Progressive und regressive Hirngrößienveränderungen bei Equiden

Progressive and regressive changes of brain size within Equidae From Hyracotherium to Equus brain size increased eightfold independently from body size. In domestication brain size is reduced; within mammals the amount of reduction depends on cephalization. Species with high cephalization show much more reductions than those with low cephalization. Among the ancestors of domesticated mammals wild horses have the highest cephalization level; reduction of brain size of more than 30% in domesticated horses could be expected. The size of the brain case of domesticated horses is only 14 % smaller than in wild Przewalski horses. We think that populations of the wild Przewalski horses have been crossbreeds between wild and domesticated animals. There is no difference in size of the brain case capacity and the brain weight between the Przewalski horses from zoological gardens and domesticated horses. This may be due to further crossbreeding between Zoo-Przewalski horses and domesticated horses and to artificial selection.     

Increased brain size in mammals is associated with size variations in gene families with cell signalling,chemotaxis and immune-related functions

Genomic determinants underlying increased encephalization across mammalian lineages are unknown. Whole genome comparisons have revealed large and frequent changes in the size of gene families, and it has been proposed that these variations could play a major role in shaping morphological and physiological differences among species. Using a genome-wide comparative approach, we examined changes in gene family size (GFS) and degree of encephalization in 39 fully sequenced mammalian species and found a significant over-representation of GFS variations in line with increased encephalization in mammals. We found that this relationship is not accounted for by known correlates of brain size such as maximum lifespan or body size and is not explained by phylogenetic relatedness. Genes involved in chemotaxis, immune regulation and cell signalling-related functions are significantly over-represented among those gene families most highly correlated with encephalization. Genes within these families are prominently expressed in the human brain, particularly the cortex, and organized in co-expression modules that display distinct temporal patterns of expression in the developing cortex. Our results suggest that changes in GFS associated with encephalization represent an evolutionary response to the specific functional requirements underlying increased brain size in mammals.     

Evolution of anuran brains: disentangling ecological and phylogenetic sources of variation

Variation in ecological selection pressures has been implicated to explain variation in brain size and architecture in fishes, birds and mammals, but little is known in this respect about amphibians. Likewise, the relative importance of constraint vs. mosaic hypotheses of brain evolution in explaining variation in brain size and architecture remains contentious. Using phylogenetic comparative methods, we studied interspecific variation in brain size and size of different brain parts among 43 Chinese anuran frogs and explored how much of this variation was explainable by variation in ecological factors (viz. habitat type, diet and predation risk). We also evaluated which of the two above‐mentioned hypotheses best explains the observed patterns. Although variation in brain size explained on average 80.5% of the variation in size of different brain parts (supporting the constraint hypothesis), none of the three ecological factors were found to explain variation in overall brain size. However, habitat and diet type explained a significant amount of variation in telencephalon size, as well in three composite measures of brain architecture. Likewise, predation risk explained a significant amount of variation in bulbus olfactorius and optic tecta size. Our results show that evolution of anuran brain accommodates features compatible with both constraint (viz. strong allometry among brain parts) and mosaic (viz. independent size changes in response to ecological factors in certain brain parts) models of brain size evolution.     

Cerebellum size is related to fear memory and domestication of chickens

Red Junglefowl (Gallus gallus) were selected for divergent levels of fear of humans during eight generations, causing the selection lines to differ in fear levels as well as in the proportional brain and cerebellum masses. Birds from the two lines were then crossed to obtain an F3 intercross in order to study the correlations between brain mass and fear learning. We exposed 105 F3-animals individually to a fear habituation and memory test at 8 days of age, where the reactions to repeated light flashes were assessed on 2 consecutive days. After culling, the absolute and relative sizes of each of four brain regions were measured. Stepwise regression was used to analyse the effects of the size of each brain region on habituation and memory. There were no effects of any brain region on the habituation on day one. However, birds with a larger absolute size of cerebellum had significantly reduced reactions to the fearful stimuli on day two, indicating a better memory of the stimuli. No other regions had significant effects. We conclude that increased cerebellum size may have been important in facilitating chicken domestication, allowing them to adapt to a life with humans.     

