontogeny of the adrenocortical response in an extremely altricial bird

Life history theory predicts that physiological and behavioral responsiveness to stress should be delayed in development until the benefits of heightened reactivity outweigh the costs of potentially chronic glucocorticoid levels. Birds often acquire stress-responsiveness at locomotor independence, however, both stress-responsiveness and locomotor ability are delayed in birds with altricial developmental strategies. Parrots (Psittacidae) are extremely altricial, but it is not known whether they also postpone physiological responsiveness to stress until locomotor independence. We quantified individual variation in baseline and stress-induced plasma corticosterone (CORT) concentrations, the main avian glucocorticoid, in wild green-rumped parrotlets (Forpus passerinus) of Venezuela at four stages of nestling development. Parrotlet neonates are very underdeveloped and compete for parental care among extreme sibling size hierarchies, a competitive scenario that might benefit from early hypothalamic-pituitary-adrenal (HPA) functionality. Nestlings that underwent a standardized restraint stress-treatment showed higher average CORT concentrations compared to baseline in all age groups sampled, and exhibited no evidence of age-related changes in the stress response. This is 2 weeks before locomotor independence and earlier than previously documented for altricial species. Results suggest that precocity of HPA function may be advantageous to growth and survivorship in extremely altricial birds.

Variation in mandible development and its relationship to dependence on parents across burying beetles

BACKGROUND

In species with parental care, there is striking variation in offspring dependence at birth, ranging from feeding independence to complete dependency on parents for nutrition. Frequently, highly dependent offspring further evolve reductions or alterations of morphological traits that would otherwise promote self-sufficiency. Here, we examine evidence for morphological evolution associated with dependence in burying beetles (Nicrophorus spp.), in which dependence upon parents appears to have several independent origins. In many species, precocial first instar larvae can survive without parenting, but several altricial species die at this stage on their own. We focused specifically on the mandibles, which are expected to be related to feeding ability and therefore independence from parents.

RESULTS

We find no evidence that the size of the mandible is related to dependence on parents. However, we do find a developmental and phylogenetic correlation between independence and the presence of serrations on the inner edge of the mandible. Mandibles of independent species bear serrations at hatching, whereas dependent species hatch with smooth mandibles, only developing serrations in the second instar when these larvae gain the ability to survive on their own. Phylogenetic evidence suggests that serrations coincide with independence repeatedly. We note a single exception to this trend, a beetle with a serrated mandible that cannot survive without parents. However, this exception occurs in a species that has recently evolved the loss of independence.

CONCLUSIONS

We argue that the absence of mandible serrations occurs due to alternative selection pressures incurred in larvae dependent upon parents to survive. We suggest that this may have led to a variable function for mandibles, perhaps related to increased competitive ability among siblings or increased efficiency in receiving nutrition from parents. Furthermore, we propose that the phylogenetic pattern we see is consistent with the long-held evolutionary hypothesis that evolutionary change in behavior and physiology precede morphological change.

Bone histology provides insights into the life history mechanisms underlying dwarfing in hipparionins

Size shifts may be a by-product of alterations in life history traits driven by natural selection. Although this approach has been proposed for islands, it has not yet been explored in continental faunas. The trends towards size decrease experienced by some hipparionins constitute a good case study for the application of a life history framework to understand the size shifts on the continent. Here, we analysed bone microstructure to reconstruct the growth of some different-sized hipparionins from Greece and Spain. The two dwarfed lineages studied show different growth strategies. The Greek hipparions ceased growth early at a small size thus advancing maturity, whilst the slower-growing Spanish hipparion matured later at a small size. Based on predictive life history models, we suggest that high adult mortality was the likely selective force behind early maturity and associated size decrease in the Greek lineage. Conversely, we infer that resource limitation accompanied by high juvenile mortality triggered decrease in growth rate and a relative late maturity in the Spanish lineage. Our results provide evidence that different selective pressures can precipitate different changes in life history that lead to similar size shifts.

