Vertebrate brain size is remarkably variable at all taxonomic levels. Brains of mammals forexample, range from 0.1 gram in small bats (Chiroptera) to about 8-9 kilos in Sperm whales(Physeter macrocephalus). But what does this variation in size really mean? The link between brainsize and cognition is debated due to, for instance the difficulties of comparing cognitive ability indifferent species. A large number of comparative studies continue to provide information aboutcorrelations found both within and between species. The relative size of the brain is an example of apopular measurement that correlates with cognitive ability. But to date, no experimental studieshave yielded any proof causality between relative brain size and cognitive ability. Here I usedguppies selected for either large or small relative brain size to investigate differences in cognitiveperformance of a quantity discrimination task. The results from this experiment provideexperimental evidence that relative brain size is important for cognitive ability, and that a differencein cognitive ability could be obtained already after two generations of selection experiments onrelative brain size in a vertebrate. / Artificial Selection on Relative Brain Size in the Guppy Reveals Costs and benefits of Evolving a Larger Brain

Phenotypic plasticity, the ability of a genotype to express different phenotypes in different environments, allows organisms to cope with variation in resources and invade novel environments. Biologists have long been fascinated with the costs and tradeoffs that generate and maintain variation in plasticity, such as possible increases in brain size and delays in reproduction associated with the evolution of learning. However, the costs of plasticity vary: many studies have failed to find costs of plasticity, the degree of costs often vary with the system or environments considered, and many costs of plasticity are variable even within the lifetime of an individual. This research adopts a developmental perspective to predict the degree and incidence of costs of plasticity, using host learning in butterflies as a case study. Learning, a mechanism of plasticity that develops through a trial-and-error sampling process, should result in developmental costs and allocation of energy towards development (at the expense of reproduction). Furthermore, costs of learning should be less pronounced in environments for which organisms have innate biases and for learned traits underlain by short-term memory, relative to long-term memory (which requires more developmental re-structuring). This research found support for all three predictions across three levels of costs: behavioral costs, tissue costs, and fecundity trade-offs. Butterflies exhibited genetic variation in their ability to learn to recognize different colored hosts. Genotypes with higher proxies for long-term memory emerged with relatively larger neural investment and smaller reproductive investment. In contrast to these costs of long-term learning, proxies of short-term learning were only correlated with increased exploration of a range of possible resources (types of non-hosts) early in the host-learning process. Family-level costs of plasticity emerged from the ability to learn to locate a red host, for which butterflies do not have an innate bias. Costs of learning were also induced by learning itself: following exposure to novel (red) host environments, individual butterflies, regardless of genetic background, increased exploratory behavior, increased neural investment, and re-allocated energy away from reproduction towards other functions (e.g., flight). Considering developmental mechanisms helps to predict how costs will influence the evolution of learning and plasticity.

Les ephrines et leurs récepteurs Eph constituent une famille multigénique de facteurs de guidage cellulaire et axonal. Ces facteurs jouent un rôle-clé dans l’établissement de cartes neurales topographiques, notamment au niveau des connexions thalamocorticales, réseau neuronal majeur du cerveau des mammifères. Notre projet visait initialement à étudier l’implication des ephrines corticales dans la génèse des connexions thalamocorticales par une approche de gain de fonction. Pour ce faire, nous avons généré des souris transgéniques présentant une expression ectopique spécifique de l’ephrine-A5 dans le cortex en développement, en utilisant une technique de transgénèse d’addition par chromosome artificiel de bactéries (BAC). De façon surprenante, l’analyse de ces souris nous a révélé que les ephrines, à côté de leurs rôles classiques de facteurs de guidage, influençaient la taille du cortex cérébral en régulant l’apoptose des progéniteurs neuronaux. En effet, nous avons pu montrer que l’expression ectopique du ligand ephrine-A5 par les progéniteurs corticaux exprimant son récepteur EphA7 résultait en une déplétion précoce en progéniteurs corticaux par apoptose, et une diminution subséquente de la taille du cortex. Cette vague apoptotique est observée en l’absence de toute altération détectable de la prolifération, la différenciation et la migration neurale dans le cortex. Nous avons étayé notre étude in vivo par des expériences in vitro, qui ont montré que l’ephrine-A5 recombinante était capable d’induire rapidement la mort des progéniteurs neuronaux dissociés. Nous avons également montré que cette mort cellulaire impliquait l’activation de la caspase-3, confirmant ainsi l’effet direct des ephrines et de leurs récepteurs sur une ou plusieurs cascades apoptotiques. Par contre, la stimulation des neurones post-mitotiques corticaux par l’ephrine-A5 est accompagnée d’une activation de la caspase-3 sans mort cellulaire apparente. La signalisation ephrine/Eph induirait donc l’activation de la caspase-3 dans différents types cellulaires, sans que celle-ci ne soit systématiquement le reflet d’une mort cellulaire programmée. Parallèlement, afin d’évaluer l’importance physiologique de cette voie pro-apoptotique dépendante des ephrines, nous avons étudié des souris présentant une perte de fonction du récepteur EphA7. L’analyse de ces mutants nous a permis de mettre en évidence une diminution de l’apoptose des progéniteurs corticaux, une augmentation de la taille du cortex, ainsi qu’une hypercroissance exencéphalique de tout le cerveau antérieur dans les cas les plus extrêmes. Ces observations indiquent donc que les ephrines sont nécessaires au contrôle de la mort cellulaire programmée des progéniteurs du cortex cérébral. Nous avons également observé le même phénotype exencéphalique dans des mutants déficients en ephrines-A2, -A3 et -A5, dont l’analyse préliminaire suggère également des défauts de processus apoptotiques. Nos diverses expériences, combinant une approche par gain et perte de fonction, à la fois in vivo et in vitro, ont ainsi permis de proposer un nouveau rôle des ephrines en marge de leur implication dans la guidance axonale, à savoir un rôle dans le contrôle de la taille cérébrale par induction de l’apoptose des progéniteurs corticaux. La mise en évidence de cette nouvelle voie de signalisation pro-apoptotique pourrait avoir des implications importantes dans d’autres aspects de la biologie du développement et des cellules souches, ainsi que dans l’oncogénèse.