The allometric approach to species differences in brain size

Allometric methods can be used to test quantitative theories of the relationship between brain size and body size across species, and to search for ecological, behavioural, life history, and ontogenetic correlates of brain size. Brain size scales with an allometric exponent of around 0.75 against body size across mammals, but is closer to 0.56 for birds and for reptiles. The slope of the allometric line often varies depending upon the taxonomic level of analysis. However, this phenomenon, at least in mammals, may be a statistical artifact. Brain size for a given body size (relative brain size) varies among orders in birds and mammals, and some dietary associations with relative brain size have been found in particular taxa. Developmental status at birth is the most consistent correlate of relative brain size: precocial neonates have larger brains for a given maternal size than altricial neonates in both birds and mammals. Altricial neonates, however, have more brain growth following birth, and in birds also have larger relative adult brain sizes. Energetic explanations for differences in neonatal brain growth, although attractive on theoretical grounds, have largely failed to stand up to empirical tests.     

Reproductive strategies,body size,and encephalization in primate evolution

In order to understand fully the generally high level of encephalization observed in living primates, we must determine why early primates exhibited moderately large relative brain sizes compared to their early Tertiary contemporaries. The relatively high degree of encephalization in early primates may be related at least in part to the fact that they were highly unusualamong mammals in combining a small body size with a strongly precocial reporductive strategy. Other small, precocial mammals also exhibit moderately high levels of encephalization; but primates appear to have been truly uniquein being the only such small-sized and highly precocial group to give rise to extensive radiations of larger descendants. This is a key element in understanding primate brain evolution, because the initial “head start” of the early primates was translated up to larger sizes in descendants. The possible relationships among encephalization, precociality, small size, and arboreality are discussed, particularly in light of recent debates concerning the validity of the superorder Archonta. This work emphasizes that we need to consider relative brain size as but one element in a complex synergistic network of morphological and life-history features.     

Comparative analysis of encephalization in mammals reveals relaxed constraints on anthropoid primate and cetacean brain scaling

There is a well-established allometric relationship between brain and body mass in mammals. Deviation of relatively increased brain size from this pattern appears to coincide with enhanced cognitive abilities. To examine whether there is a phylogenetic structure to such episodes of changes in encephalization across mammals, we used phylogenetic techniques to analyse brain mass, body mass and encephalization quotient (EQ) among 630 extant mammalian species. Among all mammals, anthropoid primates and odontocete cetaceans have significantly greater variance in EQ, suggesting that evolutionary constraints that result in a strict correlation between brain and body mass have independently become relaxed. Moreover, ancestral state reconstructions of absolute brain mass, body mass and EQ revealed patterns of increase and decrease in EQ within anthropoid primates and cetaceans. We propose both neutral drift and selective factors may have played a role in the evolution of brain-body allometry.     

Comparative allometric investigations on the skulls of wild cavies (Cavia aperea) versus domesticated guinea pigs (C. aperea f. porcellus) with comments on the domestication of this species

Bivariate allometric calculations were performed to quantitatively compare skulls of wild cavies with domesticated guinea pigs. Descendents of wild caught Cavia aperea from eastern regions of the species’ distribution area were used, as well as unselected domesticated breeds of guinea pigs differing in outer appearance. The individuals of both groups were kept under similar environmental conditions. Altogether 19 parameters on the skulls and the body weights were used for the analyses. These parameters were studied in relation to greatest skull length and to body size. As a general result the diverse parameters are in most cases significantly different between both groups which is interpreted as a special result of unconsciously selected and genetically determined intraspecific changes concomitant with domestication. The skull does not change in total under the domestication process but in a mosaic manner. However, for the mosaic changes of the diverse parameters in relation to skull length a different picture is valid as related to body weight. This is caused by the fact that the skull of guinea pigs is around 5% shorter independent of the body size, a common effect of domestication also described for other species. Thus, skull length is not an appropriate parameter for body size with respect to such intraspecific investigations, although normally used for the characterization of species in interspecific comparisons of museum materials.Altogether in relation to body weight most of the parameters describing the fascial portion of the skull are shorter in the guinea pig, especially the palatine, the diastema and the mandible but also the nasalia and frontalia lengths as well as the breadth of the rostrum and the zygomaticum are smaller. Most of the occipital skull measures are additionally smaller in the guinea pigs. This is clearly the case for the length and the breadth of the braincase and for the tympanic bulla. The braincase volume is 16.2% smaller, a value only slightly different when compared with the degree of brain size decrease due to domestication as reported for this species in other investigations.     