Formal comment on: Myhrvold (2016) Dinosaur metabolism and the allometry of maximum growth rate. PLoS ONE; 11(11): e0163205

In his 2016 paper, Myhrvold criticized ours from 2014 on maximum growth rates (Gmax, maximum gain in body mass observed within a time unit throughout an individual's ontogeny) and thermoregulation strategies (ectothermy, endothermy) of 17 dinosaurs. In our paper, we showed that Gmax values of similar-sized extant ectothermic and endothermic vertebrates overlap. This strongly questions a correct assignment of a thermoregulation strategy to a dinosaur only based on its Gmax and (adult) body mass (M). Contrary, Gmax separated similar-sized extant reptiles and birds (Sauropsida) and Gmax values of our studied dinosaurs were similar to those seen in extant similar-sized (if necessary scaled-up) fast growing ectothermic reptiles. Myhrvold examined two hypotheses (H1 and H2) regarding our study. However, we did neither infer dinosaurian thermoregulation strategies from group-wide averages (H1) nor were our results based on that Gmax and metabolic rate (MR) are related (H2). In order to assess whether single dinosaurian Gmax values fit to those of extant endotherms (birds) or of ectotherms (reptiles), we already used a method suggested by Myhrvold to avoid H1, and we only discussed pros and cons of a relation between Gmax and MR and did not apply it (H2). We appreciate Myhrvold's efforts in eliminating the correlation between Gmax and M in order to statistically improve vertebrate scaling regressions on maximum gain in body mass. However, we show here that his mass-specific maximum growth rate (kC) replacing Gmax (= MkC) does not model the expected higher mass gain in larger than in smaller species for any set of species. We also comment on, why we considered extant reptiles and birds as reference models for extinct dinosaurs and why we used phylogenetically-informed regression analysis throughout our study. Finally, we question several arguments given in Myhrvold in order to support his results.

Intraskeletal histovariability, allometric growth patterns, and their functional implications in bird-like dinosaurs

With their elongated forelimbs and variable aerial skills, paravian dinosaurs, a clade also comprising modern birds, are in the hotspot of vertebrate evolutionary research. Inferences on the early evolution of flight largely rely on bone and feather morphology, while osteohistological traits are usually studied to explore life-history characteristics. By sampling and comparing multiple homologous fore- and hind limb elements, we integrate for the first time qualitative and quantitative osteohistological approaches to get insight into the intraskeletal growth dynamics and their functional implications in five paravian dinosaur taxa, Anchiornis, Aurornis, Eosinopteryx, Serikornis, and Jeholornis. Our qualitative assessment implies a considerable diversity in allometric/isometric growth patterns among these paravians. Quantitative analyses show that neither taxa nor homologous elements have characteristic histology, and that ontogenetic stage, element size and the newly introduced relative element precocity only partially explain the diaphyseal histovariability. Still, Jeholornis, the only avialan studied here, is histologically distinct from all other specimens in the multivariate visualizations raising the hypothesis that its bone tissue characteristics may be related to its superior aerial capabilities compared to the non-avialan paravians. Our results warrant further research on the osteohistological correlates of flight and developmental strategies in birds and bird-like dinosaurs.

Development of activity rates in fledgling songbirds: when do young birds begin to behave like adults?

Logistical and technological limitations have constrained the characterization of behavioural development in songbirds, particularly during the post-fledging period. Recently, advances in radio-telemetry technology — automated radio-telemetry systems (ARTS) capable of recording continuous, high-resolution spatial-temporal data on radio-tagged birds — have opened the door to more comprehensive examinations of fledgling behaviour. We examined development of activity rates (number of times a bird was determined to have moved per number of detections, per hour) in fledgling Dickcissels using ARTS established at two grassland sites in east-central Illinois, USA. Specifically, we described general patterns of fledgling activity rates and determined the age at which fledglings began exhibiting activity rates like adults. We found that juveniles decreased activity rates immediately following fledging, likely to avoid detection by snakes and other predators, but increased activity levels throughout the rest of the post-fledging period. Peak hours of fledgling activity occurred around 0700 and 1800 h for all ages, with consistently low activity rates at night. On average, fledglings began exhibiting adult-like activity rates approximately 22 days after fledging, around roughly the same time they stopped being fed by adults. While our study provides important insights into development of fledgling behaviour in the Dickcissel, it remains unclear how patterns of behavioural development vary within and among species.