Animals in general vary immensely in body size, which greatly affects their morphology, physiology, survival, and nutritional requirements. The nervous system is also affected by variation in body size, which, in turn, shapes the perception of environmental stimuli and the behavior of animals. Comparative studies of vertebrates suggest that larger brains and their integrative centers comprise more and generally larger neurons (Jerison, 1973; Kaas, 2000), but much less is known about brain - body size relations in invertebrates. Closely related social bee species are well suited to study correlations between body size and brain composition. Different honey bee species vary in body size yet differ little in their ecological requirements and behavior and bumble bees feature a large range of body sizes even within a single colony.

<p>The pace of life history is highly variable across mammals, and several evolutionary biologists have theorized that the tempo of a species' life history is set by external factors. These factors, such as food availability and predation pressure, determine mortality rates. In turn, mortality rate determines the age at maturity. High mortality rate results in early age at maturity; individuals must grow and reproduce quickly because of the high risk of death. Conversely, a low mortality rate is allows individuals to prolong their growth period and reproduce slowly. This theory assumes that growth rates are constant across species, and thus body size is determined by mortality rates.</p><p>This project posits that the intrinsic characteristics of species set the pace of life history. Among anthropoids, there is a great deal of variation in growth rates and the pace of life history relative to body size. The hypotheses proposed by this project state that the degree of encephalization in a species determines the growth rates, the length of the growth period, and the adult lifespan. Growing a large brain is costly and requires a prolonged period of development. However, a large brain has the benefit of reducing mortality by facilitating cognitive strategies for food procurement and predator avoidance. This cost/benefit balance results in the pattern of life-history variation in which mortality rates are correlated with the length of the growth period. However, the causal arrows are reversed; instead of the mortality rate determining the age at maturity and consequently the size of the species, the relative brain size of the anthropoid determines the mortality rate and the age maturity.</p><p>These hypotheses were tested by determining the body and brain growth trajectories of thirteen anthropoids, and compiling life-history data from long-term studies of these species in the wild. Multi-variate analyses demonstrated that extensive brain growth, whether through prolonged duration or rapid growth rates, results in slow body-growth rates during the juvenile period and delayed age at maturity. In addition, encephalization results in longer adult lifespan. Therefore, this project demonstrated that intrinsic characteristics of anthropoid species determine the pace of their life histories.</p> / Dissertation

Vertebrates show dramatic interspecific variation in the size of their brains. The complexity of brains is considered to be the key factor of evolutionary success in Vertebrates, and therefore an evolutionary trend towards increasing brain size and coplexity is assumed. Large and complex brains evolved independently in birds and mammals. Birds have brains that are comparable in their relative size to the brains of mammals. However, in stark contrast to mammals, there is no general trend towards increase of brain size in birds. Relatively large brains have evolved independently in many avian lineages. Highly encephalised orders are parrots (Psittaciformes), woodpeckers and relatives (Piciformes), hornbills, hoopoe and wood hoopoes (Bucerotiformes), owls (Strigiformes), storks (Ciconiiformes) and several families of songbirds (Passeriformes), mainly bowerbirds (Ptilorhynchidae) and corvids (Corvidae). Otherhighlyencephalizedgroupsarenon-parasiticcuckoos(genusCentropus,Phaenicophaeus and Coua) and family Diomeidea and genus Pelecanus belonging to the clade water birds. Less encephalized groups include the basal lineages such as paleognaths and fowl (Galloanserae), and also pigeons (Columbiformes) and swifts, treeswifts and hummingbirds (Apodiformes). We suggest that this mosaic evolution is result of...

