Neuromorphological Changes following Selection for Tameness and Aggression in the Russian Farm-Fox experiment

How domestication, feralization and experience-dependent plasticity affect brain size variation in Sus scrofa

Among domestic species, pigs experienced the greatest brain size reduction, but the extent and factors of this reduction remain unclear. Here, we used the brain endocast volume collected from 92 adult skulls of wild, captive, feral and domestic Sus scrofa to explore the effects of domestication, feralization and captivity over the brain size variation of this species. We found a constant brain volume increase over 24 months, while body growth slowed down from month 20. We observed an 18% brain size reduction between wild boars and pigs, disagreeing with the 30%-40% reduction previously mentioned. We did not find significant sexual differences in brain volume, refuting the theory of the attenuation of male secondary sexual characteristics through the selection for reduced male aggression. Feralization in Australia led to brain size reduction-probably as an adaptation to food scarcity and drought, refuting the reversal to wild ancestral brain size. Finally, free-born wild boars raised in captivity showed a slight increase in brain size, potentially due to a constant and high-quality food supply as well as new allospecific interactions. These results support the need to further explore the influence of diet, environment and experience on brain size evolution during animal domestication.

A prominent vertical occipital white matter fasciculus unique to primate brains

Vision in humans and other primates enlists parallel processing streams in the dorsal and ventral visual cortex, known to support spatial and object processing, respectively. These streams are bridged, however, by a prominent white matter tract, the vertical occipital fasciculus (VOF), identified in both classical neuroanatomy and recent diffusion-weighted magnetic resonance imaging (dMRI) studies. Understanding the evolution of the VOF may shed light on its origin, function, and role in visually guided behaviors. To this end, we acquired high-resolution dMRI data from the brains of select mammalian species, including anthropoid and strepsirrhine primates, a tree shrew, rodents, and carnivores. In each species, we attempted to delineate the VOF after first locating the optic radiations in the occipital white matter. In all primate species examined, the optic radiation was flanked laterally by a prominent and coherent white matter fasciculus recognizable as the VOF. By contrast, the equivalent analysis applied to four non-primate species from the same superorder as primates (tree shrew, ground squirrel, paca, and rat) failed to reveal white matter tracts in the equivalent location. Clear evidence for a VOF was also absent in two larger carnivore species (ferret and fox). Although we cannot rule out the existence of minor or differently organized homologous fiber pathways in the non-primate species, the results suggest that the VOF has greatly expanded, or possibly emerged, in the primate lineage. This adaptation likely facilitated the evolution of unique visually guided behaviors in primates, with direct impacts on manual object manipulation, social interactions, and arboreal locomotion.

Comparative anatomy of the caudate nucleus in canids and felids: Associations with brain size, curvature, cross-sectional properties, and behavioral ecology

The evolutionary history of canids and felids is marked by a deep time separation that has uniquely shaped their behavior and phenotype toward refined predatory abilities. The caudate nucleus is a subcortical brain structure associated with both motor control and cognitive, emotional, and executive functions. We used a combination of three-dimensional imaging, allometric scaling, and structural analyses to compare the size and shape characteristics of the caudate nucleus. The sample consisted of MRI scan data obtained from six canid species (Canis lupus lupus, Canis latrans, Chrysocyon brachyurus, Lycaon pictus, Vulpes vulpes, Vulpes zerda), two canid subspecies (Canis lupus familiaris, Canis lupus dingo), as well as three felids (Panthera tigris, Panthera uncia, Felis silvestris catus). Results revealed marked conservation in the scaling and shape attributes of the caudate nucleus across species, with only slight deviations. We hypothesize that observed differences in caudate nucleus size and structure for the domestic canids are reflective of enhanced cognitive and emotional pathways that possibly emerged during domestication.

The endangered brain: actively preserving ex-situ animal behaviour and cognition will benefit in-situ conservation

Endangered species have small, unsustainable population sizes that are geographically or genetically restricted. Ex-situ conservation programmes are therefore faced with the challenge of breeding sufficiently sized, genetically diverse populations earmarked for reintroduction that have the behavioural skills to survive and breed in the wild. Yet, maintaining historically beneficial behaviours may be insufficient, as research continues to suggest that certain cognitive-behavioural skills and flexibility are necessary to cope with human-induced rapid environmental change (HIREC). This paper begins by reviewing interdisciplinary studies on the 'captivity effect' in laboratory, farmed, domesticated and feral vertebrates and finds that captivity imposes rapid yet often reversible changes to the brain, cognition and behaviour. However, research on this effect in ex-situ conservation sites is lacking. This paper reveals an apparent mismatch between ex-situ enrichment aims and the cognitive-behavioural skills possessed by animals currently coping with HIREC. After synthesizing literature across neuroscience, behavioural biology, comparative cognition and field conservation, it seems that ex-situ endangered species deemed for reintroduction may have better chances of coping with HIREC if their natural cognition and behavioural repertoires are actively preserved. Evaluating the effects of environmental challenges rather than captivity per se is recommended, in addition to using targeted cognitive enrichment.

