Light during embryonic development modulates patterns of lateralization strongly and similarly in both zebrafish and chick

The effect of light during embryonic development on laterality and exploration in Western Rainbowfish

Several factors affect the development of lateralization such as hormones and light exposure during early development. Laterality also often correlates with other behavioral traits. To examine whether there is a common mechanism underlying the development of laterality and other behaviors, we manipulated laterality by exposing embryos of the Western rainbowfish (Melatotaenia australis) to light or continuous darkness during early development and determined whether a shift in laterality was associated with a change in behavior in a novel environment test at two different ages. We found that exposing eggs to darkness led to offspring that displayed significantly less lateralized behavior in the mirror test two weeks after hatching than offspring from eggs exposed to light. Interestingly, the effects of rearing condition were lost by 3 months of age. These data suggest that exposure to light can influence laterality very early in development, but such bias can be overwritten by developmental processes post-hatch. Moreover, our manipulation of laterality apparently had no influence on exploration suggesting independent causal mechanisms. The experimental manipulation of light exposure during development could be a useful tool for enhancing individuals with a specific laterality and behavioral traits to aid future research into the causes and consequences of laterality.

Unfolding a sequence of sensory influences and interactions in the development of functional brain laterality

Evidence of sensory experience influencing the development of lateralized brain and behavior is reviewed. The epigenetic role of light exposure during two specific stages of embryonic development of precocial avian species is a particular focus of the research discussed. Two specific periods of light sensitivity (in early versus late incubation), each depending on different subcellular and cellular processes, affect lateralized behavior after hatching. Auditory and olfactory stimulation during embryonic development is also discussed with consideration of interactions with light-generated visual lateralization.

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.

It Is Not Just in the Genes

Asymmetries in the functional and structural organization of the nervous system are widespread in the animal kingdom and especially characterize the human brain. Although there is little doubt that asymmetries arise through genetic and nongenetic factors, an overarching model to explain the development of functional lateralization patterns is still lacking. Current genetic psychology collects data on genes relevant to brain lateralizations, while animal research provides information on the cellular mechanisms mediating the effects of not only genetic but also environmental factors. This review combines data from human and animal research (especially on birds) and outlines a multi-level model for asymmetry formation. The relative impact of genetic and nongenetic factors varies between different developmental phases and neuronal structures. The basic lateralized organization of a brain is already established through genetically controlled embryonic events. During ongoing development, hemispheric specialization increases for specific functions and subsystems interact to shape the final functional organization of a brain. In particular, these developmental steps are influenced by environmental experiences, which regulate the fine-tuning of neural networks via processes that are referred to as ontogenetic plasticity. The plastic potential of the nervous system could be decisive for the evolutionary success of lateralized brains.

Stimulus characteristics, learning bias and visual discrimination in zebrafish (Danio rerio)

Zebrafish is an emerging model in the study of brain function; however, knowledge about its behaviour and cognition is incomplete. Previous studies suggest this species has limited ability in visual learning tasks compared to other teleosts. In this study, we systematically examined zebrafish's ability to learn to discriminate colour, shape, size, and orientation of figures using an appetitive conditioning paradigm. Contrary to earlier reports, the zebrafish successfully completed all tasks. Not all discriminations were learned with the same speed and accuracy. Subjects discriminated the size of objects better than their shape or colour. In all three tasks, they were faster and more accurate when required to discriminate between outlined figures than between filled figures. With stimuli consisting of outlines, the learning performance of zebrafish was comparable to that observed in higher vertebrates. Zebrafish easily learned a horizontal-vertical discrimination task, but like many other vertebrates, they had great difficulty discriminating a figure from its mirror image. Performance was more accurate for subjects reinforced on one stimulus (green over red, triangle over circle, large over small). Unexpectedly, these stimulus biases occurred only when zebrafish were tested with filled figures, suggesting some causal relationship between stimulus preference, learning bias and performance.

Effect of exposure to different light colors on embryonic development and neurophysiological traits in the chick embryo

Background and Aim:

Many environmental factors exist that influence embryonic development which is missing in the poultry industry, such as light in incubation facilities or hatcheries. Light plays an important role in the growth and development of chick embryos, whereas dark environments can lead to hatching failure or embryo distortion. Therefore, this study aimed to demonstrate the importance of light and its various colors on the growth and development of broiler chick embryos.

