The evolution of cerebellum structure correlates with nest complexity

Phylogeny and historical biogeography of the swallow family (Hirundinidae) inferred from comparisons of thousands of UCE loci

Swallows (Hirundinidae) are a globally distributed family of passerine birds that exhibit remarkable similarity in body shape but tremendous variation in plumage, sociality, nesting behavior, and migratory strategies. As a result, swallow species have become models for empirical behavioral ecology and evolutionary studies, and variation across the Hirundinidae presents an excellent opportunity for comparative analyses of trait evolution. Exploiting this potential requires a comprehensive and well-resolved phylogenetic tree of the family. To address this need, we estimated swallow phylogeny using genetic data from thousands of ultraconserved element (UCE) loci sampled from nearly all recognized swallow species. Maximum likelihood, coalescent-based, and Bayesian approaches yielded a well-resolved phylogenetic tree to the generic level, with minor disagreement among inferences at the species level, which likely reflect ongoing population genetic processes. The UCE data were particularly useful in helping to resolve deep nodes, which previously confounded phylogenetic reconstruction efforts. Divergence time estimates from the improved swallow tree support a Miocene origin of the family, roughly 13 million years ago, with subsequent diversification of major groups in the late Miocene and Pliocene. Our estimates of historical biogeography support the hypothesis that swallows originated in the Afrotropics and have subsequently expanded across the globe, with major in situ diversification in Africa and a secondary major radiation following colonization of the Neotropics. Initial examination of nesting and sociality indicates that the origin of mud nesting - a relatively rare nest construction phenotype in birds - was a major innovation coincident with the origin of a clade giving rise to over 40% of extant swallow diversity. In contrast, transitions between social and solitary nesting appear less important for explaining patterns of diversification among swallows.

Nest complexity correlates with larger brain size but smaller body mass across bird species

Amniotes differ substantially in absolute and relative brain size after controlling for allometry, and numerous hypotheses have been proposed to explain brain size evolution. Brain size is thought to correlate with processing capacity and the brain's ability to support complex manipulation such as nest-building skills. The increased complexity of nest structure is supposed to be a measure of an ability to manipulate nesting material into the required shape. The degree of nest-structure complexity is also supposed to be associated with body mass, partly because small species lose heat faster and delicate and insulated nests are more crucial for temperature control of eggs during incubation by small birds. Here, we conducted comparative analyses to test these hypotheses by investigating whether the complexity of species-typical nest structure can be explained by brain size and body mass (a covariate also to control for allometric effects on brain size) across 1353 bird species from 147 families. Consistent with these hypotheses, our results revealed that avian brain size increases as the complexity of the nest structure increases after controlling for a significant effect of body size, and also that a negative relationship exists between nest complexity and body mass.

Investigating the role of Purkinje fibers and synaptic connectivity in balance regulation through comprehensive ultrastructural and immunohistochemical analysis of the donkey's (Equus asinus) cerebellum

The donkey's extraordinary capacity to endure substantial loads over long distances while maintaining equilibrium suggests a distinctive cerebellar architecture specialized in balance regulation. Consequently, our study aims to investigate the intricate histophysiology of the donkey's cerebellum using advanced ultrastructural and immunohistochemical methodologies to comprehend the mechanisms that govern this exceptional ability. This study represents the pioneering investigation to comprehensively describe the ultrastructure and immunohistochemistry within the donkey cerebellum. Five adult donkeys' cerebella were utilized for the study, employing stains such as hematoxylin, eosin, and toluidine blue to facilitate a comprehensive histological examination. For immunohistochemical investigation, synaptophysin (SP), calretinin, and glial fibrillary acidic protein were used and evaluated by the Image J software. Furthermore, a double immunofluorescence staining of SP and neuron-specific enolase (NSE) was performed to highlight the co-localization of these markers and explore their potential contribution to synaptic function within the donkey cerebellum. This investigation aims to understand their possible roles in regulating neuronal activity and synaptic connectivity. We observed co-expression of SP and NSE in the donkey cerebellum, which emphasizes the crucial role of efficient energy utilization for motor coordination and balance, highlighting the interdependence of synaptic function and energy metabolism. The Purkinje cells were situated in the intermediate zone of the cerebellum cortex, known as the Purkinje cell layer. Characteristically, the Purkinje cell's bodies exhibited a distinct pear-like shape. The cross-section area of the Purkinje cells was 107.7 ± 0.2 µm2 , and the Purkinje cell nucleus was 95.7 ± 0.1 µm2 . The length and diameter of the Purkinje cells were 36.4 × 23.4 µm. By scanning electron microscopy, the body of the Purkinje cell looked like a triangular or oval with a meandrous outer surface. The dendrites appeared to have small spines. The Purkinje cells' cytoplasm was rich with mitochondria, rough endoplasmic reticulum, ribosomes, Golgi apparatus, multivesicular bodies, and lysosomes. Purkinje cell dendrites were discovered in the molecular layer, resembling trees. This study sheds light on the anatomical and cellular characteristics underlying the donkey's exceptional balance-maintaining abilities.