Variation in developmental trajectories of physiological and somatic traits in a common songbird approaching fledging

In avian species, little is known about the development of physiological traits in the days preceding fledging, a critical life history transition marked by a high mortality rate. Developmental trajectory during this period may be flexible based on ecological context or hardwired, with potential costs for variation in growth in the form of oxidative stress. Patterns in development are likely to relate to variation in life history, for which seabirds and aerial insectivores have been well studied, while our focal species is a grassland ground forager, the European starling (Sturnus vulgaris). We show that changes in haematocrit, body mass and wing length are independent of year and brood quality, while changes in haemoglobin concentration are higher in low-quality broods. Moreover, we also identify higher oxidative stress in low-quality year and second broods, a potential cost for maintaining a hardwired developmental trajectory in a lower quality environment. Finally, we experimentally test the effects of food supplementation on development and maturity of chicks at fledging to show that although food increases body mass early in development, it does not change the trajectory or final maturity of chicks at fledging. Collectively this study demonstrates that some developmental changes prior to fledging may be hardwired, but may have long-term oxidative costs in low-quality environments.

INVERSE RELATIONSHIP BETWEEN FUNCTIONAL MATURITY AND EXPONENTIAL GROWTH RATE OF AVIAN SKELETAL MUSCLE: A CONSTRAINT ON EVOLUTIONARY RESPONSE

In this study, we investigate whether a tissue-level constraint can explain the general inverse relationship between growth rate and precocity of development in birds. On the whole, altricial (dependent) chicks grow three to four times faster than the less dependent, more able chicks of precocial species of similar adult mass. We suggest that an antagonism between growth and acquisition of mature function in skeletal muscle constrains postnatal growth and development in most species of birds. Altricial species, represented by European starlings in this study, hatch with skeletal muscle having low capacity for generating force but grow rapidly. Conversely, precocial species (northern bobwhite quail and Japanese quail), hatch with relatively mature skeletal muscle, especially in their legs, but grow more slowly. As development proceeds in all species, exponential growth rates decrease as muscles acquire adult levels of function. Among four variables associated with muscle function, exponential growth rate (EGR) was negatively correlated with pyruvate kinase activity (glycolysis), potassium concentration (electrical potential), and dry weight fraction (contractile proteins) in both pectoral and leg muscles but not with citrate synthase activity (aerobic metabolism) in either set of muscles. For pectoral muscle, these variables accounted for 87% of the total variation in EGR in all three species combined despite a twofold difference in growth rates between the starling and quail. EGRs of leg muscle (51% of variation accounted for) were less than predicted by the pectoral-muscle equation in quail during the early part of the postnatal period and in starlings during the late postnatal period. This result would not contradict a growth rate/maturity constraint hypothesis if EGRs were down-regulated for allometric or other considerations.

Geographical and temporal variation in environmental conditions affects nestling growth but not immune function in a year-round breeding equatorial lark

Background

Variation in growth and immune function within and among populations is often associated with specific environmental conditions. We compared growth and immune function in nestlings of year-round breeding equatorial Red-capped Lark Calandrella cinerea from South Kinangop, North Kinangop and Kedong (Kenya), three locations that are geographically close but climatically distinct. In addition, we studied growth and immune function of lark nestlings as a function of year-round variation in breeding intensity and rain within one location. We monitored mass, wing, and tarsus at hatching (day 1) and at 4, 7, and 10 days post-hatch, and we quantified four indices of immune function (haptoglobin, agglutination, lysis and nitric oxide) using blood samples collected on day 10.

