Forebrain specialization and the olfactory system in anseriform birds. An architectonic and tracing study

Comparative growth in the olfactory system of the developing chick with considerations for evolutionary studies

Despite the long-held assumption that olfaction plays a relatively minor role in the behavioral ecology of birds, crown-group avians exhibit marked phylogenetic variation in the size and form of the olfactory apparatus. As part of a larger effort to better understand the role of olfaction and olfactory tissues in the evolution and development of the avian skull, we present the first quantitative analysis of ontogenetic scaling between olfactory features [olfactory bulbs (OBs) and olfactory turbinates] and neighboring structures (cerebrum, total brain, respiratory turbinates) based on the model organism Gallus gallus. The OB develops under the predictions of a concerted evolutionary model with rapid early growth that is quickly overcome by the longer, sustained growth of the larger cerebrum. A similar pattern is found in the nasal cavity where the morphologically simple (non-scrolled) olfactory turbinates appear and mature early, with extended growth characterizing the larger and scrolled respiratory turbinates. Pairwise regressions largely recover allometric relationships among the examined structures, with a notable exception being the isometric trajectory of the OB and olfactory turbinate. Their parallel growth suggests a unique regulatory pathway that is likely driven by the morphogenesis of the olfactory nerve, which serves as a structural bridge between the two features. Still, isometry was not necessarily expected given that the olfactory epithelium covers more than just the turbinate. These data illuminate a number of evolutionary hypotheses that, moving forward, should inform tradeoffs and constraints between the olfactory and neighboring systems in the avian head.

Efferent and afferent connections of the olfactory bulb and prepiriform cortex in the pigeon (Columba livia)

Although olfaction in birds is known to be involved in a variety of behaviors, there is comparatively little detailed information on the olfactory brain. In the pigeon brain, the olfactory bulb (OB) is known to project to the prepiriform cortex (CPP), piriform cortex (CPi), and dorsolateral corticoid area (CDL), which together are called the olfactory pallium, but centrifugal pathways to the OB have not been fully explored. Fiber connections of CPi and CDL have been reported, but those of other olfactory pallial nuclei remain unknown. The present study examines the fiber connections of OB and CPP in pigeons to provide a more detailed picture of their connections using tract-tracing methods. When anterograde and retrograde tracers were injected in OB, projections to a more extensive olfactory pallium were revealed, including the anterior olfactory nucleus, CPP, densocellular part of the hyperpallium, tenia tecta, hippocampal continuation, CPi, and CDL. OB projected commissural fibers to the contralateral OB but did not receive afferents from the contralateral olfactory pallium. When tracers were injected in CPP, reciprocal ipsilateral connections with OB and nuclei of the olfactory pallium were observed, and CPP projected to the caudolateral nidopallium and the limbic system, including the hippocampal formation, septum, lateral hypothalamic nucleus, and lateral mammillary nucleus. These results show that the connections of OB have a wider distribution throughout the olfactory pallium than previously thought and that CPP provides a centrifugal projection to the OB and acts as a relay station to the limbic system.

Anatomical specializations for nocturnality in a critically endangered parrot, the Kakapo (Strigops habroptilus)

The shift from a diurnal to nocturnal lifestyle in vertebrates is generally associated with either enhanced visual sensitivity or a decreased reliance on vision. Within birds, most studies have focused on differences in the visual system across all birds with respect to nocturnality-diurnality. The critically endangered Kakapo (Strigops habroptilus), a parrot endemic to New Zealand, is an example of a species that has evolved a nocturnal lifestyle in an otherwise diurnal lineage, but nothing is known about its' visual system. Here, we provide a detailed morphological analysis of the orbits, brain, eye, and retina of the Kakapo and comparisons with other birds. Morphometric analyses revealed that the Kakapo's orbits are significantly more convergent than other parrots, suggesting an increased binocular overlap in the visual field. The Kakapo exhibits an eye shape that is consistent with other nocturnal birds, including owls and nightjars, but is also within the range of the diurnal parrots. With respect to the brain, the Kakapo has a significantly smaller optic nerve and tectofugal visual pathway. Specifically, the optic tectum, nucleus rotundus and entopallium were significantly reduced in relative size compared to other parrots. There was no apparent reduction to the thalamofugal visual pathway. Finally, the retinal morphology of the Kakapo is similar to that of both diurnal and nocturnal birds, suggesting a retina that is specialised for a crepuscular niche. Overall, this suggests that the Kakapo has enhanced light sensitivity, poor visual acuity and a larger binocular field than other parrots. We conclude that the Kakapo possesses a visual system unlike that of either strictly nocturnal or diurnal birds and therefore does not adhere to the traditional view of the evolution of nocturnality in birds.