A global database of bird nest traits

The reproductive success of birds is closely tied to the characteristics of their nests. It is crucial to understand the distribution of nest traits across phylogenetic and geographic dimensions to gain insight into bird evolution and adaptation. Despite the extensive historical documentation on breeding behavior, a structured dataset describing bird nest characteristics has been lacking. To address this gap, we have compiled a comprehensive dataset that characterizes three ecologically and evolutionarily significant nest traits-site, structure, and attachment-for 9,248 bird species, representing all 36 orders and 241 out of the 244 families. By defining seven sites, seven structures, and four attachment types, we have systematically classified the nests of each species using information from text descriptions, photos, and videos sourced from online databases and literature. This nest traits dataset serves as a valuable addition to the existing body of morphological and ecological trait data for bird species, providing a useful resource for a wide range of avian macroecological and macroevolutionary research.

A Comparative Perspective on the Cerebello-Cerebral System and Its Link to Cognition

The longstanding idea that the cerebral cortex is the main neural correlate of human cognition can be elaborated by comparative analyses along the vertebrate phylogenetic tree that support the view that the cerebello-cerebral system is suited to support non-motor functions more generally. In humans, diverse accounts have illustrated cerebellar involvement in cognitive functions. Although the neocortex, and its transmodal association cortices such as the prefrontal cortex, have become disproportionately large over primate evolution specifically, human neocortical volume does not appear to be exceptional relative to the variability within primates. Rather, several lines of evidence indicate that the exceptional volumetric increase of the lateral cerebellum in conjunction with its connectivity with the cerebral cortical system may be linked to non-motor functions and mental operation in primates. This idea is supported by diverging cerebello-cerebral adaptations that potentially coevolve with cognitive abilities across other vertebrates such as dolphins, parrots, and elephants. Modular adaptations upon the vertebrate cerebello-cerebral system may thus help better understand the neuroevolutionary trajectory of the primate brain and its relation to cognition in humans. Lateral cerebellar lobules crura I-II and their reciprocal connections to the cerebral cortical association areas appear to have substantially expanded in great apes, and humans. This, along with the notable increase in the ventral portions of the dentate nucleus and a shift to increased relative prefrontal-cerebellar connectivity, suggests that modular cerebellar adaptations support cognitive functions in humans. In sum, we show how comparative neuroscience provides new avenues to broaden our understanding of cerebellar and cerebello-cerebral functions in the context of cognition.

Variation in nest-building behaviour in birds: a multi-species approach

Researchers have long suggested that animals with greater behavioural flexibility will be more likely to survive in face of environmental changes. However, it is unknown how this varies across species. Nest building is a behaviour directly related to the reproduction and survival of species by conferring protection from external environmental conditions. The study of nests offers a window into the behaviour of birds, and variation in nest morphology is necessarily linked to variation in building behaviours. We test whether variation in nest morphology is phylogenetically conserved by using data on nest morphology from 55 passerine species (>700 specimens) and measuring intraspecific variability in nest structure. We found that species mean and within-species variation in nest morphology are phylogenetically conserved, and that species with domed nests presented higher levels of nest morphology variation than cup nest species. We also revealed that the capacity of species to present innovative behaviours is not linked with how they vary nest morphology. Moreover, we revealed that nests from species with larger variation in clutch size and that are built by single parents are more variable. Our results help in the understanding of how behaviour and extended phenotypes evolve, and highlight the importance of exploring the phylogenetic history of behavioural flexibility when trying to predict the capacity of species to respond to novel challenges. This article is part of the theme issue 'The evolutionary ecology of nests: a cross-taxon approach'.

Adding the neuro to cognition: from food storing to nest building

Typically, investigations of animal cognition couple careful experimental manipulations with examination of the animal's behavioural responses. Sometimes those questions have included attempts to describe the neural underpinnings of the behavioural outputs. Over the past 25 years, behaviours that involve spatial learning and memory (such as navigation and food storing) has been one context in which such dual or correlated investigations have been both accessible and productive. Here I review some of that work and where it has led. Because of the wealth of data and insights gained from that work and song learning before it, it seems that it might also be useful to try to add some neurobiology to other systems in animal cognition. I finish then, with a description of recent work on the cognition and neurobiology of avian nest building. It is still relatively early days but asking questions about the cognition of nest building has already shown both neural correlates of nest building and that learning and memory play a much greater role in this behaviour than previously considered. While it is not yet clear how putting these components together will be synergistic, the examples of song learning and food storing provide encouragement. Perhaps this might be true for other behaviours too?