Materials and Methods:

Four treatments were used to study the impact of various light colors on the growth of embryos and their neurophysiological traits: Dark without light (D), red light (RL), blue light (BL), and green light (GL), with three replicates per treatment (25 eggs/replicate) for a total of 300 fertile Ross 308 eggs. Each treatment was assigned to one incubator (75 eggs/incubator), whereas all other conditions were kept the same.

Results:

The results showed a significant increase (p<0.01) in embryonic development for embryo weight, chick body weight, hatchability, and embryo index for RL, BL, and especially GL. RL, BL, and especially GL significantly increased (p<0.01) neurophysiological traits of the neurons, brain weight, and brain index.

Conclusion:

The use of light during the embryonic period affects the development of the embryo and its neurophysiological traits.

Influence of light intensity and photoperiod on embryonic development, survival and growth of threatened catfish Ompok bimaculatus early larvae

Larval growth and survival of catfishes are largely influenced by the various biotic and abiotic factors. The present study investigated the effect of different light intensities and photoperiods on growth and survival of Ompok bimaculatus larvae. Three separate trials of 21 days each were carried out in an aquarium tank. The first trial investigated the embryonic changes (based on hatching rate and time) upon exposure to varied light intensity (0, 300, 500, 900 and 1200 lx) and photoperiodic regime (24l:0d, 16l:8d, 12l:12d, 8l:16d and 0l:24d). Subsequently, hatched-out larvae were subjected to the aforementioned intensities (Trial II) and photoperiod (Trial III, intensity of 300 lx) for growth and survival attributes. Eight hundred healthy larvae (average body weight = 0.003 g) were randomly distributed into five treatment groups for the last two trials. Results suggest a higher embryo hatching rate and larval survival at 0 and 300 lx, whereas the largest larval growth was observed at 900 lx. In Trial III, survival was highest in 0l:24d and growth in 24l:0d and 16l:8d was higher (P < 0.05). Performance index was higher (P < 0.05) in both 0 and 300 lx light and decreased at higher intensities. The overall interpretation from the present study concludes that a completely dark rearing environment is recommended for better survival of O. bimaculatus although growth was compromised.

Prenatal Visual Exposure to a Predator Influences Lateralization in Goldbelly Topminnows

The role of genetic and environmental factors in modulating the development of brain lateralization is far from being fully understood, and the presence of individual differences in several lateralized functions is still an open question. In goldbelly topminnows, the genetic basis of asymmetrical functions in the brain has been studied, and recently it has been found that light stimulation influences the expression of lateralization of newborns. Here, we investigated whether prenatal exposure to predators affects the development of lateralization in 10-day-old topminnows born from females exposed to a real or to a simulated predator during pregnancy. Offspring from females exposed to a real predator were lateralized in both visual and motor tests, whereas fish from females exposed to a simulated predator were not and did not differ from controls. Prenatal exposure to a real predator might promote the alignment of lateralization in the same direction in different individuals.

Brain and Behavioral Asymmetry: A Lesson From Fish

It is widely acknowledged that the left and right hemispheres of human brains display both anatomical and functional asymmetries. For more than a century, brain and behavioral lateralization have been considered a uniquely human feature linked to language and handedness. However, over the past decades this idea has been challenged by an increasing number of studies describing structural asymmetries and lateralized behaviors in non-human species extending from primates to fish. Evidence suggesting that a similar pattern of brain lateralization occurs in all vertebrates, humans included, has allowed the emergence of different model systems to investigate the development of brain asymmetries and their impact on behavior. Among animal models, fish have contributed much to the research on lateralization as several fish species exhibit lateralized behaviors. For instance, behavioral studies have shown that the advantages of having an asymmetric brain, such as the ability of simultaneously processing different information and perform parallel tasks compensate the potential costs associated with poor integration of information between the two hemispheres thus helping to better understand the possible evolutionary significance of lateralization. However, these studies inferred how the two sides of the brains are differentially specialized by measuring the differences in the behavioral responses but did not allow to directly investigate the relation between anatomical and functional asymmetries. With respect to this issue, in recent years zebrafish has become a powerful model to address lateralization at different level of complexity, from genes to neural circuitry and behavior. The possibility of combining genetic manipulation of brain asymmetries with cutting-edge in vivo imaging technique and behavioral tests makes the zebrafish a valuable model to investigate the phylogeny and ontogeny of brain lateralization and its relevance for normal brain function and behavior.