Results

Nestling body mass and size at hatching, which presumably reflect the resources that females allocated to their eggs, were lowest in the most arid location, Kedong. Contrary to our predictions, nestlings in Kedong grew faster than nestlings in the two other cooler and wetter locations of South and North Kinangop. During periods of peak reproduction within Kedong, nestlings were heavier at hatching, but they did not grow faster over the first 10 days post-hatch. In contrast, rainfall, which did not relate to timing of breeding, had no effect on hatching mass, but more rain did coincide with faster growth post-hatch. Finally, we found no significant differences in nestling immune function, neither among locations nor with the year-round variation within Kedong.

Conclusion

Based on these results, we hypothesize that female body condition determines nestling mass and size at hatching, but other independent environmental conditions subsequently shape nestling growth. Overall, our results suggest that environmental conditions related to food availability for nestlings are relatively unimportant to the timing of breeding in equatorial regions, while these same conditions do have consequences for nestling size and growth.

Ontogeny of sex differences in the energetics and kinematics of terrestrial locomotion in leghorn chickens (Gallus gallus domesticus)

Sex differences in locomotor performance may precede the onset of sexual maturity and/or arise concomitantly with secondary sex characteristics. Here, we present the first study to quantify the terrestrial locomotor morphology, energetics and kinematics in a species, either side of sexual maturation. In domestic leghorn chickens (Gallus gallus domesticus) sexual maturation brings about permanent female gravidity and increased male hind limb muscle mass. We found that the sexes of a juvenile cohort of leghorns shared similar maximum sustainable speeds, while in a sexually mature cohort maximum sustainable speeds were greater by 67% (males) and 34% (females). Furthermore, relative to that in juveniles of the same sex, the absolute duration of leg swing was longer in mature males and shorter in mature females. Consequently, the proportion of a stride that each limb was in contact with the ground (duty factor) was higher in sexually mature females compared to males. Modulation of the duty factor with the development of secondary sex characteristics may act to minimize mechanical work in males; and minimise mechanical power and/or peak force in females. A greater incremental response of mass-specific metabolic power to speed in males compared to females was common to both age cohorts and, therefore, likely results from physiological sexual dimorphisms that precede sexual maturation.

A long-term experimental study demonstrates the costs of begging that were not found over the short term

Parent-offspring conflict theory predicts that begging behaviour could escalate continuously over evolutionary time if it is not prevented by costliness of begging displays. Three main potential physiological costs have been proposed: growth, immunological and metabolic costs. However, empirical evidence on this subject remains elusive because published results are often contradictory. In this study, we test for the existence of these three potential physiological costs of begging in house sparrow (Passer domesticus) nestlings by stimulating a group of nestlings to beg for longer and another group for shorter periods than in natural conditions. All nestlings were fed with the same quantity of food. Our study involves a long-term experimental treatment for begging studies (five consecutive days). Long-term studies frequently provide clearer results than short-term studies and, sometimes, relevant information not reported by the latter ones. Our long-term experiment shows (i) a clear effect on the immune response even since the first measurement (6 hours), but it was higher during the second (long-term) than during the first (short-term) test; (ii) evidence of a growth cost of begging in house sparrow nestlings not previously found by other studies; (iii) body condition was affected by our experimental manipulation only after 48 hour; (iv) a metabolic cost of begging never previously shown in any species, and (v) for the first time, it has shown a simultaneous effect of the three potential physiological costs of begging: immunocompetence, growth, and metabolism. This implies first, that a multilevel trade-off can occur between begging and all physiological costs and, second, that a lack of support in a short-term experiment for the existence of a tested cost of begging does not mean absence of that cost, because it can be found in a long-term experiment.

