Gerd Rehkämper

Quantitative Development of Brain and Brain Structures in Birds (Galliformes and Passeriformes) Compared to that in Mammals (Insectivores and Primates) (Part 2 of 2)

The brain weight and brain structure volumes of galliform and passeriform birds were calculated and related to body weight. The total brains and 14 brain regions were investigated in order to calculate factors by which these structures in passeriforms exceed those in galliforms in size. The larger passeriform brains have larger telencephala, especially ventral hyperstriata and neostriata. The enlargement of total brain and telencephalon resembles that in primates, compared to insectivores, within mammals. The enlargement of the ventral hyperstriata + neostriata in passeriforms is fundamentally similar to that of the isocortex in mammals: it reflects an expansion of multimodal integrational capacities, as the ventral hyperstriatum and neostriatum are occupied exclusively or primarily by multimodal integrational areas as is the isocortex.

Allometric Comparison of Brain Weight and Brain Structure Volumes in Different Breeds of the Domestic Pigeon, Columba livia f.d.(Fantails, Homing Pigeons, Strassers)

In three breeds of domestic pigeons (fantails, homing pigeons, and strassers) the volumes of fresh, i.e. unfixed tissue of 14 brain structures were determined (telencephalon, diencephalon, nervus opticus, tectum, cerebellum, tegmentum and hyperstriatum accessorium, hyperstriatum ventrale, neostriatum, paleostriatum, hippocampus, septum, regio praepiriformis, bulbus olfactorius). Allometric comparisons that take into account differences in body weight and size were made among these three breeds. The tectum, hippocampus, paleostriatum and especially the neostriatum and olfactory bulb are remarkably larger in homing pigeons. These data are discussed in a functional context, in which the homing ability of homing pigeons is considered.

Tool-making New Caledonian crows have large associative brain areas.

Animals with a high rate of innovative and associative-based behavior usually have large brains. New Caledonian (NC) crows stand out due to their tool manufacture, their generalized problem-solving abilities and an extremely high degree of encephalization. It is generally assumed that this increased brain size is due to the ability to process, associate and memorize diverse stimuli, thereby enhancing the propensity to invent new and complex behaviors in adaptive ways. However, this premise lacks firm empirical support since encephalization could also result from an increase of only perceptual and/or motor areas. Here, we compared the brain structures of NC crows with those of carrion crows, jays and sparrows. The brains of NC crows were characterized by a relatively large mesopallium, striatopallidal complex, septum and tegmentum. These structures mostly deal with association and motor-learning. This supports the hypothesis that the evolution of innovative or complex behavior requires a brain composition that increases the ability to associate and memorize diverse stimuli in order to execute complex motor output. Since apes show a similar correlation of cerebral growth and cognitive abilities, the evolution of advanced cognitive skills appears to have evolved independently in birds and mammals but with a similar neural orchestration.

Electrophysiological mapping of body representation in the cortex of the blind mole rat.

The cortex of the blind mole rat (Spalax ehrenbergi) was explored for somatosensory responses with special reference to an extension into the occipital cortex which serves vision in sighted mammals. Head and body representation was similar as in other rodents or mammals. However, the somatosensory area extended far into the occipital cortex. No responses to auditory or visual stimulation were found caudal to the somatosensory area. However, auditory responses were recorded in an area lateral to and slightly caudal to the head representation. It is concluded that in this naturally blind animal the area normally occupied by the visual cortex serves somatosensory function.

Extraordinary large brains in tool-using New Caledonian crows (Corvus moneduloides).

A general correlation exists between brain weight and higher cognitive ability in birds and mammals. In birds this relationship is especially evident in corvids. These animals are well-known for their flexible behavior and problem-solving abilities, and have relatively large brains associated with a pallial enlargement. At the behavioral level, New Caledonian crows stand out amongst corvids because of their impressive object manipulation skills both in the wild and in the laboratory. However, nothing is known about the relative size of their brains. Here we show that NC crows have highly encephalised brains relative to most other birds that have been studied. We compared the relative brain size of five NC crows with combined data for four passerine species (7 European carrion crows, 2 European magpies, 3 European jays and 4 domestic sparrows) and found that NC crows had significantly larger brains. A comparison only with the seven carrion crows also revealed significantly larger brains for NC crows. When compared with brain data for 140 avian species from the literature, the NC crow had one of the highest degrees of encephalisation, exceeding that of the 7 other Corvidae in the data set.

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.

Allometric Comparison of the Brain and Brain Structures in the White Crested Polish Chicken with Uncrested Domestic Chicken Breeds

The feather crest on the head of the White Crested Polish Chicken covers a bony protuberance, a skull modification typical of crested chickens. The telencephalon is displaced into this protuberance, giving the brain the shape of an hour-glass. Allometric comparison (i.e., consideration of the influence of body weight on brain size) shows that the brain is relatively larger in crested chickens. This enlargement is partly due to enlarged ventricles, which are observed in some individuals. Among the brain structures measured, the tegmentum, cerebellum, tectum, paleostriatum, hippocampus, septum and olfactory bulb are not significantly larger in White Crested Polish chickens in comparison to those structures in seven uncrested chicken breeds; the optic tract, diencephalon, telencephalon, accessory hyperstriatum, dorsal and ventral hyperstriatum, and neostriatum, however, are significantly enlarged in this breed.

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.

Navigational Experience Affects Hippocampus Size in Homing Pigeons

Homing (racing) pigeons (Columba livia f.d.) are well-known for their homing abilities, which are thought to be based on a genetic predisposition, multimodal learning and spatial cognition. On average, the hippocampus, a forebrain structure that processes spatial information, is larger in homing pigeons compared to other non-homing pigeon breeds or their wild ancestor, the rock dove. Here we show that this characteristic hippocampus volume is dependent on flying and navigational experience. Twenty homing pigeons originating from the same breeding stock were raised in the same loft under identical constraints. After fledging, 10 of them were allowed to fly around the loft, gain navigational experience and participate successfully in races. The other 10 stayed permanently in the loft and thus did not share the navigational skill experienced by the first group. After reaching sexual maturity, individuals of both groups were sacrificed and morphometric analyses were carried out to measure the volumes of total brain, telencephalon, hippocampus and 12 other brain structures. Individuals with experience in flying and navigation had an 11.2% larger hippocampus relative to the telencephalon compared to non-experienced individuals (p = 0.028). This effect is not seen in any of the other measured brain subdivisions. Given that plasticity in hippocampal volume has a genetic component, our results confirm that there is also an experience component, and that has certain implications for navigational ability. Evidently, experience is a precondition to full hippocampal development.

Discontinuous Variability of Brain Composition among Domestic Chicken Breeds

In 80 domestic chickens from 8 breeds, the volumes of 12 brain parts were identified as dimensions in a cluster analysis. Based on Euclidean metrics and the Ward algorithm at least 2 groups were found that are congruent with the breeds ‘White Crested Polish chicken’ and ‘Breda’, although the breed identity was not a variable used in the cluster analysis. Domestication is interpreted as evolution which includes the possibility of speciation. It is hypothesized that White Crested Polish chickens and Bredas are becoming new species in terms of a biospecies concept.