Vegetation cover induces developmental plasticity of lateralization in tadpoles

Lateralization of cognitive functions influences a large number of fitness-related behaviors and shows, in most species, substantial variation in strength and direction. Laboratory works and field data have suggested that this variation is often due to adaptive phenotypic plasticity. Strong lateralization should be favored in some ecological conditions, for example, under high risk of predation. For anuran tadpoles, the presence of cover affects predation risk, with tadpoles being more exposed to predators in environments with reduced cover. We tested the hypothesis that the amount of cover experienced early in life affects lateralization in the edible frog, Pelophylax esculentus, tadpoles. We exposed embryos and larvae to high or low vegetation cover environments. For half of the subjects, the treatment was constant whereas the remaining subjects were switched to the opposite treatment after hatching. In agreement with the theoretical expectation, tadpoles exposed to low vegetation cover for the entire development were more lateralized and showed a stronger alignment in directionality of lateralization compared with tadpoles exposed to high vegetation cover. This indicates a possible role of natural variation in vegetation abundance and developmental plasticity as determinants of between-population and between-individual differences in lateralization. We also found that shifting from high to low vegetation cover treatments and vice versa disrupted lateralization alignment, suggesting that developmental trajectories for this trait are determined at the embryonic stage and need environmental stability to be fully expressed.

Distinct effect of early and late embryonic light-stimulation on chicks' lateralization

Embryonic light exposure affects similarly functional lateralization in fish and birds. While the light acts on an asymmetric habenular system during the first post fertilization hours in zebrafish, in the domestic chicks it shapes the thalamofugal visual pathway affecting the right retinal photoreceptors in the last stages before hatching. However, recent evidence has shown that also in chicks a precocial embryonic time window seems open to light action. Here we addressed the issue of whether the light effect is comparable in the early and late sensitive periods of chicks' embryonic development by testing birds coming from early (EL) and late (LL) light stimulated eggs compared to dark maintained eggs (DK) under different conditions of vision in a gravel-grain task. The perseveration of pecks directed to irrelevant elements revealed that in all chicks the right hemisphere was heavily attracted by the novel elements when tested with the left eye. When using the right eye, instead, only DK chicks attended repeatedly to distractors whereas LL and EL chicks showed a left hemisphere advantage for fine discrimination and sustained attention; conversely, when tested binocularly, LL chicks perseverated significantly more than both DK and EL chicks, likely compensating the distraction with the analysis carried out by both hemispheres. For the first time, we unveiled a fine graded difference between the light modulation exerted during the two time windows, adding evidence to the idea that genes and environmental factors interplay in several separate routes to the modulation of the neurodevelopment of cerebral lateralization in vertebrates.

Effects of different light spectra on embryo development and the performance of newly hatched turbot (Scophthalmus maximus) larvae

