“Let There be Light!” pigeon eggs are regularly exposed to light during breeding

Covariation between glucocorticoid levels and receptor expression modulates embryo development and postnatal phenotypes in gulls

The hypothalamic-pituitary-adrenocortical axis can translate, through glucocorticoid secretion, the prenatal environment to development to produce phenotypes that match prevailing environmental conditions. However, whether developmental plasticity is modulated by the interaction between circulating glucocorticoids and receptor expression remains unclear. Here, we tested whether covariation between plasma corticosterone (CORT) and glucocorticoid receptor gene (Nr3c1) expression in blood underlies embryonic developmental programming in yellow-legged gulls (Larus michahellis). We examined variations in circulating levels of CORT and the expression and DNA methylation patterns of Nr3c1 in response to two ecologically relevant prenatal factors: adult alarm calls (a cue of predator presence) and changes in prenatal light environment (a cue of competitive disadvantage). We then determined whether embryonic development and postnatal phenotypes were associated with CORT levels and Nr3c1 expression, and explored direct and indirect relationships between the prenatal environment, hormone-receptor covariation, and postnatal phenotypes. Prenatal exposure to alarm calls increased CORT levels and up-regulated Nr3c1 expression in gull chicks, while exposure to light cues reduced both hormone levels and receptor expression. Chicks prenatally exposed to alarm calls showed altered DNA methylation profiles in the Nr3c1 regulatory region, but patterns varied throughout the breeding season and between years. Moreover, our results suggest a negative relationship between DNA methylation and expression in Nr3c1 , at least at specific CpG sites. The interplay between circulating CORT and Nr3c1 expression affected embryo developmental timing and vocalizations, as well as hatchling mass and fitness-relevant behaviours. These findings provide a link between prenatal inputs, glucocorticoid function and phenotypic outcomes, suggesting that hormone-receptor interaction may underlie developmental programming in free-living animals.

Visual categories and concepts in the avian brain

Birds are excellent model organisms to study perceptual categorization and concept formation. The renewed focus on avian neuroscience has sparked an explosion of new data in the field. At the same time, our understanding of sensory and particularly visual structures in the avian brain has shifted fundamentally. These recent discoveries have revealed how categorization is mediated in the avian brain and has generated a theoretical framework that goes beyond the realm of birds. We review the contribution of avian categorization research-at the methodical, behavioral, and neurobiological levels. To this end, we first introduce avian categorization from a behavioral perspective and the common elements model of categorization. Second, we describe the functional and structural organization of the avian visual system, followed by an overview of recent anatomical discoveries and the new perspective on the avian 'visual cortex'. Third, we focus on the neurocomputational basis of perceptual categorization in the bird's visual system. Fourth, an overview of the avian prefrontal cortex and the prefrontal contribution to perceptual categorization is provided. The fifth section outlines how asymmetries of the visual system contribute to categorization. Finally, we present a mechanistic view of the neural principles of avian visual categorization and its putative extension to concept learning.

Light received by embryos promotes postnatal junior phenotypes in a seabird

Light is a salient and variable ecological factor that can impact developmental trajectories of vertebrate embryos, yet whether prenatal light environment can act as an anticipatory cue preparing organisms to cope with postnatal conditions is still unclear. In asynchronous birds, last-laid eggs are particularly exposed to sunlight as parental incubation behavior becomes intermittent after the hatching of senior chicks. Here, we explore whether natural variations in prenatal light exposure shape the distinctive phenotype showed by last-hatched chicks of a semi-precocial seabird, the yellow-legged gull (Larus michahellis), potentially preparing them to cope with the postnatal competitive context. To do this, we manipulated the amount of light received by last-laid eggs (within a natural range) during last stages of embryonic development. Prenatal exposure to light cues promoted the development of the resilient “junior phenotype” exhibited by last-hatched gull chicks, characterized by accelerated hatching, increased begging behavior and a slower growth rate. These developmental and behavioral adjustments were accompanied by down-regulation of genes involved in metabolism and development regulation (SOD2 and TRalpha), as well as changes in the HPA-axis functioning (lower baseline corticosterone and robust adrenocortical response). Junior chicks exposed to light cues during the embryonic development showed longer telomeres during the early postnatal period, suggesting that light-induced adjustments could allow them to buffer the competitive disadvantages associated with hatching asynchrony. Our study provides evidence that postnatal junior phenotypes are, at least in part, prenatally shaped by light cues that act during a critical temporal window of developmental sensitivity.