Telencephalic organization of the olfactory system in homing pigeons (Columba livia)

Pigeons use olfactory cues to navigate over unfamiliar areas, and any impairment of the olfactory system generates remarkable reduction of homing performance. Lesion and deprivation studies suggest a critical involvement of the right nostril and thus, the right olfactory bulb (OB) and the left piriform cortex (CPi) for initial orientation. This functional pattern suggests that OB and CPi are asymmetrically connected with a stronger projection from the right OB to the left CPi. However, the structural organization of the olfactory system is not unequivocally clarified yet. Thus, we re-analyzed the system by antero- and retrograde tract tracing with biotinylated dextran amine and choleratoxin subunit B, and we especially evaluated quantitative differences in the number of cells in the OB innervating the left and right CPi. Our anterograde tracing data verified a strong bilateral input to the CPi, and the prepiriform cortex (CPP), as well as small projections to the ipsilateral medial septum and the dorsolateral corticoid area and the nucleus taeniae of the amygdala in both hemispheres. Apart from the bilateral bulbar afferents, CPi in turn receives unequivocal input from the ipsilateral CPP, hyperpallium densocellulare, dorsal arcopallium, and from a cluster of cells located within the frontolateral nidopallium. Thus, an indirect connection between OB and CPi is only mediated by the CPP. For quantitative analysis of bulbar input to the CPi, we counted the number of ipsi- and contralaterally projecting neurons located in the OB after injections into the left or right CPi. Retrogradely labeled cells were found bilaterally in the OB with a higher number of ipsilaterally located cells. The bilaterality index did not differ after left- or right-sided CPi injections indicating that the functional lateralization of the olfactory system is not simply based on differences in the number of projecting axons of the major processing stream.

Cladistic analysis of olfactory and vomeronasal systems

Most tetrapods possess two nasal organs for detecting chemicals in their environment, which are the sensory detectors of the olfactory and vomeronasal systems. The seventies’ view that the olfactory system was only devoted to sense volatiles, whereas the vomeronasal system was exclusively specialized for pheromone detection was challenged by accumulating data showing deep anatomical and functional interrelationships between both systems. In addition, the assumption that the vomeronasal system appeared as an adaptation to terrestrial life is being questioned as well. The aim of the present work is to use a comparative strategy to gain insight in our understanding of the evolution of chemical “cortex.” We have analyzed the organization of the olfactory and vomeronasal cortices of reptiles, marsupials, and placental mammals and we have compared our findings with data from other taxa in order to better understand the evolutionary history of the nasal sensory systems in vertebrates. The olfactory and vomeronsasal cortices have been re-investigated in garter snakes (Thamnophis sirtalis), short-tailed opossums (Monodelphis domestica), and rats (Rattus norvegicus) by tracing the efferents of the main and accessory olfactory bulbs using injections of neuroanatomical anterograde tracers (dextran-amines). In snakes, the medial olfactory tract is quite evident, whereas the main vomeronasal-recipient structure, the nucleus sphaericus is a folded cortical-like structure, located at the caudal edge of the amygdala. In marsupials, which are acallosal mammals, the rhinal fissure is relatively dorsal and the olfactory and vomeronasal cortices relatively expanded. Placental mammals, like marsupials, show partially overlapping olfactory and vomeronasal projections in the rostral basal telencephalon. These data raise the interesting question of how the telencephalon has been re-organized in different groups according to the biological relevance of chemical senses.

Neurobiology of the homing pigeon--a review

Homing pigeons are well known as good homers, and the knowledge of principal parameters determining their homing behaviour and the neurological basis for this have been elucidated in the last decades. Several orientation mechanisms and parameters-sun compass, earth's magnetic field, olfactory cues, visual cues-are known to be involved in homing behaviour, whereas there are still controversial discussions about their detailed function and their importance. This paper attempts to review and summarise the present knowledge about pigeon homing by describing the known orientation mechanisms and factors, including their pros and cons. Additionally, behavioural features like motivation, experience, and track preferences are discussed. All behaviour has its origin in the brain and the neuronal basis of homing and the neuroanatomical particularities of homing pigeons are a main topic of this review. Homing pigeons have larger brains in comparison to other non-homing pigeon breeds and particularly show increased size of the hippocampus. This underlines our hypothesis that there is a relationship between hippocampus size and spatial ability. The role of the hippocampus in homing and its plasticity in response to navigational experience are discussed in support of this hypothesis.