Defining hippocampal area CA2 in the fox (Vulpes vulpes) brain

Since 1959, the Russian Farm-Fox study has bred foxes to be either tame or, more recently, aggressive, and scientists have used them to gain insight into the brain structures associated with these behavioral features. In mice, hippocampal area CA2 has emerged as one of the essential regulators of social aggression, and so to eventually determine whether we could identify differences in CA2 between tame and aggressive foxes, we first sought to identify CA2 in foxes (Vulpes vulpes). As no clearly defined area of CA2 has been described in species such as cats, dogs, or pigs, it was not at all clear whether CA2 could be identified in foxes. In this study, we cut sections of temporal lobes from male and female red foxes, perpendicular to the long axis of the hippocampus, and stained them with markers of CA2 pyramidal cells commonly used in tissue from rats and mice. We observed that antibodies against Purkinje cell protein 4 best stained the pyramidal cells in the area spanning the end of the mossy fibers and the beginning of the pyramidal cells lacking mossy fibers, resembling the pattern seen in rats and mice. Our findings indicate that foxes do have a "molecularly defined" CA2, and further, they suggest that other carnivores like dogs and cats might as well. With this being the case, these foxes could be useful in future studies looking at CA2 as it relates to aggression.

The brain of the silver fox (Vulpes vulpes): a neuroanatomical reference of cell-stained histological and MRI images

Although the silver fox (Vulpes vulpes) has been largely overlooked by neuroscientists, it has the potential to serve as a powerful model for the investigation of brain-behavior relationships. The silver fox is a melanistic variant of the red fox. Within this species, the long-running Russian farm-fox experiment has resulted in different strains bred to show divergent behavior. Strains bred for tameness, aggression, or without selection on behavior present an excellent opportunity to investigate neuroanatomical changes underlying behavioral characteristics. Here, we present a histological and MRI neuroanatomical reference of a fox from the conventional strain, which is bred without behavioral selection. This can provide an anatomical basis for future studies of the brains of foxes from this particular experiment, as well as contribute to an understanding of fox brains in general. In addition, this can serve as a resource for comparative neuroscience and investigations into neuroanatomical variation among the family Canidae, the order Carnivora, and mammals more broadly.

The evolutionary neuroscience of domestication

How does domestication affect the brain? This question has broad relevance. Domesticated animals play important roles in human society, and substantial recent work has addressed the hypotheses that a domestication syndrome links phenotypes across species, including Homo sapiens. Surprisingly, however, neuroscience research on domestication remains largely disconnected from current knowledge about how and why brains change in evolution. This article aims to bridge that gap. Examination of recent research reveals some commonalities across species, but ultimately suggests that brain changes associated with domestication are complex and variable. We conclude that interactions between behavioral, metabolic, and life-history selection pressures, as well as the role the role of experience and environment, are currently largely overlooked and represent important directions for future research.

Genetics and epigenetics of human aggression

There is substantial variation between humans in aggressive behavior, with its biological etiology and molecular genetic basis mostly unknown. This review chapter offers an overview of genomic and omics studies revealing the genetic contribution to aggression and first insights into associations with epigenetic and other omics (e.g., metabolomics) profiles. We allowed for a broad phenotype definition including studies on "aggression," "aggressive behavior," or "aggression-related traits," "antisocial behavior," "conduct disorder," and "oppositional defiant disorder." Heritability estimates based on family and twin studies in children and adults of this broadly defined phenotype of aggression are around 50%, with relatively small fluctuations around this estimate. Next, we review the genome-wide association studies (GWAS) which search for associations with alleles and also allow for gene-based tests and epigenome-wide association studies (EWAS) which seek to identify associations with differently methylated regions across the genome. Both GWAS and EWAS allow for construction of Polygenic and DNA methylation scores at an individual level. Currently, these predict a small percentage of variance in aggression. We expect that increases in sample size will lead to additional discoveries in GWAS and EWAS, and that multiomics approaches will lead to a more comprehensive understanding of the molecular underpinnings of aggression.

Endocranial volume increases across captive generations in the endangered Mexican wolf

Endangered animals in captivity may display reduced brain sizes due to captive conditions and limited genetic diversity. Captive diets, for example, may differ in nutrition and texture, altering cranial musculature and alleviating constraints on cranial shape development. Changes in brain size are associated with biological fitness, which may limit reintroduction success. Little is known about how changes in brain size progress in highly managed carnivoran populations and whether such traits are retained among reintroduced populations. Here, we measured the endocranial volume of preserved Mexican wolf skulls across captive generations and between captive, wild, and reintroduced populations and assessed endocranial volume dependence on inbreeding and cranial musculature. Endocranial volume increased across captive generations. However, we did not detect a difference among captive, wild, and reintroduced groups, perhaps due to the variability across captive generations. We did not find a relationship between endocranial volume and either inbreeding or cranial musculature, although the captive population displayed an increase in the cross-sectional area of the masseter muscle. We hypothesize that the increase in endocranial volume observed across captive generations may be related to the high-quality nutrition provided in captivity.