Proportional Cerebellum Size Predicts Fear Habituation in Chickens

The cerebellum has a highly conserved neural structure across species but varies widely in size. The wide variation in cerebellar size (both absolute and in proportion to the rest of the brain) among species and populations suggests that functional specialization is linked to its size. There is increasing recognition that the cerebellum contributes to cognitive processing and emotional control in addition to its role in motor coordination. However, to what extent cerebellum size reflects variation in these behavioral processes within species remains largely unknown. By using a unique intercross chicken population based on parental lines with high divergence in cerebellum size, we compared the behavior of individuals repeatedly exposed to the same fear test (emergence test) early in life and after sexual maturity (eight trials per age group) with proportional cerebellum size and cerebellum neural density. While proportional cerebellum size did not predict the initial fear response of the individuals (trial 1), it did increasingly predict adult individuals response as the trials progressed. Our results suggest that proportional cerebellum size does not necessarily predict an individual’s fear response, but rather the habituation process to a fearful stimulus. Cerebellum neuronal density did not predict fear behavior in the individuals which suggests that these effects do not result from changes in neuronal density but due to other variables linked to proportional cerebellum size which might underlie fear habituation.

Do sex differences in construction behavior relate to differences in physical cognitive abilities?

Nest-building behaviour in birds may be particularly relevant to investigating the evolution of physical cognition, as nest building engages cognitive mechanisms for the use and manipulation of materials. We hypothesized that nest-building ecology may be related to physical cognitive abilities. To test our hypothesis, we used zebra finches, which have sex-differentiated roles in nest building. We tested 16 male and 16 female zebra finches on three discrimination tasks in the following order: length discrimination, flexibility discrimination, and color discrimination, using different types of string. We predicted that male zebra finches, which select and deposit the majority of nesting material and are the primary nest builders in this species, would learn to discriminate string length and flexibility-structural traits relevant to nest building-in fewer trials compared to females, but that the sexes would learn color discrimination (not structurally relevant to nest building) in a similar number of trials. Contrary to these predictions, male and female zebra finches did not differ in their speed to learn any of the three tasks. There was, however, consistent among-individual variation in performance: learning speed was positively correlated across the tasks. Our findings suggest that male and female zebra finches either (1) do not differ in their physical cognitive abilities, or (2) any cognitive sex differences in zebra finches are more specific to tasks more closely associated with nest building. Our experiment is the first to examine the potential evolutionary relationship between nest building and physical cognitive abilities.

A quantitative analysis of cerebellar anatomy in birds

The cerebellum is largely conserved in its circuitry, but varies greatly in size and shape across species. The extent to which differences in cerebellar morphology is driven by changes in neuron numbers, neuron sizes or both, remains largely unknown. To determine how species variation in cerebellum size and shape is reflective of neuron sizes and numbers requires the development of a suitable comparative data set and one that can effectively separate different neuronal populations. Here, we generated the largest comparative dataset to date on neuron numbers, sizes, and volumes of cortical layers and surface area of the cerebellum across 54 bird species. Across different cerebellar sizes, the cortical layers maintained relatively constant proportions to one another and variation in cerebellum size was largely due to neuron numbers rather than neuron sizes. However, the rate at which neuron numbers increased with cerebellum size varied across Purkinje cells, granule cells, and cerebellar nuclei neurons. We also examined the relationship among neuron numbers, cerebellar surface area and cerebellar folding. Our estimate of cerebellar folding, the midsagittal foliation index, was a poor predictor of surface area and number of Purkinje cells, but surface area was the best predictor of Purkinje cell numbers. Overall, this represents the first comprehensive, quantitative analysis of cerebellar anatomy in a comparative context of any vertebrate. The extent to which these relationships occur in other vertebrates requires a similar approach and would determine whether the same scaling principles apply throughout the evolution of the cerebellum.

Sex-specific contributions to nest building in birds

The causes and consequences of interspecific variation in sex-specific contributions to animal parental care are relatively well understood during pregnancy or incubation and during offspring provisioning, but comparative patterns of sex-biased investment during nest-, den-, or other shelter-building have been almost completely overlooked. This is surprising because birthing shelters’ protective properties have important fitness consequences for both parents and offspring. Here, we address this gap in our knowledge by testing predictions concerning sex-specific contributions to avian nest building in more than 500 species of Western Palearctic birds in relation to the time available to breed and sex-specific reproductive effort, while also examining correlates with nesting site and nest structure. Using multivariate phylogenetic comparative and path analysis approaches, we found that, opposite to what had been predicted, species in which females build nests alone have shorter breeding seasons and breed at higher latitudes. In addition, species in which females lay larger clutch sizes and incubate eggs alone are more likely to have nests built by females alone, again countering predictions that reproductive contributions are not traded-off between the sexes. Finally, however, sex-specific nest building contributions were predictably related to nest site and structure, as species in which females built nests alone were more likely to have open cup nests relative to enclosed, domed nests of species in which both parents build. Our study provides important new insights, and generates several new questions for experimental research into the adaptive dynamics of sex-specific contributions prior or at the onset of parental care.