The role of uropygial gland on sexual behavior in domestic chicken Gallus gallus domesticus

Recent studies have indicated that avian social behavior is influenced by olfactory cues. During the reproductive season a change in the chemical composition of uropygial gland secretion has been reported in some species and the hypothesis that olfactory signals may be produced by this gland has been proposed. To examine this hypothesis we performed two behavioral experiments to determine whether a female's uropygial gland produces chemical signals that stimulate mating behaviors in domestic chickens. In Experiment 1 the role of the female's uropygial gland in male mating behavior was examined by removing and examining the female's uropygial gland. The frequency of mounts and copulations of intact male birds with sham-operated female birds was significantly higher than with uropygial glandectomized female birds. With respect to the number of waltzing that is one of the courtship displays intact males showed no significant difference between sham-operated female birds and uropygial glandectomized female birds. In Experiment 2 the relationship between male olfaction and the female's uropygial gland was investigated using olfactory bulbectomized male birds. The number of mounts and copulations of sham-operated male birds with sham-operated female bird was significantly higher than with uropygial glandectomized female birds. In contrast olfactory bulbectomized male birds showed no significant differences in the number of mounts and copulations between sham-operated female birds and uropygial glandectomized female birds. These results indicate that intact and sham-operated male birds prefer to mate with female birds with the uropygial gland. The number of courtship waltzing of sham-operated male birds showed no significant difference. However olfactory bulbectomized male birds significantly courted to uropygial glandectomized female birds. Summarizing our results show that while anosmic males did not have any preference, both intact and sham-operated male birds chose to mate with female birds having an intact uropygial gland, suggesting that mate preference involves in male olfaction and that the female's uropygial gland acts as a source of social odor cues in domestic chickens.

Mosaic evolution and adaptive brain component alteration under domestication seen on the background of evolutionary theory

Brain sizes and brain component sizes of five domesticated pigeon breeds including homing (racing) pigeons are compared with rock doves (Columba livia) based on an allometric approach to test the influence of domestication on brain and brain component size. Net brain volume, the volumes of cerebellum and telencephalon as a whole are significantly smaller in almost all domestic pigeons. Inside the telencephalon, mesopallium, nidopallium (+ entopallium + arcopallium) and septum are smaller as well. The hippocampus is significantly larger, particularly in homing pigeons. This finding is in contrast to the predictions of the 'regression hypothesis' of brain alteration under domestication. Among the domestic pigeons homing pigeons have significantly larger olfactory bulbs. These data are interpreted as representing a functional adaptation to homing that is based on spatial cognition and sensory integration. We argue that domestication as seen in domestic pigeons is not principally different from evolution in the wild, but represents a heuristic model to understand the evolutionary process in terms of adaptation and optimization.

Anseriform brain and its parts versus taxonomic and ecological categories

The size of the brain and its macro-anatomical parts in 206 birds representing 19 anseriform species and 4 tribes (Anserini, Anatini, Aythyini and Mergini) was the subject of a comparative analysis. The comparisons involved two aspects: taxonomic (differences among species within tribes and differences among tribes) and ecological (diet composition: vegetation, invertebrates, or fish and the foraging mode: browsing, dabbling, shallow diving, and deep diving). The relative size of the encephalon (E) and its parts (optic tectum, OT; cerebellum, C; brain stem, BS; hemispheres, H) were described using appropriate indices. Five of them, called the cerebral-body indices (E/BW, OT/BW, C/BW, BS/BW, H/BW), involved a ratio between the weight of E or its parts and that of the body (BW). Four intracerebral indices (OT/E, C/E, BS/E, H/E) and allometric equations were used as well. Almost all the indices showed a high intraspecific variability within the Anserini and Mergini; on the other hand, the intracerebral indices did not differ between the species of the Anatini and Aythyini (except for OT/E in the Aythyini). Between-tribe differences were reflected in all 9 indices. The birds feeding on different diets were found to differ in their OT/E and H/E. The herbivorous anserifom OT/E was clearly lower than that of those birds feeding on invertebrates and fish. The highest OT/E was that of the piscivorous birds. In terms of foraging mode, significant differences were revealed in 7 out of the 9 indices used (differences in OT/BW and C/BW proved non-significant). OT/E of the browsing birds was clearly lower than that of the deep diving ducks; BS/E of the browsers was much lower than that of the dabbling and shallow diving ducks. Geese and swans (browsers) showed much higher H/E compared to the deep diving sea ducks. The latter revealed the highest C/E, but significant differences were detected only in comparison with C/E of the shallow diving ducks. The taxonomic (among tribes) and ecological comparisons showed more differences in the intracerebral indices than in the cerebral-body indices.