Light is a key environmental factor that synchronizes various life stages from embryo development to sexual maturation in fish. For turbot, light spectra have the most influence at the larval and juvenile stages. In the current study, differences in the development of embryos and the performance of newly hatched turbot larvae exposed to five different spectra: full spectrum (LDF), blue (LDB, peak at 450 nm), green (LDG, peak at 533 nm), orange (LDO, peak at 595 nm) and red (LDR, peak at 629 nm), were examined. At 62.8 h post fertilization, a higher number of embryos exposed to short-wavelengths (LDG and LDB) had developed a heartbeat in comparison with embryos exposed to other wavelengths. Larvae exposed to the green spectrum had higher malformation rates than larvae exposed to the other spectra, indicating that larvae exposed to green light may have significantly reduced survival rates. The results of non-specific immunity parameters showed that the mRNA expression levels of cathepsin D (CTSD), cathepsin F (CTSF), catalase (CAT) and metallothionein (MT) in larvae exposed to LDB were significantly higher than those exposed to other spectra, but CAT activity in larvae exposed to LDB was significantly lower than larvae exposed to the other spectra. There was no significant difference in MT activity in larvae exposed to the five different spectra. The mRNA expression level of lysozyme (LZM) in larvae exposed to LDR was significantly higher than other spectra, while there was no significant difference in LZM activity observed in larvae exposed to LDR, LDG, LDB and LDF. The difference of the enzyme activity of total superoxide dismutase (T-SOD) was not significant among larvae exposed to the five spectra. mRNA expression of the heat shock protein 70 (HSP70) was significantly higher in newly hatched larvae exposed to LDB, LDR and LDG, indicating that larvae exposed to LDB, LDG and LDR exhibited a stress response. The mRNA expression level of the insulin-like growth factor-1 (IGF-1) and growth parameters in the newly hatched larvae exposed to the different spectra were not significantly different. The results of the present study indicate that LDO and LDF should be used for embryo incubation and newly hatched larvae when rearing turbot. This study provides a theoretical basis for optimizing the incubation light environment for fertilized turbot eggs, promoting immunity and reducing stress responses in newly hatched larvae.

Lateral asymmetry in the freely occurring behaviour of budgerigars (Melopsittacus undulatus) and its relation to cognitive performance

Hemispheric laterality and its provision of potential fitness advantages to aves is a widespread topic of research in budgerigars and other parrot species [e.g., Magat, M. & Brown, C. (2009). Laterality enhances cognition in Australian parrots. Proceedings of the Royal Society B: Biological Sciences, 276, 4155-4162]. In the current study, lateral preferences were tracked for 11 captive-reared budgerigars that subsequently underwent 2 cognitive problem-solving tests: a tool-use problem and a dig-discrimination task. Two significant individual-level lateral preferences (a leftward unipedal support preference and a rightward preening side preference) and 1 significant population-level preference (a right-side preening preference) were obtained. Lateral preening side preference indices for individual subjects correlated with various measures of performance on the dig task, such that right-side preening preferences and stronger lateral preferences irrespective of direction were associated with enhanced dig task performance. The data generally support the idea that laterality in aves serves as a good predictor of increased cognitive capability, while also offering new evidence for population-level lateral preferences in the species.

Lateralized scale-eating behaviour of cichlid is acquired by learning to use the naturally stronger side

The scale-eating cichlid Perissodus microlepis exhibits significant lateralised predation behaviour using an asymmetric mouth. But how the acquisition of the behavioural laterality depends, if at all, on experience during development remains obscure. Here, naïve juveniles were tested in a series of predation sessions. Initially, they attacked both sides of the prey, but during subsequent sessions, attack direction gradually lateralised to the skewed mouth (dominant) side. Attack side preference of juveniles that had accumulated scale-eating experience during successive sessions was significantly higher than that of naïve juveniles at the same age and naïve adults. Thus, the lateralised behaviour was a learned experience, and did not develop with age. Surprisingly, however, both maximum amplitude and angular velocity of body flexion during attack of naïve fish was dominant on one side. Therefore, scale-eating fish have a naturally stronger side for attacking prey fish, and they learn to use the dominant side through experience.

The effects of light exposure during incubation on embryonic development and hatchling traits in lizards

Light is an environmental factor that is known to profoundly affect embryonic development in some oviparous vertebrates, but such effects are unstudied in reptiles. We investigated the light sensitivity of lizard embryos by examining the thickness and light transmittance of eggshells as well as the effect of light on embryonic development and hatchling traits in four lizard species, the Chinese skink (Plestiodon chinensis), the northern grass lizard (Takydromus septentrionalis), the oriental leaf-toed gecko (Hemidactylus bowringii) and the Japanese gecko (Gekko japonicus). The eggshells were thinner and thus had higher light transmittance in Chinese skink than the other three species. Light exposure during incubation significantly accelerated the embryonic development in all species, with higher light intensity resulting in faster embryonic development. Interestingly, light stimulation negatively influenced hatchling size and survival in skinks, but had no effect in lacertids and geckos. This interspecific discrepancy not only relates to the differences in thickness and light transmittance of eggshells, but might also reflect the differences in the reproductive habits of these species. Given the diversity of light conditions that reptile embryos face during development, studies on the response of reptile embryos to light may offer a unique opportunity to understand the mechanisms of embryonic light sensitivity in animals.