Effects of hatching time on behavior and weight development of chickens

The length of the embryonic period varies both among and within species and can affect the individual phenotype in many ways, both physiologically and behaviorally. In chickens, the hatch window may last 24–48 hours (up to 10% of the incubation time), and studies have shown that incubation length may affect post-hatch growth and physiology. However, little is known about effects on behavior. We therefore investigated how behavior variation correlates with hatching time in the early life of chickens. We also measured egg weight and egg weight loss in relation to hatching time, as well as post-hatch growth. For females, there was a negative correlation between hatch time and body weight from day 4 and throughout the experiment. For males, such a correlation was only observed when testing all hatched males up until day 10. The birds were exposed to a number of behavioral tests, and a principal components analysis was performed on the variables, resulting in four components. For the largest component, termed “Passivity”, a tendency of a difference was found between early and middle male hatchers. Furthermore, a significant difference between early and middle male hatchers was found in the second component, termed “Response to novelty”. In a spatial learning test, late hatchers tended to learn slower. The behavior of females was not significantly affected by hatching time in any of these tests. This study is among the first to demonstrate a link between time of hatching and early behavior in a precocial species like the chicken, and may help shedding light on the evolutionary trade-offs between incubation length and post-hatch traits. The results may also be relevant from a perspective of stress coping and therefore also for animal welfare and productivity in the chicken industry. The mechanisms linking hatching time with post-hatch phenotype remain to be investigated.

Immediate and delayed effects of growth conditions on ageing parameters in nestling zebra finches

Conditions experienced during development and growth are of crucial importance as they can have significant influence on the optimisation of life histories. Indeed, the ability of an organism to grow fast and achieve a large body size often confers short and long term fitness benefits. However, there is good evidence that organisms do not grow at their maximal rates as growth rates seem to have potential costs on subsequent lifespan. Several proximate causes of such a reduced lifespan might be involved. Among them, one emerging hypothesis is that growth impacts adult survival and/or longevity through a shared, endpoint, ageing mechanism: telomere erosion. In this study, we manipulated brood size in order to investigate if rapid growth (chicks in reduced broods) is effectively done at the cost of a short (end of growth) and long term (at adulthood) increase of oxidative damage and telomere loss. Contrary to what we expected, chicks from the enlarged broods displayed more oxidative damage and had shorter telomeres at the end of the growth period and at adulthood. Our study extends the understanding of the proximate mechanisms involved in the trade-off between growth and ageing. It highlights that adverse environmental conditions during growth can come at a cost via transient increased oxidative stress and pervasive eroded telomeres. Indeed, it suggests that telomeres are not only controlled by intrinsic growth rates per se but may also be under the control of some extrinsic environmental factors that may get our understanding of the growth ageing interaction more complicated.

Ontogeny of muscle bioenergetics in Adélie penguin chicks (Pygoscelis adeliae)

The ontogeny of pectoralis muscle bioenergetics was studied in growing Adélie penguin chicks during the first month after hatching and compared with adults using permeabilized fibers and isolated mitochondria. With pyruvate-malate-succinate or palmitoyl-carnitine as substrates, permeabilized fiber respiration markedly increased during chick growth (3-fold) and further rose in adults (1.4-fold). Several markers of muscle fiber oxidative activity (cytochrome oxidase, citrate synthase, hydroxyl-acyl-CoA dehydrogenase) increased 6- to 19-fold with age together with large rises in intermyofibrillar (IMF) and subsarcolemmal (SS) mitochondrial content (3- to 5-fold) and oxidative activities (1.5- to 2.4-fold). The proportion of IMF relative to SS mitochondria increased with chick age but markedly dropped in adults. Differences in oxidative activity between mitochondrial fractions were reduced in adults compared with hatched chicks. Extrapolation of mitochondrial to muscle respirations revealed similar figures with isolated mitochondria and permeabilized fibers with carbohydrate-derived but not with lipid-derived substrates, suggesting diffusion limitations of lipid substrates with permeabilized fibers. Two immunoreactive fusion proteins, mitofusin 2 (Mfn2) and optic atrophy 1 (OPA1), were detected by Western blots on mitochondrial extracts and their relative abundance increased with age. Muscle fiber respiration was positively related with Mfn2 and OPA1 relative abundance. Present data showed by two complementary techniques large ontogenic increases in muscle oxidative activity that may enable birds to face thermal emancipation and growth in childhood and marine life in adulthood. The concomitant rise in mitochondrial fusion protein abundance suggests a role of mitochondrial networks in the skeletal muscle processes of bioenergetics that enable penguins to overcome harsh environmental constraints.