Interspecific Allometry of the Brain and Brain Regions in Parrots (Psittaciformes): Comparisons with Other Birds and Primates

Despite significant progress in understanding the evolution of the mammalian brain, relatively little is known of the patterns of evolutionary change in the avian brain. In particular, statements regarding which avian taxa have relatively larger brains and brain regions are based on small sample sizes and statistical analyses are generally lacking. We tested whether psittaciforms (parrots, cockatoos and lorikeets) have larger brains and forebrains than other birds using both conventional and phylogenetically based methods. In addition, we compared the psittaciforms to primates to determine if cognitive similarities between the two groups were reflected by similarities in brain and telencephalic volumes. Overall, psittaciforms have relatively larger brains and telencephala than most other non-passerine orders. No significant difference in relative brain or telencephalic volume was detected between psittaciforms and passerines. Comparisons of other brain region sizes between psittaciforms and other birds, however, exhibited conflicting results depending upon whether body mass or a brain volume remainder (total brain volume - brain region volume) was used as a scaling variable. When compared to primates, psittaciforms possessed similar relative brain and telencephalic volumes. The only exception to this was that in some analyses psittaciforms had significantly larger telencephala than primates of similar brain volume. The results therefore provide empirical evidence for previous claims that psittaciforms possess relatively large brains and telencephala. Despite the variability in the results, it is clear that psittaciforms tend to possess large brains and telencephala relative to non-passerines and are similar to primates in this regard. Although it could be suggested that this reflects the advanced cognitive abilities of psittaciforms, similar studies performed in corvids and other avian taxa will be required before this claim can be made with any certainty.

Fate map of the avian anterior forebrain at the four-somite stage, based on the analysis of quail-chick chimeras

To better understand the topological organization of the primordia within the anterior forebrain, we made a fate map of the rostral neural plate in the chick. Homotopic grafts at the four-somite stage were allowed to survive for up to 9 days to enable an analysis of definitive brain structures. In some cases, the topography of the grafted neuroepithelia was compared with gene expression patterns. The midpoint of the anterior neural ridge maps upon the anterior commissure in the closed neural tube, continuing concentrically into the preoptic area and optic field. Non-neural epithelium just in front of this median ridge gives rise to the adenohypophysis. Areas for the presumptive pallial commissure, septum, and prosencephalic choroidal tissue lie progressively more posteriorly along the ridge, peripheral to the telencephalic entopeduncular and striatopallidal primordia (the subpallium), and the pallium (olfactory bulb, dorsal ventricular ridge, and cortical domains). Subpallial structures lie topologically anterior to the pallial formations, and both are concentric to the septum. Within the pallium, the major cortical domains (Wulst and caudolateral, parahippocampal, and hippocampal cortices) appear posterior to the dorsal ventricular ridge. The amygdaloid region appears concentrically across both the subpallial and pallial regions. This fate map shows that the arrangement of the prospective primordia in the neural plate is basically a flattened representation of topological relationships present in the mature brain, though marked phenomena of differential growth and selective tangential migration of some cell populations complicate the histogenetic constitution of the mature telencephalon.

Magnetic resonance imaging of intracranial tissue accumulations in domestic ducks (Anas platyrhynchos f. dom.) with feather crests

The crested breed of domestic duck (Anas platyrhynchos f. dom.) has been described as a variety which has high pre- and postnatal mortalities, malformations in skull and brain anatomy, and several central nervous deficiencies. In addition, intracranial tissue accumulations have been diagnosed in purebred Crested ducks. The incidence, heredity and inheritance of these accumulations as well as their pathogenesis are still generally unknown. The aims of this study were to examine the head of Crested ducks, plain-headed duck breeds, and their crossbreeding relating to the incidence of intracranial alterations. These examinations were performed using magnetic resonance imaging. We found a high incidence of intracranial tissue accumulations in domestic ducks with feather crests. Creasted ducks had more intracranial tissue deposits than plain-headed ducks (p < 0.001). In the present study a correlation between the volume of the crest cushion and the volume of the intracranial tissue deposit could not be found (r = 0.014). Some of the Crested ducks had encephaloceles in addition to the crest cushion.

Electroencephalographic study of odor responses in the domestic fowl

Although avian olfaction has been studied behaviorally and anatomically, few electrophysiological (EEG) studies exist. The purpose of this study is to examine the characteristics of the olfactory function of the fowl with EEG recording. We found that slow waves decreased and the rapid waves increased in response to a variety of odors.