Endocranial morphology of the piciformes (Aves, Coraciimorphae): Functional and ecological implications

We used three-dimensional digital models to investigate the brain and endosseous labyrinth morphology of selected Neotropical Piciformes (Picidae, Ramphastidae, Galbulidae and Bucconidae). Remarkably, the brain morphology of Galbulidae clearly separates from species of other families. The eminentiae sagittales of Galbulidae and Bucconidae (insectivorous with high aerial maneuverability abilities) are smaller than those of the toucans (scansorial frugivores). Galbula showed the proportionally largest cerebellum, and Ramphastidae showed the least foliated one. Optic lobes ratio relative to the telencephalic hemispheres showed a strong phylogenetic signal. Three hypotheses were tested: (a) insectivorous taxa that need precise and fast movements to catch their prey, have well developed eminentiae sagittales compared to fruit eaters, (b) species that require high beak control would show larger cerebellum compared to other brain regions and higher number of visible folia and (c) there are marked differences between the brain shape of the four families studied here that bring valuable information of this interesting bird group. Hypotheses H1 and H2 are rejected, meanwhile H3 is accepted.

Correlated evolution of acrobatic display and both neural and somatic phenotypic traits in manakins (Pipridae)

Brightly colored manakin (Aves: Pipridae) males are known for performing acrobatic displays punctuated by non-vocal sounds (sonations) in order to attract dull colored females. The complexity of the display sequence and assortment of display elements involved (e.g., sonations, acrobatic maneuvers, and cooperative performances) varies considerably across manakin species. Species-specific display elements coevolve with display-distinct specializations of the neuroanatomical, muscular, endocrine, cardiovascular, and skeletal systems in the handful of species studied. Conducting a broader comparative study, we previously found positive associations between display complexity and both brain mass and body mass across 8 manakin genera, indicating selection for neural and somatic expansion to accommodate display elaboration. Whether this gross morphological variation is due to overall brain and body mass expansion (concerted evolution) versus size increases in only functionally relevant brain regions and growth of particular body ("somatic") features (mosaic evolution) remains to be explored. Here we test the hypothesis that cross-species variation in male brain mass and body mass is driven by mosaic evolution. We predicted positive associations between display complexity and variation in the volume of the cerebellum and sensorimotor arcopallium, brain regions which have roles in sensorimotor processes, and learning and performance of precisely timed and sequenced thoughts and movements, respectively. In contrast, we predicted no associations between the volume of a limbic arcopallial nucleus or a visual thalamic nucleus and display complexity as these regions have no-specific functional relationship to display behavior. For somatic features, we predicted that the relationship between body mass and complexity would not include contributions of tarsus length based on a recent study suggesting selection on tarsus length is less labile than body mass. We tested our hypotheses in males from 12 manakin species and a closely related flycatcher. Our analyses support mosaic evolution of neural and somatic features functionally relevant to display and indicate sexual selection for acrobatic complexity may increase the capacity for procedural learning via cerebellar enlargement and maneuverability via a reduction in tarsus length in species with lower overall complexity scores.

Object manipulation without hands

Our current understanding of manipulation is based on primate hands, resulting in a detailed but narrow perspective of ways to handle objects. Although most other animals lack hands, they are still capable of flexible manipulation of diverse objects, including food and nest materials, and depend on dexterity in object handling to survive and reproduce. Birds, for instance, use their bills and feet to forage and build nests, while insects carry food and construct nests with their mandibles and legs. Bird bills and insect mandibles are much simpler than a primate hand, resembling simple robotic grippers. A better understanding of manipulation in these and other species would provide a broader comparative perspective on the origins of dexterity. Here we contrast data from primates, birds and insects, describing how they sense and grasp objects, and the neural architectures that control manipulation. Finally, we outline techniques for collecting comparable manipulation data from animals with diverse morphologies and describe the practical applications of studying manipulation in a wide range of species, including providing inspiration for novel designs of robotic manipulators.

Climate as an Evolutionary Driver of Nest Morphology in Birds: A Review

Avian nests are critical for successful reproduction in birds. Nest microclimate can affect egg development, chick growth and fledgling success, suggesting that nest building behavior should be under strong selective pressure to nesting conditions. Given that the internal microclimate of the nest is critical for avian fitness, it is expected that nest morphology is shaped by the local environment. Here we review the relationship between nest morphology and climate across species’ distributions. We collate growing evidence that supports a link between environmental conditions and particular nest traits, within species and across species. We discuss the degree to which phenotypic plasticity in nesting behavior can contribute to observed variation in nest traits, the role of phylogenetic history in determining nest morphology, and which nest traits are likely to be influenced by climatic conditions. Finally, we identify gaps in our understanding of the evolution of nest morphology and suggest topics for future research. Overall, we argue that nests are part of the extended phenotype of a bird, they play a crucial role in their reproductive success, and may be an important factor in determining which species will be able to persist in the face of ongoing climate change.

Baldwin effects in early stone tools

A sizeable dataset comprising millions of lithic artifacts sampling over two million years of early paleolithic tool technology from Africa and Eurasia is now available. The widespread presupposition of an exclusively cultural, that is, socially learned, nature of early stone tools from at least Acheulean times onwards has been challenged by researchers who hypothesize that these tools, a crucial element of early hominin survival strategies, may partly have been under genetic control, next to the effects of various other determinants. The discussion this hypothesis has sparked off in the present journal is here explored somewhat further, focusing on the Baldwin effect.