Acquisition of Lateralized Predation Behavior Associated with Development of Mouth Asymmetry in a Lake Tanganyika Scale-Eating Cichlid Fish

The scale-eating cichlid Perissodus microlepis with asymmetric mouth is an attractive model of behavioral laterality: each adult tears off scales from prey fishes' left or right flanks according to the direction in which its mouth is skewed. To investigate the development of behavioral laterality and mouth asymmetry, we analyzed stomach contents and lower jaw-bone asymmetry of various-sized P. microlepis (22 ≤ SL<115 mm) sampled in Lake Tanganyika. The shapes of the pored scales found in each specimen's stomach indicated its attack side preference. Early-juvenile specimens (SL<45 mm) feeding mainly on zooplankton exhibited slight but significant mouth asymmetry. As the fish acquired scale-eating (45 mm ≤ SL), attack side preference was gradually strengthened, as was mouth asymmetry. Among size-matched individuals, those with more skewed mouths ate more scales. These findings show that behavioral laterality in scale-eating P. microlepis is established in association with development of mouth asymmetry which precedes the behavioral acquisition, and that this synergistic interaction between physical and behavioral literalities may contribute to efficient scale-eating.

Early-light embryonic stimulation suggests a second route, via gene activation, to cerebral lateralization in vertebrates

Genetic factors determine the asymmetrical position of vertebrate embryos allowing asymmetric environmental stimulation to shape cerebral lateralization. In birds, late-light stimulation, just before hatching, on the right optic nerve triggers anatomical and functional cerebral asymmetries. However, some brain asymmetries develop in absence of embryonic light stimulation. Furthermore, early-light action affects lateralization in the transparent zebrafish embryos before their visual system is functional. Here we investigated whether another pathway intervenes in establishing brain specialization. We exposed chicks' embryos to light before their visual system was formed. We observed that such early stimulation modulates cerebral lateralization in a comparable vein of late-light stimulation on active retinal cells. Our results show that, in a higher vertebrate brain, a second route, likely affecting the genetic expression of photosensitive regions, acts before the development of a functional visual system. More than one sensitive period seems thus available to light stimulation to trigger brain lateralization.

Laterality influences cognitive performance in rainbowfish Melanotaenia duboulayi

Cerebral lateralization has been suggested to convey a selective advantage to individuals by enhancing their cognitive abilities. Few, however, have explicitly compared the cognitive ability of animals with strongly contrasting laterality. Here, we examined the influence of laterality on learning performance in the crimson spotted rainbowfish, Melanotaenia duboulayi, using a classical conditioning paradigm. We also compared the learning ability of wild caught and captive-reared fish to examine the influence of rearing environment on cognitive performance. Laterality was established by observing which eye fish preferred to use while viewing their mirror image. Subjects were then conditioned to associate the appearance of a red light with a food reward over 7 days. Our results revealed that left-lateralized fish learned the conditioning task faster than right-lateralized. These results provide further evidence that cerebral lateralization can play important roles in cognitive function which likely have diverse fitness consequences for animals in their natural environments.

Waste nitrogen metabolism and excretion in zebrafish embryos: effects of light, ammonia, and nicotinamide

Bony fish primarily excrete ammonia as adults however the persistence of urea cycle genes may reflect a beneficial role for urea production during embryonic stages in protecting the embryo from toxic effects of ammonia produced from a highly nitrogenous yolk. This study aimed to examine the dynamic scope for changes in rates of urea synthesis and excretion in one such species (zebrafish, Danio rerio) by manipulating the intrinsic developmental rate (by alteration of light:dark cycles), as well as by direct chemical manipulation via ammonia injection (to potentially activate urea production) and nicotinamide exposure (to potentially inhibit urea production). Continuous dark exposure delayed development in embryos as evidenced by delayed appearance of hallmark anatomical features (heartbeat, eye pigmentation, body pigmentation, lateral line, fin buds) at 30 and 48 hr post-fertilization, as well by a lower hatching rate compared to embryos reared in continuous light. Both ammonia and urea excretion were similarly effected and were generally higher in embryos continuously exposed to light. Ammonia injection resulted in significant increases (up to fourfold) of urea N excretion and no changes to ammonia excretion rates along with modest increases in yolk ammonia content during 2-6 hr post-injection. Nicotinamide (an inhibitor of urea synthesis in mammals) reduced the ammonia-induced increase in urea excretion and led to retention of ammonia in the yolk and body of the embryo. Our results indicate that there is a relatively rapid and large scope for increases in urea production/excretion rates in developing embryos. Potential mechanisms for these increases are discussed.