Component analysis of craniologic characters of silver foxes (Vulpes fulvus Desm.) and their changes arising under domestication]

The data on cranial measurements performed in silver foxes indicate that there are differences in sizes measured between the farm foxes--bred population and the population selected for domestication. A method of principal components was used to analyse the cranial measurements and their changes under domestication. The first component covers about 50% of cranial diversity, which is interpreted as variation in the total skull size. This component clearly separates the two sexes, but not different populations. The second component presumably reflects the growth rate allometry between the skull length and width. The third and fourth components are measurements of skull width; the fifth one reflects the sizes of brain skull. None of these components clearly separate the foxes from farm--bred and domesticated populations. However, some differences in distribution are observed.     

Brain size, development and metabolism in birds and mammals

Recent hypotheses that variation in brain size among birds and mammals result from differences in metabolic allocation during ontogeny are tested.
Indices of embryonic and post-embryonic brain growth are defined. Precocial birds and mammals have high embryonic brain growth indices which are compensated for by low post-embryonic indices (with the exception of Homo supiens ). In contrast, altricial birds and mammals have low embryonic and high post-embryonic indices. Altricial birds have relatively small brains at hatching and develop relatively large brains as adults, but among mammals there is no equivalent correlation between variation in adult relative brain sizes and state of neonatal development.
Compensatory brain development in both birds and mammals is associated with compensatory parental metabolic allocation. In comparison with altricial development, precocial development is characterized by higher levels of brain growth and parental metabolic allocation prior to hatching or birth and lower levels subsequently. Differences between degrees of postnatal investment by the parents in the young of precocial birds versus precocial mammals may result in the different patterns of adult brain size associated with precociality versus altriciality in the two groups.
The allometric exponent scaling brain on body size differs among taxonomic levels in birds. The exponent is higher for some parts of the brain than others, irrespective of taxonomic level. Unlike mammals, the exponents for birds do not show a general increase with taxonomic level. These pattcrns call into question recent interpretations of the allometric exponent in birds. and the reason for changes in exponent with taxonomic level.     

Comparative quantitative investigations on brains of wild cavies (Cavia aperea) and guinea pigs (Cavia aperea f. porcellus). A contribution to size changes of CNS structures due to domestication

Intraspecific allometric calculations of the brain to body size relation revealed distinct differences between 127 (67; 60) ancestral wild cavies and 82 (37; 45) guinea pigs, their domesticated relatives. The dependency of both measures from one another remained the same in both animal groups but the brains of guinea pigs were by 14.22% smaller at any net body weight. Consistent with results in other species the domestication of Cavia aperea is also characterized by a decrease of brain size. Fresh tissue sizes of the five brain parts medulla oblongata, cerebellum, mesencephalon, diencephalon and telencephalon were determined for 6 cavies and 6 guinea pigs by the serial section method. Additionally the sizes of 16 endbrain structures and those of the optic tract, the lateral geniculate body and the cochlear nucleus were measured. Different decrease values resulted for all these structures concomitant with domestication as was calculated from the amount of total brain size decrease and average relative structure values in the wild as well as the domesticated brain. The size decrease of the entire telencephalon (−13.7%) was within the range of the mean overall reduction as similarly was the case for the total neocortex (−10.7%) whereas the total allocortex (−20.9%) clearly was more strongly affected. The size decrease of the olfactory bulb (−41.9%) was extreme and clearly higher than found for the secondary olfactory structures (around −11%). The primary nuclei of other sensory systems (vision, audition) were decreased to less extent (lateral geniculate: −18.1%; cochlear nucleus: −12.6%). Mass decreases of pure white matter parts were nearly twice as high in contrast to associated grey matter parts (neocortex white versus grey matter; tractus opticus versus lateral geniculate body). The relatively great decrease values found for the limbic structures hippocampus (−26.9%) and schizocortex (−25.9%) are especially notable since they are in good conformity with domestication effects in other mammalian species. The findings of this study are discussed with regard to results of similar investigations on wild and domesticated gerbils (Meriones unguiculatus), the encephalization of the wild form, the special and species-specific mode and duration of domestication and in connection with certain behavioral changes as resulted from comparative investigations in ethology, socio-biology, endocrinology and general physiology.     