Selection for Divergent Reproductive Investment Affects Neuron Size and Foliation in the Cerebellum

The cerebellum has a highly conserved internal circuitry, but varies greatly in size and morphology within and across species. Despite this variation, the underlying volumetric changes among the layers of the cerebellar cortex or their association with Purkinje cell numbers and sizes is poorly understood. Here, we examine intraspecific scaling relationships and variation in the quantitative neuroanatomy of the cerebellum in Japanese quail (Coturnix japonica) selected for high or low reproductive investment. As predicted by the circuitry of the cerebellum, the volumes of the constituent layers of the cerebellar cortex were strongly and positively correlated with one another and with total cerebellar volume. The number of Purkinje cells also significantly and positively co-varied with total cerebellar volume and the molecular layer, but not the granule cell layer or white matter volumes. Purkinje cell size and cerebellar foliation did not significantly covary with any cerebellar measures, but differed significantly between the selection lines. Males and females from the high-investment lines had smaller Purkinje cells than males and females from the low-investment lines and males from the high-investment lines had less folded cerebella than quail from the low-investment lines. These results suggest that within species, the layers of the cerebellum increase in a coordinated fashion, but Purkinje cell size and cerebellar foliation do not increase proportionally with overall cerebellum size. In contrast, selection for differential reproductive investment affects Purkinje cell size and cerebellar foliation, but not other quantitative measures of cerebellar anatomy.

Allometric Scaling Rules of the Cerebellum in Galliform Birds

Although the internal circuitry of the cerebellum is highly conserved across vertebrate species, the size and shape of the cerebellum varies considerably. Recent comparative studies have examined the allometric rules between cerebellar mass and number of neurons, but data are lacking on the numbers and sizes of Purkinje and granule cells or scaling of cerebellar foliation. Here, we investigate the allometric rules that govern variation in the volumes of the layers of the cerebellum, the numbers and sizes of Purkinje cells and granule cells and the degree of the cerebellar foliation across 7 species of galliform birds. We selected Galliformes because they vary greatly in body and brain sizes. Our results show that the molecular, granule and white matter layers all increase in volume at the same rate relative to total cerebellum volume. Both numbers and sizes of Purkinje cells increased with cerebellar volume, but numbers of Purkinje cells increased at a much faster rate than size. Granule cell numbers increased with cerebellar volume, but size did not. Sizes and numbers of Purkinje cells as well as numbers of granule cells were positively correlated with the degree of cerebellar foliation, but granule cell size decreased with higher degrees of foliation. The concerted changes among the volumes of cerebellar layers likely reflects the conserved neural circuitry of the cerebellum. Also, our data indicate that the scaling of cell sizes can vary markedly across neuronal populations, suggesting that evolutionary changes in cell sizes might be more complex than what is often assumed.

Neural Circuits Underlying Nest Building in Male Zebra Finches

Nest building consists of a series of motor actions, which are concomitant with activity in regions of the anterior motor pathway, the social behavior network, and the reward circuity in nest building adult male zebra finches (Taeniopygia guttata). It is not clear, however, whether this activity is due to nest building, collection, and/or manipulation of nest material. To identify which areas of the brain are specifically involved, we used immunohistochemistry to quantify the immediate early gene c-Fos in male zebra finches that were nest building (Building), birds given a nest box but could interact only with tied down nest material (Fixed), and birds that were not given a nest box or nest material (Control). We investigated the following brain regions: the anterior motor pathway (anterior ventral mesopallium [AMV], AN, anterior striatum [ASt]), areas of the social behavior network (bed nucleus of the stria terminalis, dorsomedial subdivision [BSTmd], lateral septum [LS]), the dopaminergic reward circuitry (ventral tegmental area), and the cerebellum. We found that there was greater Fos immunoreactivity expression in the BSTmd, LS, and AMV with increased material deposition; in LS, AMV ASt, and Folium VI with increased material carrying; in LS, AMV, and ASt with increased nest material tucking; and in LS and all folia (except Folium VIII) with increased tugging at tied down material. These data confirm a functional role for areas of the anterior motor pathway, social behavior network, and the cerebellum in nest material collection and manipulation by birds.

Comparative analysis of squamate brains unveils multi-level variation in cerebellar architecture associated with locomotor specialization

Ecomorphological studies evaluating the impact of environmental and biological factors on the brain have so far focused on morphology or size measurements, and the ecological relevance of potential multi-level variations in brain architecture remains unclear in vertebrates. Here, we exploit the extraordinary ecomorphological diversity of squamates to assess brain phenotypic diversification with respect to locomotor specialization, by integrating single-cell distribution and transcriptomic data along with geometric morphometric, phylogenetic, and volumetric analysis of high-definition 3D models. We reveal significant changes in cerebellar shape and size as well as alternative spatial layouts of cortical neurons and dynamic gene expression that all correlate with locomotor behaviours. These findings show that locomotor mode is a strong predictor of cerebellar structure and pattern, suggesting that major behavioural transitions in squamates are evolutionarily correlated with mosaic brain changes. Furthermore, our study amplifies the concept of ‘cerebrotype’, initially proposed for vertebrate brain proportions, towards additional shape characters.