Body temperatures in dinosaurs: what can growth curves tell us?

To estimate the body temperature (BT) of seven dinosaurs Gillooly, Alleen, and Charnov (2006) used an equation that predicts BT from the body mass and maximum growth rate (MGR) with the latter preserved in ontogenetic growth trajectories (BT-equation). The results of these authors evidence inertial homeothermy in Dinosauria and suggest that, due to overheating, the maximum body size in Dinosauria was ultimately limited by BT. In this paper, I revisit this hypothesis of Gillooly, Alleen, and Charnov (2006). I first studied whether BTs derived from the BT-equation of today's crocodiles, birds and mammals are consistent with core temperatures of animals. Second, I applied the BT-equation to a larger number of dinosaurs than Gillooly, Alleen, and Charnov (2006) did. In particular, I estimated BT of Archaeopteryx (from two MGRs), ornithischians (two), theropods (three), prosauropods (three), and sauropods (nine). For extant species, the BT value estimated from the BT-equation was a poor estimate of an animal's core temperature. For birds, BT was always strongly overestimated and for crocodiles underestimated; for mammals the accuracy of BT was moderate. I argue that taxon-specific differences in the scaling of MGR (intercept and exponent of the regression line, log-log-transformed) and in the parameterization of the Arrhenius model both used in the BT-equation as well as ecological and evolutionary adaptations of species cause these inaccuracies. Irrespective of the found inaccuracy of BTs estimated from the BT-equation and contrary to the results of Gillooly, Alleen, and Charnov (2006) I found no increase in BT with increasing body mass across all dinosaurs (Sauropodomorpha, Sauropoda) studied. This observation questions that, due to overheating, the maximum size in Dinosauria was ultimately limited by BT. However, the general high inaccuracy of dinosaurian BTs derived from the BT-equation makes a reliable test of whether body size in dinosaurs was ultimately limited by overheating impossible.

Growth prior to thermogenesis for a quick fledging of Adélie penguin chicks (Pygoscelis adeliae)

The evolutionary trade-off between tissue growth and mature function restricts the post natal development of polar birds. The present study uses an original integrative approach as it includes gene expression, plus biochemical and physiological analysis to investigate how Adélie penguin chicks achieve a rapid growth despite the energetic constraints linked to the cold and the very short breeding season in Antarctica. In pectoralis muscle, the main thermogenic tissue in birds, our data show that the transition from ectothermy to endothermy on Day 15 post- hatching is associated with substantial and coordinated changes in the transcription of key genes. While the early activation of genes controlling cell growth and differentiation (avGHR, avIGF-1R, T3Rβ) is rapidly down-regulated after hatching, the global increase in the relative expression of genes involved in thermoregulation (avUCP, avANT, avLPL) and transcriptional regulation (avPGC1α, avT3Rβ) underlie the muscular acquisition of oxidative metabolism. Adélie chicks only become real endotherms at 15 days of age with the development of an oxidative muscle phenotype and the ability to shiver efficiently. The persistent muscular expression of IGF-1 throughout growth probably acts as a local mediator to adjust muscle size and its oxidative capacity to anticipate the new physiological demands of future Dives in cold water. The up-regulation of T3Rβ mRNA levels suggests that circulating T3 may play an important role in the late maturation of skeletal muscle by reinforcing, at least in part, the paracrine action of IGF-1. From day 30, the metabolic shift from mixed substrate to lipid metabolism, with the markedly increased mRNA levels of muscle avLPL, avANT and avUCP, suggests the late development of a fatty acid-enhanced muscle non-shivering thermogenesis mechanism. This molecular control is the key to this finely-tuned strategy by which the Adélie penguin chick successfully heads for the sea on schedule.