A Comparison of Brain Gene Expression Levels in Domesticated and Wild Animals

Domestication has led to similar changes in morphology and behavior in several animal species, raising the question whether similarities between different domestication events also exist at the molecular level. We used mRNA sequencing to analyze genome-wide gene expression patterns in brain frontal cortex in three pairs of domesticated and wild species (dogs and wolves, pigs and wild boars, and domesticated and wild rabbits). We compared the expression differences with those between domesticated guinea pigs and a distant wild relative (Cavia aperea) as well as between two lines of rats selected for tameness or aggression towards humans. There were few gene expression differences between domesticated and wild dogs, pigs, and rabbits (30–75 genes (less than 1%) of expressed genes were differentially expressed), while guinea pigs and C. aperea differed more strongly. Almost no overlap was found between the genes with differential expression in the different domestication events. In addition, joint analyses of all domesticated and wild samples provided only suggestive evidence for the existence of a small group of genes that changed their expression in a similar fashion in different domesticated species. The most extreme of these shared expression changes include up-regulation in domesticates of SOX6 and PROM1, two modulators of brain development. There was almost no overlap between gene expression in domesticated animals and the tame and aggressive rats. However, two of the genes with the strongest expression differences between the rats (DLL3 and DHDH) were located in a genomic region associated with tameness and aggression, suggesting a role in influencing tameness. In summary, the majority of brain gene expression changes in domesticated animals are specific to the given domestication event, suggesting that the causative variants of behavioral domestication traits may likewise be different.     

Bemerkungen zur Evolution von., Sprachen bei Säugetieren1 Zur Variabilität innerartlicher Kommunikation bei Caniden

Remarks on the evolution of “languages” of mammals. The variability of the intraspecific communication of canids The canid species mainly considered in this study, the golden jackal (Canis aureus), the cojote (Canis latrans), and especially the wolf (Canis lupus) have an extremely variable species-specific intraspecific communication. Communities, but also differences between the species become evident; the wolf with its distinct social behavior has the greatest differentiation. These results make also clear, that forms of communication in mammals develop in evolutionary processes and events. A safe statement on the beginning of the forms of communication in canids cannot be given so far. This is also true for humans. The ideas of Lenneberg (1967) of the parallel and disjunct origin of the forms of communication in mammals has a high probability for higher systematic units, but on lower systematic level a gradual change can be observed. The forms of communication in mammals are highly differentiated structures, where specific pecularities of the brain are of a special importance. The observations on the variability of intraspecific communication of canids, representing nonhuman mammals, suggest to define “language” in a very general form. In this way, similarities, analogies, and differences between the species of mammals, inclusive man, can be seen more clearly. In only this way the material can be gained to answer the question, if the language of man has in principle exceptional position biologically. It is important for the valuation of forms of intraspecific communication, the “languages”, of mammals not to choose human theory of life and human thinking as a basis, but to start with the biological pecularities of the species. Also canids, as higher mammals, have the ability to abstract, a form of thinking. In their special, species-specific way also non-human mammals arrange their area of experience and thinking. They form for themselves a species-specific “theory of live”. This will of course be different from the human one, according to the systematic position and the biological environment of the species. It is generally accepted that man has special abilities of thinking, he possesses an extraordinary developed brain, compared to all other mammals. This must effect his communication, his language, and result in differences to other mammal species. Gipper (1977) pointed out especially these differences, but they should not be interpreted as differences in principle. Canids use different gestures and sounds for communication within the species; dialog relations can be observed. Different connections of gestures and sounds have different contents of information. It is remarkable that canids do not use their total abilities in using the means of communication in the same way. This is very striking in the change from the wolf, as the only ancestor, to the domestic dog. It is evident that the brain has a greater importance in developing intraspecific communication, than do the peculuarities of organs causing the understanding. A dependanc of intraspecific communication from the environment can be observed either in differences of wird species of canids as well as in the domestication. In the domestic dog, but also in other domestic animals, there is an increase in vocalisations. But the content of information of sounds and gestures decreases in domestication, expressions of arousal become more important. A remarkable plasticity of intraspecific communication becomes evident by domestication. The transformations can be evaluated in the sense of Kosswig (1963, 1965) as first stages of a regressive evolution. Experiments of cross-breeding canid species show exemplarily that forms of communication within these mammal species have a highly polygene regulation, and that processes of learning have an influence. Also from this point of view, no basic differences between nonhuman mammals and humans can be assured, especially when taking into account the facts and thoughts described by Lenneberg. The findings in the canids give useful hints for a better understanding of the human language and its relations to the forms of communication in other mammals. Altogether one can conclude that genetic variability, numerous recombinations, the synorganisation of the processes caused in this way, and natural selection plays definite role in the evolution of “languages”.     