The cerebellum is critical in sensory-motor control and is structurally diverse across vertebrates. Here, the authors investigate the evolutionary relationship between locomotory mode and cerebellum architecture across squamates by integrating study of gene expression, cell distribution, and 3D morphology.

Co-evolution of cerebral and cerebellar expansion in cetaceans

Cetaceans possess brains that rank among the largest to have ever evolved, either in terms of absolute mass or relative to body size. Cetaceans have evolved these huge brains under relatively unique environmental conditions, making them a fascinating case study to investigate the constraints and selection pressures that shape how brains evolve. Indeed, cetaceans have some unusual neuroanatomical features, including a thin but highly folded cerebrum with low cortical neuron density, as well as many structural adaptations associated with acoustic communication. Previous reports also suggest that at least some cetaceans have an expanded cerebellum, a brain structure with wide‐ranging functions in adaptive filtering of sensory information, the control of motor actions, and cognition. Here, we report that, relative to the size of the rest of the brain, both the cerebrum and cerebellum are dramatically enlarged in cetaceans and show evidence of co‐evolution, a pattern of brain evolution that is convergent with primates. However, we also highlight several branches where cortico‐cerebellar co‐evolution may be partially decoupled, suggesting these structures can respond to independent selection pressures. Across cetaceans, we find no evidence of a simple linear relationship between either cerebrum and cerebellum size and the complexity of social ecology or acoustic communication, but do find evidence that their expansion may be associated with dietary breadth. In addition, our results suggest that major increases in both cerebrum and cerebellum size occurred early in cetacean evolution, prior to the origin of the major extant clades, and predate the evolution of echolocation.

The role of the environment in the evolution of nest shape in Australian passerines

Avian nests present great variation in structure but, after excluding cavity nesters, probably the most obvious difference is that between open and domed nests. Some species lay their eggs in open structures, exposed to environmental variables, while other species build domed, enclosed nests with a roof, which are suggested to protect eggs and nestlings from weather conditions, high radiation levels, and predation. To date it is unclear which variables drove the evolution of different nest types. In this study, environmental and nest type information was extracted for continental Australian passerines, showing that species with open and closed nests are distributed in similar climates. However, species with open nests have larger ranges and are distributed in a wider variety of climatic conditions, suggesting open nests could be an evolutionary key innovation. This analysis was complemented with a detailed study of the evolution of particular nest traits in the largest Australasian avian radiation (Meliphagoidea), confirming that adult body size - but not environment - is an important factor in nest architecture, and larger species tend to build nests that are shallow and supported from underneath. Nest structure is a multidimensional trait that has probably evolved to match the phenotype of the nest owner, but that could also constrain or facilitate establishment in different environments.

Asynchronous evolution of interdependent nest characters across the avian phylogeny

Nest building is a widespread behavior among birds that reflects their adaptation to the environment and evolutionary history. However, it remains unclear how nests evolve and how their evolution relates to the bird phylogeny. Here, by examining the evolution of three nest characters—structure, site, and attachment—across all bird families, we reveal that nest characters did not change synchronically across the avian phylogeny but had disparate evolutionary trajectories. Nest structure shows stronger phylogenetic signal than nest site, while nest attachment has little variation. Nevertheless, the three characters evolved interdependently. For example, the ability of birds to explore new nest sites might depend on the emergence of novel nest structure and/or attachment. Our results also reveal labile nest characters in passerines compared with other birds. This study provides important insights into avian nest evolution and suggests potential associations between nest diversification and the adaptive radiations that generated modern bird lineages.

Nearly all bird species build nests; however, there is extensive variation in both how and where their nests are built. Here, Fang and colleagues reconstruct the evolution of nest structure, nest site and nest attachment across all extant bird families.

The Molecular Pathway Regulating Bergmann Glia and Folia Generation in the Cerebellum

Evolution of complex behaviors in higher vertebrates and primates require the development of sophisticated neuronal circuitry and the expansion of brain surface area to accommodate the vast number of neuronal and glial populations. To achieve these goals, the neocortex in primates and the cerebellum in amniotes have developed specialized types of basal progenitors to aid the folding of their cortices. In the cerebellum, Bergmann glia constitute such a basal progenitor population, having a distinctive morphology and playing a critical role in cerebellar corticogenesis. Here, we review recent studies on the induction of Bergmann glia and their crucial role in mediating folding of the cerebellar cortex. These studies uncover a key function of FGF-ERK-ETV signaling cascade in the transformation of Bergmann glia from radial glia in the ventricular zone. Remarkably, in the neocortex, the same signaling axis operates to facilitate the transformation of ventricular radial glia into basal radial glia, a Bergmann glia-like basal progenitor population, which have been implicated in the establishment of neocortical gyri. These new findings draw a striking similarity in the function and ontogeny of the two basal progenitor populations born in distinct brain compartments.