Precocial hindlimbs and altricial forelimbs: partitioning ontogenetic strategies in mallards (Anas platyrhynchos)

Precocial development, in which juveniles are relatively mature at hatching or birth, is more common among vertebrates than altricial development, and is likely to be the basal condition. Altricial development characterizes many birds and mammals and is generally viewed as an alternate strategy, promoting fast growth rates, short developmental periods and relatively poor locomotor performance prior to attaining adult size. Many aquatic birds such as Anseriformes (ducks, geese and swans), Charadriformes (gulls and terns) and Gruiformes (rails) undergo distinctive developmental trajectories, in that hatchlings are able to run and swim the day they hatch, yet they do not begin to fly until fully grown. We hypothesized that there should be tradeoffs in apportioning bone and muscle mass to the hindlimb and forelimb that could account for these patterns in locomotor behavior within the mallard (Anas platyrhynchos). Growth of the musculoskeletal system in the forelimbs and hindlimbs was measured and compared with maximal aquatic and terrestrial sprint speeds and aerial descent rates throughout the 2-month-long mallard ontogenetic period. At 30 days post hatching, when body mass is 50% of adult values, hindlimb muscle mass averages 90% and forelimb muscle mass averages 10% of adult values; similarly, bone growth (length and width) in the hindlimbs and forelimbs averages 90 and 60% of adult values, respectively. The attainment of mallard locomotor performance parallels the morphological maturation of forelimb and hindlimb morphometrics - hindlimb performance initiates just after hatching at a relatively high level (~50% adult values) and gradually improves throughout the first month of development, while forelimb performance is relatively non-existent at hatching (~10% adult values), experiencing delayed and dramatic improvement in function, and maturing at the time of fledging. This divergence in ontogenetic strategy between locomotor modules could allow developing Anseriformes to inhabit aquatic, predator-reduced refuges without relying on flight for juvenile escape. Furthermore, by freeing the forelimbs from locomotor demand early in ontogeny, Anseriformes may bypass the potential canalization (i.e. retention) of juvenile form present within their precocial hindlimbs, to dramatically depart in forelimb form and function in the adult.

Life history of Rhamphorhynchus inferred from bone histology and the diversity of pterosaurian growth strategies

BACKGROUND

Rhamphorhynchus from the Solnhofen Limestones is the most prevalent long tailed pterosaur with a debated life history. Whereas morphological studies suggested a slow crocodile-like growth strategy and superprecocial volant hatchlings, the only histological study hitherto conducted on Rhamphorhynchus concluded a relatively high growth rate for the genus. These controversial conclusions can be tested by a bone histological survey of an ontogenetic series of Rhamphorhynchus.

METHODOLOGY/PRINCIPAL FINDINGS

Our results suggest that Bennett's second size category does not reflect real ontogenetic stage. Significant body size differences of histologically as well as morphologically adult specimens suggest developmental plasticity. Contrasting the 'superprecocial hatchling' hypothesis, the dominance of fibrolamellar bone in early juveniles implies that hatchlings sustained high growth rate, however only up to the attainment of 30-50% and 7-20% of adult wingspan and body mass, respectively. The early fast growth phase was followed by a prolonged, slow-growth phase indicated by parallel-fibred bone deposition and lines of arrested growth in the cortex, a transition which has also been observed in Pterodaustro. An external fundamental system is absent in all investigated specimens, but due to the restricted sample size, neither determinate nor indeterminate growth could be confirmed in Rhamphorhynchus.

CONCLUSIONS/SIGNIFICANCE

The initial rapid growth phase early in Rhamphorhynchus ontogeny supports the non-volant nature of its hatchlings, and refutes the widely accepted 'superprecocial hatchling' hypothesis. We suggest the onset of powered flight, and not of reproduction as the cause of the transition from the fast growth phase to a prolonged slower growth phase. Rapidly growing early juveniles may have been attended by their parents, or could have been independent precocial, but non-volant arboreal creatures until attaining a certain somatic maturity to get airborne. This study adds to the understanding on the diversity of pterosaurian growth strategies.