Comparative Neuroanatomical Study of the Main Olfactory Bulb in Domestic and Wild Canids: Dog, Wolf and Red Fox

Simple Summary

The study of the morphological, physiological and molecular changes associated with the domestication process has been one of the most interesting unresolved neuroanatomical issues. The olfactory system deserves special attention since both wild and domestic canids are macrosmatic mammals with very high olfactory capacities. Nevertheless, the question remains open as to whether domestication involuted the sense of smell in domestic dogs. Further, there is a lack of comparative morphological information on the olfactory bulb, the first structure integrating olfactory sensory information in the brain. To provide comparative information on the domestication process, we studied the olfactory bulb of dogs and their two most important wild ancestors: the wolf and the fox. The study was carried out by macroscopic dissection and histological and immunohistochemical techniques and has allowed us to verify, first of all, that the three species present olfactory bulbs corresponding to a macrosmatic animal, but that there are noticeable differences not only in size, which was already known, but also in the cellularity and intensity of the immunohistochemical pattern characteristic of each species. These variations point to a reduction of the olfactory system as a consequence of the selection pressure associated with the domestication of animals.

Abstract

The sense of smell plays a fundamental role in mammalian survival. There is a considerable amount of information available on the vomeronasal system of both domestic and wild canids. However, much less information is available on the canid main olfactory system, particularly at the level of the main olfactory bulb. Comparative study of the neuroanatomy of wild and domestic canids provides an excellent model for understanding the effects of selection pressure associated with domestication. A comprehensive histological (hematoxylin–eosin, Nissl, Tolivia and Gallego’s Trichrome stains), lectin (UEA, LEA) and immunohistochemical (Gαo, Gαi2, calretinin, calbindin, olfactory marker protein, glial fibrillary acidic protein, microtubule-associated protein 2) study of the olfactory bulbs of the dog, fox and wolf was performed. Our study found greater macroscopic development of the olfactory bulb in both the wolf and fox compared to the dog. At the microscopic level, all three species show a well-developed pattern of lamination and cellularity typical of a macrosmatic animal. However, greater development of cellularity in the periglomerular and mitral layers of wild canids is characteristic. Likewise, the immunohistochemical study shows comparable results between the three species, but with a noticeably higher expression of markers in wild canids. These results suggest that the reduction in encephalization experienced in dogs due to domestication also corresponds to a lower degree of morphological and neurochemical differentiation of the olfactory bulb.

Can domestication shape Canidae brain morphology? The accessory olfactory bulb of the red fox as a case in point

BACKGROUND

The accessory olfactory bulb (AOB) is the first integrative center of the vomeronasal system (VNS), and the general macroscopic, microscopic, and neurochemical organizational patterns of the AOB differ fundamentally among species. Therefore, the low degree of differentiation observed for the dog AOB is surprising. As the artificial selection pressure exerted on domestic dogs has been suggested to play a key role in the involution of the dog VNS, a wild canid, such as the fox, represents a useful model for studying the hypothetical effects of domestication on the AOB morphology.

METHODS

A comprehensive histological, lectin-histochemical, and immunohistochemical study of the fox AOB was performed. Anti-Gαo and anti-Gαi2 antibodies were particularly useful, as they label the transduction cascade of the vomeronasal receptor types 1 (V1R) and 2 (V2R), respectively. Other employed antibodies included those against proteins such as microtubule-associated protein 2 (MAP-2), tubulin, glial fibrillary acidic protein, growth-associated protein 43 (GAP-43), olfactory marker protein (OMP), calbindin, and calretinin.

RESULTS

The cytoarchitecture of the fox AOB showed a clear lamination, with neatly differentiated layers; a highly developed glomerular layer, rich in periglomerular cells; and large inner cell and granular layers. The immunolabeling of Gαi2, OMP, and GAP-43 delineated the outer layers, whereas Gαo and MAP-2 immunolabeling defined the inner layers. MAP-2 characterized the somas of AOB principal cells and their dendritic trees. Anti-calbindin and anti-calretinin antibodies discriminated neural subpopulations in both the mitral-plexiform layer and the granular cell layer, and the lectin Ulex europeus agglutinin I (UEA-I) showed selectivity for the AOB and the vomeronasal nerves.

CONCLUSION

The fox AOB presents unique characteristics and a higher degree of morphological development compared with the dog AOB. The comparatively complex neural basis for semiochemical information processing in the fox compared with that observed in dogs suggests loss of AOB anatomical complexity during the evolutionary history of dogs and opens a new avenue of research for studying the effects of domestication on brain structures.