Seasonal changes in hippocampus size and spatial behaviour in mammals and birds

Hippocampus is involved in processing of environmental spatial information, and its size is known to correlate positively with spatial abilities in mammals and birds. Comparisons between species suggest that amount of spatial information processed (the mean area of home range in particular) is related with hippocampus size. Do seasonal and age changes in hippocampus size correlate with seasonal dynamics of spatial behaviour during ontogenesis? The data obtained through observational and experimental studies confirm the possibility that hippocampus size may be subjected to adaptive modifications along with cyclic changes in spatial behavior. In course of seasonal dynamics, strong positive correlation was found between hippocampus mass, home range size, and mobility of small mammals. Recently, first facts demonstrating seasonal changes of hippocampus and spatial behaviour (in connection with food-storing and brood parasitism) were found in birds. A lot of facts obtained for different taxonomical groups shows parallel seasonal changes in spatial behaviour and morphology of brain region functionally related to such behaviour. Thus, in adult birds and mammals, not only behaviour but also brain structure is phenotypically flexible in response to seasonally changing environment. Morphophysiological mechanisms of hippocampus seasonal changes are also discussed.     

Problems of ontogeny and phylogeny in brain-size evolution

Fundamental ambiguities in the interpretation of brain/body allometric trends can only be resolved by analyzing relationships between ontogenetic brain/body growth processes in different groups. The ambiguous concept of adult encephalization confuses at least three distinct types of transformation of a common mammalian growth curve: scalar magnification, total curve didplacement, and changes in proportions of the pre- and postnatal phases of the curve. The conservative ratio between pre- and postnatal growth phases determines the apparent linearity of comparative brain/body allometry and can be explained by assuming that embyological neurogenetic processes ultimately determine both target brain and body size—the first directly and the second indirectly via neurohormonal regulation of somatic growth. Uneven taxonomic distribution of different ontogenetic growth patterns may explain many differences in the allometric trends at different taxonomic levels of analysis. The human brain grows exactly as if it was in a giant ape body; however, because of decoupled growth in different brain regions, it regulates body growth as though it were the size of a chimpanzee brain. Human encephalization exhibits an ontogenetic transformation not found in other mammalian groups.     

The effect of domestication on brain size and composition in the mink (Mustela vison)

The sizes of total brain, the five fundamental brain parts, and certain telencephalic structures were measured in wild mink ( Mustela vison energumenos ) and ranch mink of a Dark Standard strain of the same species. By means of intraspecific allometric methods for analysing the relationship between brain weight and body weight (net carcass weight), the volumes of the brain parts were compared in both groups. In general, total brain, as well as all the parts measured, were smaller in size in ranch mink independent of body size, age, and sex, indicating that domestication has led to a decrease in size. There were differences in the amount of decrease in various brain parts. These are discussed in connection with domestication time, with comparable results obtained in other species, and with regard to the functional importance of the brain parts.     

Competition,niche specialization and the evolution of brain size in the genus Peromyscus

Previous studies of relative brain size in mammals have suggested an association with complex habitats and with low reproductive rate. In order to examine the causal relationships more thoroughly, a detailed examination of relative brain size variation in the genus Peromyscus was undertaken. Endocranial volumes were used to estimate brain weight for 32 species including 161 subspecies, and relative brain size calculated as the species deviation from the allometric relationship between brain and body size. The intrageneric allometric coefficient was higher than most values previously reported from low taxonomic levels, but intraspecific coefficients were generally lower than this. Island species, and relict species isolated on mountain tops, which may be ecological ‘islands’, had consistently small relative brain sizes, but peninsular species were large brained. Among the remaining species there were significant correlations between litter size and relative brain size, and between the number of competitor species and relative brain size. Species with many competitor species have relatively large brains and small litters. It is concluded that the nature of the geographical distribution, the pattern of species formation and habitat complexity all influence relative brain size in existing forms.     