Light-induced asymmetries in embryonic retinal gene expression are mediated by the vascular system and extracellular matrix

Left–right asymmetries in the nervous system (lateralisation) influence a broad range of behaviours, from social responses to navigation and language. The role and pathways of endogenous and environmental mechanisms in the ontogeny of lateralisation remains to be established. The domestic chick is a model of both endogenous and experience-induced lateralisation driven by light exposure. Following the endogenous rightward rotation of the embryo, the asymmetrical position in the egg results in a greater exposure of the right eye to environmental light. To identify the genetic pathways activated by asymmetric light stimulation, and their time course, we exposed embryos to different light regimes: darkness, 6 h of light and 24 h of light. We used RNA-seq to compare gene expression in the right and left retinas and telencephalon. We detected differential gene expression in right vs left retina after 6 h of light exposure. This difference was absent in the darkness condition and had already disappeared by 24 h of light exposure, suggesting that light-induced activation is a self-terminating phenomenon. This transient effect of light exposure was associated with a downregulation of the sensitive-period mediator gene DIO2 (iodothyronine deiodinase 2) in the right retina. No differences between genes expressed in the right vs. left telencephalon were detected. Gene networks associated with lateralisation were connected to vascularisation, cell motility, and the extracellular matrix. Interestingly, we know that the extracellular matrix—including the differentially expressed PDGFRB gene—is involved in morphogenesis, sensitive periods, and in the endogenous chiral mechanism of primary cilia, that drives lateralisation. Our data show a similarity between endogenous and experience-driven lateralisation, identifying functional gene networks that affect lateralisation in a specific time window.

Light-incubation effects on lateralisation of single unit responses in the visual Wulst of domestic chicks

Since the ground-breaking discovery that in-egg light exposure triggers the emergence of visual lateralisation, domestic chicks became a crucial model for research on the interaction of environmental and genetic influences for brain development. In domestic chick embryos, light exposure induces neuroanatomical asymmetries in the strength of visual projections from the thalamus to the visual Wulst. Consequently, the right visual Wulst receives more bilateral information from the two eyes than the left one. How this impacts visual Wulst's physiology is still unknown. This paper investigates the visual response properties of neurons in the left and right Wulst of dark- and light-incubated chicks, studying the effect of light incubation on bilaterally responsive cells that integrate information from both eyes. We recorded from a large number of visually responsive units, providing the first direct evidence of lateralisation in the neural response properties of units of the visual Wulst. While we confirm that some forms of lateralisation are induced by embryonic light exposure, we found also many cases of light-independent asymmetries. Moreover, we found a strong effect of in-egg light exposure on the general development of the functional properties of units in the two hemispheres. This indicates that the effect of embryonic stimulation goes beyond its contribution to the emergence of some forms of lateralisation, with influences on the maturation of visual units in both hemispheres.

The importance of illumination in nest site choice and nest characteristics of cavity nesting birds

Light has a significant impact on many aspects of avian biology, physiology and behaviour. An increasing number of studies show that illumination may positively influences birds' offspring fitness by e.g. acceleration of embryo development, stimulation of skeleton growth or regulation of circadian rhythm. Because nest cavities have especially low illumination, suitable light levels may be especially important for species which nest there. We may therefore expect that birds breeding in relatively dim conditions should prefer brighter nest sites and/or evolve behavioral mechanisms to secure sufficient light levels in the nest. Using nest boxes with modified internal illumination, we experimentally tested whether light regime is a cue for nest site selection of secondary cavity-nesting species. Additionally, we investigated whether nest building strategies are tuned to internal illumination. Our results demonstrate that, nest boxes with elevated illumination were chosen twice as often as dark nest boxes. Moreover, birds built higher nests in dark nest boxes than birds in boxes with elevated illumination, which suggests a mechanism of compensating for low light conditions. Our results provide the first experimental support for the idea that nest site choice and nest building behaviour in cavity-nesting birds are influenced by ambient illumination.

Cryptochrome 1b: a possible inducer of visual lateralization in pigeons?