Embryonic exposure to predator odour modulates visual lateralization in cuttlefish

Predation pressure acts on the behaviour and morphology of prey species. In fish, the degree of lateralization varies between high- and low-predation populations. While lateralization appears to be widespread in invertebrates, we do not know whether heredity and early experience interact during development as in vertebrates. Here we show, for the first time, that an exposure to predator odour prior to hatching modulates visual lateralization in newly hatched cuttlefish. Only cuttlefish that have been exposed to predator odour display a left-turning bias when tested with blank seawater in a T-shaped apparatus. Exposure to predator odour all the incubation long could appear as an acute predictor of a high-predation surrounding environment. In addition, cuttlefish of all groups display a left-turning preference when tested with predator odour in the apparatus. This suggests the ability of cuttlefish to innately recognize predator odour. To our knowledge, this is the first clear demonstration that lateralization is vulnerable to ecological challenges encountered during embryonic life, and that environmental stimulation of the embryo through the olfactory system could influence the development of subsequent visual lateralization.

Cerebral correlates of visual lateralization in Sepia

The common cuttlefish, Sepia officinalis (cephalopod mollusc) has recently become a relevant model for studying the setting-up of brain asymmetry among invertebrates. As the animals age from 3 to 30 days post hatching, they progressively develop a left-turning bias resulting from an eye-use preference. The aim of this study is to investigate whether anatomical (vertical, peduncle, inferior buccal, and optic lobes) or neurochemical (monoamines in optic lobes) brain asymmetries are present in the cuttlefish brain at 3 or at 30 post hatching days; and whether these correlate with side-turning preferences. We here find brain and behavioral asymmetry only at 30 post hatching days. Cuttlefish displayed a significant population bias towards a larger right peduncle lobe, and higher monoamine concentration in the left optic lobe (i.e. serotonin, dopamine and noradrenaline). None of these brain asymmetries were correlated to the studied side-turning bias. However, we found individual variation in the magnitude of the vertical and optic lobes asymmetry. A striking correlation was found with the behavioral results: the larger the right optic lobe and the right part of the vertical lobe, the stronger the bias to turn leftwards. To our knowledge, this is the first study to demonstrate a relationship at the individual level between brain and behavioral asymmetries in invertebrates.

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.

Asymmetry of different brain structures in homing pigeons with and without navigational experience

Homing pigeons (Columba livia f.d.) are well-known for their homing abilities, and their brains seem to be functionally adapted to homing as exemplified, e.g. by their larger hippocampi and olfactory bulbs. Their hippocampus size is influenced by navigational experience, and, as in other birds, functional specialisation of the left and right hemispheres (‘lateralisation’) occurs in homing pigeons. To show in what way lateralisation is reflected in brain structure volume, and whether some lateralisation or asymmetry in homing pigeons is caused by experience, we compared brains of homing pigeons with and without navigational experience referring to this. Fourteen homing pigeons were raised under identical constraints. After fledging, seven of them were allowed to fly around the loft and participated successfully in races. The other seven stayed permanently in the loft and thus did not share the navigational experiences of the first group. After reaching sexual maturity, all individuals were killed and morphometric analyses were carried out to measure the volumes of five basic brain parts and eight telencephalic brain parts. Measurements of telencephalic brain parts and optic tectum were done separately for the left and right hemispheres. The comparison of left/right quotients of both groups reveal that pigeons with navigational experience show a smaller left mesopallium in comparison with the right mesopallium and pigeons without navigational experience a larger left mesopallium in comparison with the right one. Additionally, there are significant differences between left and right brain subdivisions within the two pigeon groups, namely a larger left hyperpallium apicale in both pigeon groups and a larger right nidopallium, left hippocampus and right optic tectum in pigeons with navigational experience. Pigeons without navigational experience did not show more significant differences between their left and right brain subdivisions. The results of our study confirm that the brain of homing pigeons is an example for mosaic evolution and indicates that lateralisation is correlated with individual life history (experience) and not exclusively based on heritable traits.