Longevity is associated with relative brain size in birds

Brain size of vertebrates has long been recognized to evolve in close association with basic life‐history traits, including lifespan. According to the cognitive buffer hypothesis, large brains facilitate the construction of behavioral responses against novel socioecological challenges through general cognitive processes, which should reduce mortality and increase lifespan. While the occurrence of brain size–lifespan correlation has been well documented in mammals, much less evidence exists for a robust link between brain size and longevity in birds. The aim of this study was to use phylogenetically controlled comparative approach to test for the relationship between brain size and longevity among 384 avian species from 23 orders. We used maximum lifespan and maximum reproductive lifespan as the measures of longevity and accounted for a set of possible confounding effects, such as allometry, sampling effort, geographic patterns, and life‐history components (clutch size, incubation length, and mode of development). We found that both measures of longevity positively correlated with relative (residual) brain size. We also showed that major diversification of brain size preceded diversification of longevity in avian evolution. In contrast to previous findings, the effect of brain size on longevity was consistent across lineages with different development patterns, although the relatively low strength of this correlation could likely be attributed to the ubiquity of allomaternal care associated with the altricial mode of development. Our study indicates that the positive relationship between brain size and longevity in birds may be more general than previously thought.     

Geographic patterns in seasonal changes of body mass,skull, and brain size of common shrews

1. Some small mammals exhibit Dehnel''s Phenomenon, a drastic decrease in body mass, braincase, and brain size from summer to winter, followed by a regrowth in spring. This is accompanied by a re‐organization of the brain and changes in other organs. The evolutionary link between these changes and seasonality remains unclear, although the intensity of change varies between locations as the phenomenon is thought to lead to energy savings during winter.
2. Here we explored geographic variation of the intensity of Dehnel''s Phenomenon in Sorex araneus. We compiled literature on seasonal changes in braincase size, brain, and body mass, supplemented by our own data from Poland, Germany, and Czech Republic.
3. We analyzed the effect of geographic and climate variables on the intensity of change and patterns of brain re‐organization.
4. From summer to winter, the braincase height decreased by 13%, followed by 10% regrowth in spring. For body mass, the changes were −21%/+82%, respectively. Changes increased toward northeast. Several climate variables were correlated with these transformations, confirming a link of the intensity of the changes with environmental conditions. This relationship differed for the decrease versus regrowth, suggesting that they may have evolved under different selective pressures.
5. We found no geographic trends explaining variability in the brain mass changes although they were similar (−21%/+10%) to those of the braincase size. Underlying patterns of change in brain organization in northeastern Poland were almost identical to the pattern observed in southern Germany. This indicates that local habitat characteristics may play a more important role in determining brain structure than broad scale geographic conditions.
6. We discuss the techniques and criteria used for studying this phenomenon, as well as its potential presence in other taxa and the importance of distinguishing it from other kinds of seasonal variation.

     

Sperm competition and brain size evolution in mammals

The ‘expensive tissue hypothesis’ predicts a size trade‐off between the brain and other energetically costly organs. A specific version of this hypothesis, the ‘expensive sexual tissue hypothesis’, argues that selection for larger testes under sperm competition constrains brain size evolution. We show here that there is no general evolutionary trade‐off between brain and testis mass in mammals. The predicted negative relationship between these traits is not found for rodents, ungulates, primates, carnivores, or across combined mammalian orders, and neither does total brain mass vary according to the level of sperm competition as determined by mating system classifications. Although we are able to confirm previous reports of a negative relationship between brain and testis mass in echolocating bats, our results suggest that mating system may be a better predictor of brain size in this group. We conclude that the expensive sexual tissue hypothesis accounts for little or none of the variance in brain size in mammals, and suggest that a broader framework is required to understand the costs of brain size evolution and how these are met.