Social behaviour: can it change the brain?

Gonadotropin-inhibitory hormone as a regulator of social interactions in vertebrates

The social environment changes circulating hormone levels and expression of social behavior in animals. Social information is perceived by sensory systems, leading to cellular and molecular changes through neural processes. Peripheral reproductive hormone levels are regulated by activity in the hypothalamic-pituitary-gonadal (HPG) axis. Until the end of the last century, the neurochemical systems that convey social information to the HPG axis were not well understood. Gonadotropin-inhibitory hormone (GnIH) was the first hypothalamic neuropeptide shown to inhibit gonadotropin release, in 2000. GnIH is now regarded as a negative upstream regulator of the HPG axis, and it is becoming increasingly evident that it responds to social cues. In addition to controlling reproductive physiology, GnIH seems to modulate the reproductive behavior of animals. Here, we review studies investigating how GnIH neurons respond to social information and describe the mechanisms through which GnIH regulates social behavior.

Physical interactions facilitate sex change in the protogynous orange-spotted grouper, Epinephelus coioides

Sex change in teleost fishes is commonly regulated by social factors. In species that exhibit protogynous sex change, such as the orange-spotted grouper Epinephelus coioides, when the dominant males are removed from the social group, the most dominant female initiates sex change. The aim of this study was to determine the regulatory mechanisms of socially controlled sex change in E. coioides. We investigated the seasonal variation in social behaviours and sex change throughout the reproductive cycle of E. coioides, and defined the behaviour pattern of this fish during the establishment of a dominance hierarchy. The social behaviours and sex change in this fish were affected by season, and only occurred during the prebreeding season and breeding season. Therefore, a series of sensory isolation experiments was conducted during the breeding season to determine the role of physical, visual and olfactory cues in mediating socially controlled sex change. The results demonstrated that physical interactions between individuals in the social groups were crucial for the initiation and completion of sex change, whereas visual and olfactory cues alone were insufficient in stimulating sex change in dominant females. In addition, we propose that the steroid hormones 11-ketotestosterone and cortisol are involved in regulating the initiation of socially controlled sex change.

Exploring the relationship between stress coping styles and sex, origin and reproductive success, in Senegalese sole (Solea senegalensis) breeders in captivity

Individual animals commonly adopt different stress coping styles that have been shown to impact reproductive success and differ between sexes (female/male) and origin (wild/hatchery). Hatchery reared Senegalese sole (Solea senegalensis) exhibit a behavioural reproductive dysfunction and a complete failure to spawn viable eggs. Hence, the present study examined whether 1) reproductive success of Senegalese sole was linked to coping styles and 2) behavioural differences exist in relation to sex or origin. A total of 198 breeders held in two research institutions were submitted to three individual tests (restraining, new environment and confinement) and one grouping test (risk taking). In addition, a blood sample to quantify cortisol, glucose and lactate levels was obtained from each individual after completing the individual tests. Senegalese sole breeders showed individual differences in behaviour across the different behavioural tests that were consistent with proactive and reactive coping styles traits. However, the most striking result was that reproductive success, sex and origin of Senegalese sole was not biased to any specific coping style. Indeed, the behavioural responses were similar and consistent between fish grouped by reproductive success, sex and origin. This study presented information that contrasts with different studies on dominant aggressive species and indicated that social non-aggressive species such as Senegalese sole follow a cooperative strategy that favours equal opportunities between stress coping styles and sexes. Therefore, results suggest that maintaining both coping styles strategies are fundamental for a sustainable breeder population approach.

Understanding the evolution of personality requires the study of mechanisms behind the development and life history of personality traits

Research on animal personality explains the coexistence of distinct behavioural phenotypes within a species and demonstrates limits to individual plasticity. However, the mechanisms guiding the lifelong development of personality should receive more attention, because many elements of personality are emergent properties of interactions between the environment and an individual's genetic background. In these interactions, mechanisms (e.g. genetic regulatory networks, epigenetic processes and neuroendocrine regulation) influencing personality may be modified. An approach integrating proximate mechanisms with a view of lifelong personality development will crucially improve understanding stability, plasticity and inter-individual variability of personalities and clarify the effects of selection on the phenomenon.

Recent experience impacts social behavior in a novel context by adult zebrafish (Danio rerio)

Many animals exhibit behavioral plasticity as they move between habitats seasonally, reside in fluctuating environments, or respond to human-induced environmental change. We know that physical environment during early development can have a lasting impact on behavior, and on the neural mechanisms that shape behavior. In adults, social context can have similar persistent effects on behavior and the brain. Here, we asked whether physical context impacts adult social behavior in a novel environment. We placed groups of adult zebrafish (Danio rerio) in two different physical contexts. After two weeks, we measured group behavior in a novel context, and found that zebrafish with recent experience in a more-complex physical environment charged each other more often and tended to form tighter shoals than did fish that had been housed in less-complex environments. These differences were present regardless of the novel context in which we assayed behavior, and were not easily explained by differences in activity level. Our results demonstrate the impact of recent experiences on adult behavior, and highlight the importance of physical as well as social history in predicting animal behavior in novel situations.