The costs of hemispheric specialization in a fish

Laboratory and field studies have documented better cognitive performance associated with marked hemispheric specialization in organisms as diverse as chimpanzees, domestic chicks and topminnows. While providing an evolutionary explanation for the emergence of cerebral lateralization, this evidence represents a paradox because a large proportion of non-lateralized (NL) individuals is commonly observed in animal populations. Hemispheric specialization often determines large left-right differences in perceiving and responding to stimuli. Using topminnows selected for a high or low degree of lateralization, we tested the hypothesis that individuals with greater functional asymmetry pay a higher performance cost in situations requiring matching information from the two eyes. When trained to use the middle door in a row of a nine, NL fish correctly chose the central door in most cases, while lateralized fish showed systematic leftward or rightward biases. When choosing between two shoals, each seen with a different eye, NL fish chose the high-quality shoal significantly more often than the lateralized fish, whose performance was affected by eye preference for analysing social stimuli. These findings suggest the existence of a trade-off between computational advantages of hemispheric specialization and the ecological cost of making suboptimal decisions whenever relevant information is located on both sides of the body.

Lateralised visual processing in domestic cattle herds responding to novel and familiar stimuli

We investigated whether cattle exhibit preferences to monitor challenging and novel stimuli. Experiments were conducted on dairy and beef cattle herds and revealed significant left eye preferences in the cattle for viewing an experimenter walking to repeatedly split the herd through its centre. Visual lateralisation was demonstrated in the preference to use the left monocular field to monitor the experimenter, alone or equipped with a range of novel stimuli. This finding is consistent with left eye preferences found in various species of mammals, birds, and amphibians responding to predators and novel stimuli. A cohort of the familiarized cattle herds was then subjected to additional herd-splitting tests with the same stimuli and demonstrated a reversal of viewing preferences, preferring to monitor the experimenter and stimuli within the right and not left monocular field. This directional shift in viewing preferences is consistent with experience-dependent learning found in lateralised visual processing in other, non-mammalian, species, and to our knowledge is the first of such studies to suggest that such lateralised learning processes also exist in mammals. Together the data support a number of key hypotheses concerning the evolution and conservation of lateralised brain function in vertebrates, and also provide important considerations for livestock handling.

The habenular nuclei: a conserved asymmetric relay station in the vertebrate brain

The dorsal diencephalon, or epithalamus, contains the bilaterally paired habenular nuclei and the pineal complex. The habenulae form part of the dorsal diencephalic conduction (DDC) system, a highly conserved pathway found in all vertebrates. In this review, we shall describe the neuroanatomy of the DDC, consider its physiology and behavioural involvement, and discuss examples of neural asymmetries within both habenular circuitry and the pineal complex. We will discuss studies in zebrafish, which have examined the organization and development of this circuit, uncovered how asymmetry is represented at the level of individual neurons and determined how such left–right differences arise during development.

Mechanisms and functions of brain and behavioural asymmetries

For almost a century the field of brain and behavioural asymmetries has been dominated by studies on humans, resting on the evidence that the anatomical structures underlying language functions are asymmetrical, and that human handedness is lateralized at the population level. Today, there is not only evidence of population-level lateralization of brain and behaviour across a variety of vertebrate and invertebrate species, but also a growing consensus that the comparative analysis of the environmental and developmental factors that give origin to neural and behavioural laterality in animal models, together with theoretical analyses of their costs and benefits, will be crucial for understanding the evolutionary pathways that led to such a multifaceted phenomenon. The present theme issue provides a survey of theoretical, review and research work cutting across the biological and the cognitive sciences, focusing on various species of fishes, birds and primates (including humans) and emphasizing an integrative approach to the study of lateralization encompassing neural, behavioural, cognitive, developmental and environmental aspects.