Transient vimentin expression during the embryonic development of the chicken cerebellum

Complex morphogenetic events, critical for the development of normal cerebellum foliation and layering, are known to involve type III intermediate filament protein such as vimentin expressed by Bergmann glia. The present study aimed to determine aspects of intermediate and late embryonic pattern of vimentin expression during the corticogenesis of chicken cerebellum at embryonic days 10-19 (E10-E19), using single and double immunohistochemistry/immunofluorescence. Vimentin expression showed partial co-localization with the glial markers GFAP and BLBP. Within cerebellar cortex, vimentin+ fibers were first found within lobules I and X (E10) and gradually extended to all folia (E15-E17), located within the external granule (EGL) the molecular cell layer, showing a radial orientation towards the inner granular layer and the cerebellar white matter oriented longitudinally. Interestingly, within the immature fissures base of most lobules, vimentin+ fibers radiate in a fan shape. Short-term BrdU experiments revealed that EGL cell proliferation was higher in the fissure base compared to folia apex. In addition, following 24-h survival, BrdU+ cells were found in close association to vimentin+ fibers in the EGL pre-migratory zone and within immature molecular layer. Paralleling cerebellum development, vimentin expression gradually extended to all folia sub-regions (base, wall, apex), but, at day E19, it was mainly confined to the folia apex and secondary fissure base. Taken together our data further support the possible role of vimentin+ fibers in the structural events of cerebellum corticogenesis, suggesting the participation of radial/Bergmann glia in chicken cerebellum foliation, similarly to that described for mammalian cerebellum morphogenesis.

Analogous mechanism regulating formation of neocortical basal radial glia and cerebellar Bergmann glia

Neocortical basal radial glia (bRG) and cerebellar Bergmann glia (BG) are basal progenitors derived from ventricular apical radial glia (aRG) that selectively lose their apical processes. bRG and BG have been implicated in the expansion and folding of the cerebrum and cerebellum, respectively. Here, we analyzed the molecular characteristics and development of bRG and BG. Transcriptomic comparison revealed striking similarity of the molecular features of bRG and BG. We found that heightened ERK signaling activity in aRG is tightly linked to the temporal formation and the relative abundance of bRG in human and mouse cortices. Forced activation of an FGF-ERK-ETV axis that is crucial to BG induction specifically induced bRG with canonical human bRG features in mice. Therefore, our data point to a common mechanism of bRG and BG generation, bearing implications to the role for these basal progenitors in the evolution of cortical folding of the cerebrum and cerebellum.

DOI:http://dx.doi.org/10.7554/eLife.23253.001

The outer layer of the brain of a mammal, called the cortex, helps support mental abilities such as memory, attention and thought. In rodents, the cortex is smooth whereas in primates it is organized into folds. These folds increase the surface area of the brain and thus the number of neurons it can contain, which may in turn increase its processing power. Folding occurs as the brain develops in the womb. Specialized cells called basal or outer radial glia, which are more abundant in humans than in rodents, are believed to trigger the folding process.

Another area of the brain, called the cerebellum, is intricately folded in both rodents and humans. As the brain develops, cells within the cerebellum called Bergmann glia cause the tissue to fold. Bergmann glia and basal radial glia share a number of similarities, but it was not known whether the same molecular pathway might regulate both types of cell.

Now, Heng et al. show that Bergmann glia in the cerebellums of mice and basal radial glia in human cortex contain similar sets of active genes. Moreover, the molecular pathway that gives rise to Bergmann glia in mice is also active in the cortex of both mice and humans. However, it is much more active in humans, leading Heng et al. to speculate that high levels of activity in this pathway might give rise to basal radial glia. Consistent with this prediction, artificially activating the pathway at high levels in mouse cortex triggered the formation of basal radial glia in mice too. These results thus suggest that a common mechanism generates both types of glial cells involved in brain folding.

The work of Heng et al. lays the foundations for further studies into how these cells fold the brain and thus how they contribute to more complex mental abilities. Remaining questions to address are whether other species with Bergmann glia also have folded cerebellums, and whether incorrect development of basal radial glia in humans leads to disorders in which the cortex folds abnormally.

DOI:http://dx.doi.org/10.7554/eLife.23253.002

Brain regions associated with visual cues are important for bird migration

Long-distance migratory birds have relatively smaller brains than short-distance migrants or residents. Here, we test whether reduction in brain size with migration distance can be generalized across the different brain regions suggested to play key roles in orientation during migration. Based on 152 bird species, belonging to 61 avian families from six continents, we show that the sizes of both the telencephalon and the whole brain decrease, and the relative size of the optic lobe increases, while cerebellum size does not change with increasing migration distance. Body mass, whole brain size, optic lobe size and wing aspect ratio together account for a remarkable 46% of interspecific variation in average migration distance across bird species. These results indicate that visual acuity might be a primary neural adaptation to the ecological challenge of migration.