The effect of incubation temperature on different morphological, physiological, cognitive and behavioral characteristics in reptiles is a well-studied topic, although the underlying mechanism leading to the differences between individuals incubated at different temperatures remains largely unknown. In this thesis I studied the effect of incubation temperature on cognitive abilities and the number of neurons and non-neuronal cells in the gecko Paroedura picta incubated at two different temperatures, 24řC and 30řC. The geckos were tested in two cognitive tasks with simulated predatory attack. 14-day-old hatchlings were tested in a Y-maze, while 6-months-old geckos were tested in an arena with shelters of different colors. After testing, the number of neurons and non- neuronal cells in several parts of the brain were estimated using the isotropic fractionator in selected individuals. Although incubation temperature did not affect the success in the cognitive task in hatchlings, it did affect the total time needed to find the shelter. This difference remained significant also in adult geckos. The number of neurons, which was used as a proxy for the information processing capacity of the brain, did not affect success in the cognitive tasks. However, absolute brain size correlated with success in the...

This study investigated selective pressures associated with encephalization in mammals and discussed broader implications. Relative brain size as measured by EQ (Encephalization Quotient) was compared between ecological categories. Omnivores had higher average EQ than ecological specialists. Since specialists are disproportionately affected by extinction events, selection for ecological generalism is proposed as encephalization mechanism. This mechanism may reinforce the more widely known Cognitive Buffer Hypothesis (CBH)—the idea that possessing relatively large brains has buffered lineages against environmental change. CBH is tested here by comparing EQs in Procyon lotor (raccoon) in urban and rural environments. CBH predicts that raccoons in the most radically altered environment, the city, experience the strongest selection for encephalization. Urban raccoons studied here exhibit a higher EQ. Although results are preliminary, data suggest that encephalization is accelerated during abrupt periods of environmental change. Finally, implications for the evolution of biological complexity more generally are discussed.

So far, research on inter- and intraspecific teleost brain plasticity across different freshwater environments has been widely conducted. However, insights of brain morphological variation on social and predator avoidance behaviours are lacking. Here, we investigated variation in shape and size of the brain and its six major regions of European minnows (Phoxinus phoxinus) inhabiting Lake Ånnsjön and its tributaries, using geometric morphometrics methods. We also experimentally compared stream and lake fish activity and social behaviour under different feeding and predation regimes. Contrary to our predictions of lake minnows having evolved smaller brains because of living in habitats with reduced environmental complexity compared to their conspecifics in the streams, we found that overall brain size generally did not differ between locations. Instead, brain morphology differed between minnows caught in the lake and streams, with stream minnows showing larger dorsal medulla, telencephalon and olfactory bulbs, and lake minnows presenting larger optic tecta and hypothalamus. Experimental results showed that lake minnows were more likely to engage in social behaviour than those from streams. Our results indicate that while overall allocation of energy to the brain does not change, habitat-specific differences in activity and trophic divergence might predict specialization for different senses, allocating more resources towards different brain regions. In addition, we show how various ecological factors, such as environmental complexity and social organization seem to be reflected in brain shape.

Many passerine birds, particularly corvids, are known to express complex cognitive skills comparable to those observed in primates. In order to examine how these similarities are reflected at the cellular level, I counted neurons and nonneuronal cells in passerine brains using the isotropic fractionator method. I show that, in these birds, neuronal numbers scale almost isometrically with telencephalic size, i.e., the average neuron size shows little increase and neuronal density decreases minimally as brains get larger. Neuronal densities in the passerine telencephalon exceed those observed in the primate cerebral cortex by a factor of 3-6. As a result, the number of telencephalic neurons in the Common Raven (Corvus corax) equals those observed in the cerebral cortex of small monkeys. The cerebellum features similar scaling rules. However, because the relative size of the cerebellum is smaller than in mammalian brains, cerebellar neurons make a much smaller proportion of total brain neurons than in mammals. In contrast to the little variation in neuronal densities in telencephalon and cerebellum, the density of neurons rapidly decreases with increasing structure size in the diencephalon, optic tectum and brain stem. For all examined brain structures, the densities of nonneuronal cells remain constant...