The visual system of adult pigeons shows a lateralization of object discrimination with a left hemispheric dominance on the behavioural, physiological and anatomical levels. The crucial trigger for the establishment of this asymmetry is the position of the embryo inside the egg, which exposes the right eye to light falling through the egg shell. As a result, the right-sided retina is more strongly stimulated with light during embryonic development. However, it is unknown how this embryonic light stimulation is transduced to the brain as rods and cones are not yet functional. A possible solution could be the blue-light-sensitive molecule cryptochrome 1 (Cry1), which is expressed in retinal ganglion cells (RGCs) of several mammalian and avian species. RGCs have been shown to be functional during the time of induction of asymmetry and possess projections to primary visual areas. Therefore, Cry1-containing RGCs could be responsible for induction of asymmetry. The aim of this study was to identify the expression pattern of the Cry1 subtype Cry1b in the retina of embryonic, post-hatch and adult pigeons by immunohistochemical staining and to show whether Cry1b-containing RGCs project to the optic tectum. Cry1b-positive cells were indeed mainly found in the RGC layer and to lesser extent in the inner nuclear layer at all ages, including the embryonic stage. Tracing in adult animals revealed that at least a subset of Cry1b-containing RGCs project to the optic tectum. Thus, Cry1b-containing RGCs within the embryonic retina could be involved in the induction of asymmetries in the visual system of pigeons.

Hemispheric asymmetries: the comparative view

Hemispheric asymmetries play an important role in almost all cognitive functions. For more than a century, they were considered to be uniquely human but now an increasing number of findings in all vertebrate classes make it likely that we inherited our asymmetries from common ancestors. Thus, studying animal models could provide unique insights into the mechanisms of lateralization. We outline three such avenues of research by providing an overview of experiments on left-right differences in the connectivity of sensory systems, the embryonic determinants of brain asymmetries, and the genetics of lateralization. All these lines of studies could provide a wealth of insights into our own asymmetries that should and will be exploited by future analyses.

Posture in ovo as a precursor of footedness in ostriches (Struthio camelus)

Two categories of behaviour involving lateralized posture were observed in semi-natural conditions in ostriches (Struthio camelus). Observing preferences for left or right foot, both in the forward foot posture (the foot standing in front at rest) and the starting foot used to initiate locomotion, a population-level right-foot preference was shown for the whole group and for each of the three age ranges considered (chick, young and adult). Ostriches are known to rely upon a lateralized behaviour during hatching (using their right foot to break the egg shell) suggesting the hypothesis that the precocious motor laterality observed at hatching might stand as a precursor of limb preference later in development, as already observed in other avian species.

Epigenesis of behavioural lateralization in humans and other animals

Despite several decades of research, the epigenesis of behavioural and brain lateralization is still elusive, although its knowledge is important in understanding developmental plasticity, function and evolution of lateralization, and its relationship with developmental disorders. Over the last decades, it has become clear that behavioural lateralization is not restricted to humans, but a fundamental principle in the organization of behaviour in vertebrates. This has opened the possibility of extending descriptive studies on human lateralization with descriptive and experimental studies on other vertebrate species. In this review, we therefore explore the evidence for the role of genes and environment on behavioural lateralization in humans and other animals. First, we discuss the predominant genetic models for human handedness, and conclude that their explanatory power alone is not sufficient, leaving, together with ambiguous results from adoption studies and selection experiments in animals, ample opportunity for a role of environmental factors. Next, we discuss the potential influence of such factors, including perinatal asymmetrical perception induced by asymmetrical head position or parental care, and social modulation, both in humans and other vertebrates, presenting some evidence from our own work on the domestic chick. We conclude that both perinatal asymmetrical perception and later social modulation are likely candidates in influencing the degree or strength of lateralization in both humans and other vertebrates. However, in most cases unequivocal evidence for this is lacking and we will point out further avenues for research.

Laterality and flight: concurrent tests of side-bias and optimality in flying tree swallows

Behavioural side-bias occurs in many vertebrates, including birds as a result of hemispheric specialization and can be advantageous by improving response times to sudden stimuli and efficiency in multi-tasking. However, behavioural side-bias can lead to morphological asymmetries resulting in reduced performance for specific activities. For flying animals, wing asymmetry is particularly costly and it is unclear if behavioural side-biases will be expressed in flight; the benefits of quick response time afforded by side-biases must be balanced against the costs of less efficient flight due to the morphological asymmetry side-biases may incur. Thus, competing constraints could lead to context-dependent expression or suppression of side-bias in flight. In repeated flight trials through an outdoor tunnel with obstacles, tree swallows (Tachycineta bicolor) preferred larger openings, but we did not detect either individual or population-level side-biases. Thus, while observed behavioural side-biases during substrate-foraging and copulation are common in birds, we did not see such side-bias expressed in obstacle avoidance behaviour in flight. This finding highlights the importance of behavioural context for investigations of side-bias and hemispheric laterality and suggests both proximate and ultimate trade-offs between species-specific cognitive ecology and flight biomechanics.