A stab in the dark: chick killing by brood parasitic honeyguides

The most virulent avian brood parasites obligately kill host young soon after hatching, thus ensuring their monopoly of host parental care. While the host eviction behaviour of cuckoos (Cuculidae) is well documented, the host killing behaviour of honeyguide (Indicatoridae) chicks has been witnessed only once, 60 years ago, and never in situ in host nests. Here, we report from the Afrotropical greater honeyguide the first detailed observations of honeyguides killing host chicks with their specially adapted bill hooks, based on repeated video recordings (available in the electronic supplementary material). Adult greater honeyguides puncture host eggs when they lay their own, but in about half of host nests at least one host egg survived, precipitating chick killing by the honeyguide hatchling. Hosts always hatched after honeyguide chicks, and were killed within hours. Despite being blind and in total darkness, honeyguides attacked host young with sustained biting, grasping and shaking motions. Attack time of 1–5 min was sufficient to cause host death, which took from 9 min to over 7 h from first attack. Honeyguides also bit unhatched eggs and human hands, but only rarely bit the host parents feeding them.

Differential muscular myosin heavy chain expression of the pectoral and pelvic girdles during early growth in the king penguin (Aptenodytes patagonicus) chick

Continuous growth, associated with a steady parental food supply, is a general pattern in offspring development. So that young chicks can acquire their locomotor independence, this period is usually marked by a fast maturation of muscles, during which different myosin heavy chain (MyHC) isoforms are expressed. However, parental food provisioning may fluctuate seasonally, and offspring therefore face a challenge to ensure the necessary maturation of their tissues when energy is limited. To address this trade-off we investigated muscle maturation in both the pectoral and pelvic girdles of king penguin chicks. This species has an exceptionally long rearing period (1 year), which is prolonged when parental food provisioning is drastically reduced during the sub-Antarctic winter. Approximately 1 month post hatching, chicks acquire a functional pedestrian locomotion, which uses pelvic muscles, whereas swimming, which uses the pectoral muscles, only occurs 1 year later. We therefore tested the hypothesis that the MyHC content of the leg muscles reaches a mature state before those of the pectoral muscles. We found that leg muscle MyHC composition changed with the progressive acquisition of pedestrian locomotion, whereas pectoral muscle fibres reached their mature MyHC profile as early as hatching. Contrary to our predictions, the acquisition of the adult profile in pectoral muscles could be related to an early maturation of the contractile muscular proteins, presumably associated with early thermoregulatory capacities of chicks, necessary for survival in their cold environment. This differential maturation appears to reconcile both the locomotor and environmental constraints of king penguin chicks during growth.

When hatchlings outperform adults: locomotor development in Australian brush turkeys (Alectura lathami, Galliformes)

Within Galliformes, megapods (brush turkey, malleefowl, scrubfowl) exhibit unique forms of parental care and growth. Hatchlings receive no post-hatching parental care and exhibit the most exaggerated precocial development of all extant birds, hatching with fully developed, flight-capable forelimbs. Rather than flying up to safety, young birds preferentially employ wing-assisted incline running. Newly hatched Australian brush turkeys (Alectura lathami) are extraordinarily proficient at negotiating all textured inclined surfaces and can flap-walk up inclines exceeding the vertical. Yet, as brush turkeys grow, their forelimb-dependent locomotor performance declines. In an attempt to elucidate how hatchlings perform so well, we analysed hindlimb forces and forelimb kinematics. We measured ground reaction forces (GRFs) for animals spanning the entire growth range (110-2000 g) as they ascended a variably positioned inclined ramp that housed a forceplate. These data are compared with a similar dataset for a chukar partridge (Alectoris chukar) that exhibit a growth strategy typical of most other Galliformes and that demonstrate improved incline performance with increasing age. The brush turkeys' ontogenetic decline in incline running performance is accompanied by loss of traction at steep angles, reduced GRFs and increased wing-loading. We hypothesize that Australian brush turkeys, in contrast to other Galliformes, develop from forelimb-dominated young that exploit a variable terrain (e.g. mound nests, boulders, embankments, cliffs, bushes and trees) into hindlimb-dominated adults dependent on size and running speed to avoid predation.