Honey bee neurogenomic responses to affiliative and agonistic social interactions

Social interactions can be divided into two categories, affiliative and agonistic. How neurogenomic responses reflect these opposing valences is a central question in the biological embedding of experience. To address this question, we exposed honey bees to a queen larva, which evokes nursing, an affiliative alloparenting interaction, and measured the transcriptomic response of the mushroom body brain region at different times after exposure. Hundreds of genes were differentially expressed at distinct time points, revealing a dynamic temporal patterning of the response. Comparing these results to our previously published research on agonistic aggressive interactions, we found both shared and unique transcriptomic responses to each interaction. The commonly responding gene set was enriched for nuclear receptor signaling, the set specific to nursing was enriched for olfaction and neuron differentiation, and the set enriched for aggression was enriched for cytoskeleton, metabolism, and chromosome organization. Whole brain histone profiling after the affiliative interaction revealed few changes in chromatin accessibility, suggesting that the transcriptomic changes derive from already accessible areas of the genome. Although only one stimulus of each type was studied, we suggest that elements of the observed transcriptomic responses reflect molecular encoding of stimulus valence, thus priming individuals for future encounters. This hypothesis is supported by behavioral analyses showing that bees responding to either the affiliative or agonistic stimulus exhibited a higher probability of repeating the same behavior but a lower probability of performing the opposite behavior. These findings add to our understanding of the biological embedding at the molecular level.

Social Control of Hypothalamus-Mediated Male Aggression

How environmental and physiological signals interact to influence neural circuits underlying developmentally programmed social interactions such as male territorial aggression is poorly understood. We have tested the influence of sensory cues, social context, and sex hormones on progesterone receptor (PR)-expressing neurons in the ventromedial hypothalamus (VMH) that are critical for male territorial aggression. We find that these neurons can drive aggressive displays in solitary males independent of pheromonal input, gonadal hormones, opponents, or social context. By contrast, these neurons cannot elicit aggression in socially housed males that intrude in another male's territory unless their pheromone-sensing is disabled. This modulation of aggression cannot be accounted for by linear integration of environmental and physiological signals. Together, our studies suggest that fundamentally non-linear computations enable social context to exert a dominant influence on developmentally hard-wired hypothalamus-mediated male territorial aggression.

Mouse Social Network Dynamics and Community Structure are Associated with Plasticity-Related Brain Gene Expression

Laboratory studies of social behavior have typically focused on dyadic interactions occurring within a limited spatiotemporal context. However, this strategy prevents analyses of the dynamics of group social behavior and constrains identification of the biological pathways mediating individual differences in behavior. In the current study, we aimed to identify the spatiotemporal dynamics and hierarchical organization of a large social network of male mice. We also sought to determine if standard assays of social and exploratory behavior are predictive of social behavior in this social network and whether individual network position was associated with the mRNA expression of two plasticity-related genes, DNA methyltransferase 1 and 3a. Mice were observed to form a hierarchically organized social network and self-organized into two separate social network communities. Members of both communities exhibited distinct patterns of socio-spatial organization within the vivaria that was not limited to only agonistic interactions. We further established that exploratory and social behaviors in standard behavioral assays conducted prior to placing the mice into the large group was predictive of initial network position and behavior but were not associated with final social network position. Finally, we determined that social network position is associated with variation in mRNA levels of two neural plasticity genes, DNMT1 and DNMT3a, in the hippocampus but not the mPOA. This work demonstrates the importance of understanding the role of social context and complex social dynamics in determining the relationship between individual differences in social behavior and brain gene expression.

Epigenetic DNA Methylation Linked to Social Dominance

Social status hierarchies are ubiquitous in vertebrate social systems, including humans. It is well known that social rank can influence quality of life dramatically among members of social groups. For example, high-ranking individuals have greater access to resources, including food and mating prerogatives that, in turn, have a positive impact on their reproductive success and health. In contrast low ranking individuals typically have limited reproductive success and may experience lasting social and physiological costs. Ultimately, social rank and behavior are regulated by changes in gene expression. However, little is known about mechanisms that transduce social cues into transcriptional changes. Since social behavior is a dynamic process, we hypothesized that a molecular mechanism such as DNA methylation might play a role these changes. To test this hypothesis, we used an African cichlid fish, Astatotilapia burtoni, in which social rank dictates reproductive access. We show that manipulating global DNA methylation state strongly biases the outcomes of social encounters. Injecting DNA methylating and de-methylating agents in low status animals competing for status, we found that animals with chemically increased methylation states were statistically highly likely to ascend in rank. In contrast, those with inhibited methylation processes and thus lower methylation levels were statistically highly unlikely to ascend in rank. This suggests that among its many roles, DNA methylation may be linked to social status and more generally to social behavior.