Quantitative Classification of Cerebellar Foliation in Cartilaginous Fishes (Class: Chondrichthyes) Using Three-Dimensional Shape Analysis and Its Implications for Evolutionary Biology

A true cerebellum appeared at the onset of the chondrichthyan (sharks, batoids, and chimaerids) radiation and is known to be essential for executing fast, accurate, and efficient movement. In addition to a high degree of variation in size, the corpus cerebellum in this group has a high degree of variation in convolution (or foliation) and symmetry, which ranges from a smooth cerebellar surface to deep, branched convexities and folds, although the functional significance of this trait is unclear. As variation in the degree of foliation similarly exists throughout vertebrate evolution, it becomes critical to understand this evolutionary process in a wide variety of species. However, current methods are either qualitative and lack numerical rigor or they are restricted to two dimensions. In this paper, a recently developed method for the characterization of shapes embedded within noisy, three-dimensional data called spherical wave decomposition (SWD) is applied to the problem of characterizing cerebellar foliation in cartilaginous fishes. The SWD method provides a quantitative characterization of shapes in terms of well-defined mathematical functions. An additional feature of the SWD method is the construction of a statistical criterion for the optimal fit, which represents the most parsimonious choice of parameters that fits to the data without overfitting to background noise. We propose that this optimal fit can replace a previously described qualitative visual foliation index (VFI) in cartilaginous fishes with a quantitative analog, i.e. the cerebellar foliation index (CFI). The capability of the SWD method is demonstrated in a series of volumetric images of brains from different chondrichthyan species that span the range of foliation gradings currently described for this group. The CFI is consistent with the qualitative grading provided by the VFI, delivers a robust measure of cerebellar foliation, and can provide a quantitative basis for brain shape characterization across taxa.

Is Cerebellar Architecture Shaped by Sensory Ecology in the New Zealand Kiwi (Apteryx mantelli)

Among some mammals and birds, the cerebellar architecture appears to be adapted to the animal's ecological niche, particularly their sensory ecology and behavior. This relationship is, however, not well understood. To explore this, we examined the expression of zebrin II (ZII) in the cerebellum of the kiwi (Apteryx mantelli), a fully nocturnal bird with auditory, tactile, and olfactory specializations and a reduced visual system. We predicted that the cerebellar architecture, particularly those regions receiving visual inputs and those that receive trigeminal afferents from their beak, would be modified in accordance with their unique way of life. The general stripe-and-transverse region architecture characteristic of birds is present in kiwi, with some differences. Folium IXcd was characterized by large ZII-positive stripes and all Purkinje cells in the flocculus were ZII positive, features that resemble those of small mammals and suggest a visual ecology unlike that of other birds. The central region in kiwi appeared reduced or modified, with folium IV containing ZII+/- stripes, unlike that of most birds, but similar to that of Chilean tinamous. It is possible that a reduced visual system has contributed to a small central region, although increased trigeminal input and flightlessness have undoubtedly played a role in shaping its architecture. Overall, like in mammals, the cerebellar architecture in kiwi and other birds may be substantially modified to serve a particular ecological niche, although we still require a larger comparative data set to fully understand this relationship.

From neurons to nests: nest-building behaviour as a model in behavioural and comparative neuroscience

Despite centuries of observing the nest building of most extant bird species, we know surprisingly little about how birds build nests and, specifically, how the avian brain controls nest building. Here, we argue that nest building in birds may be a useful model behaviour in which to study how the brain controls behaviour. Specifically, we argue that nest building as a behavioural model provides a unique opportunity to study not only the mechanisms through which the brain controls behaviour within individuals of a single species but also how evolution may have shaped the brain to produce interspecific variation in nest-building behaviour. In this review, we outline the questions in both behavioural and comparative neuroscience that nest building could be used to address, summarize recent findings regarding the neurobiology of nest building in lab-reared zebra finches and across species building different nest structures, and suggest some future directions for the neurobiology of nest building.

Neural correlates of nesting behavior in zebra finches (Taeniopygia guttata)

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We compare markers of neural activity to nesting behavior in zebra finches.

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We visualized immediate early gene (Fos) expression in nesting and control finches.

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Fos production in motor, social, and reward neural circuits correlated with nesting.

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Fos production correlated with material pick-up in male nesting finches.

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Fos production correlated with time spent in the nest in female nesting finches.

Nest building in birds involves a behavioral sequence (nest material collection and deposition in the nest) that offers a unique model for addressing how the brain sequences motor actions. In this study, we identified brain regions involved in nesting behavior in male and female zebra finches (Taeniopygia guttata). We used Fos immunohistochemistry to quantify production of the immediate early gene protein product Fos (a molecular indicator of neuronal activity) in the brain correlated this expression with the variation in nesting behavior. Using this technique, we found that neural circuitry involved in motor sequencing, social behavior, reward and motivation were active during nesting. Within pairs of nesting birds, the number of times a male picked up or deposited nesting material and the amount of time a female spent in the nest explained the variation in Fos expression in the anterior motor pathway, social behavior network, and reward neural circuits. Identification of the brain regions that are involved in nesting enables us to begin studying the roles of motor sequencing, context, and reward in construction behavior at the neural level.