Seasonal changes in open-field behavior in wild male meadow voles (Microtus pennsylvanicus)

Stress in groups: Lessons from non-traditional rodent species and housing models

A major feature of life in groups is that individuals experience social stressors of varying intensity and type. Social stress can have profound effects on health, social behavior, and ongoing relationships. Relationships can also buffer the experience of exogenous stressors. Social stress has most commonly been investigated in dyadic contexts in mice and rats that produce intense stress. Here we review findings from studies of diverse rodents and non-traditional group housing paradigms, focusing on laboratory studies of mice and rats housed in visible burrow systems, prairie and meadow voles, and mole-rats. We argue that the use of methods informed by the natural ecology of rodent species provides novel insights into the relationship between social stress, behavior and physiology. In particular, we describe how this ethologically inspired approach reveals how individuals vary in their experience of and response to social stress, and how ecological and social contexts impact the effects of stress. Social stress induces adaptive changes, as well as long-term disruptive effects on behavior and physiology.

Frank Beach award winner: Neuroendocrinology of group living

Why do members of some species live in groups while others are solitary? Group living (sociality) has often been studied from an evolutionary perspective, but less is known about the neurobiology of affiliation outside the realms of mating and parenting. Colonial species offer a valuable opportunity to study nonsexual affiliative behavior between adult peers. Meadow voles (Microtus pennsylvanicus) display environmentally induced variation in social behavior, maintaining exclusive territories in summer months, but living in social groups in winter. Research on peer relationships in female meadow voles demonstrates that these selective preferences are mediated differently than mate relationships in socially monogamous prairie voles, but are also impacted by oxytocin and HPA axis signaling. This review addresses day-length dependent variation in physiology and behavior, and presents the current understanding of the mechanisms supporting selective social relationships in meadow voles, with connections to lessons from other species.

Neural Circuits Underlying Rodent Sociality: A Comparative Approach

All mammals begin life in social groups, but for some species, social relationships persist and develop throughout the course of an individual's life. Research in multiple rodent species provides evidence of relatively conserved circuitry underlying social behaviors and processes such as social recognition and memory, social reward, and social approach/avoidance. Species exhibiting different complex social behaviors and social systems (such as social monogamy or familiarity preferences) can be characterized in part by when and how they display specific social behaviors. Prairie and meadow voles are closely related species that exhibit similarly selective peer preferences but different mating systems, aiding direct comparison of the mechanisms underlying affiliative behavior. This chapter draws on research in voles as well as other rodents to explore the mechanisms involved in individual social behavior processes, as well as specific complex social patterns. Contrasts between vole species exemplify how the laboratory study of diverse species improves our understanding of the mechanisms underlying social behavior. We identify several additional rodent species whose interesting social structures and available ecological and behavioral field data make them good candidates for study. New techniques and integration across laboratory and field settings will provide exciting opportunities for future mechanistic work in non-model species.

Stress impairs new but not established relationships in seasonally social voles

Affiliative social relationships are impacted by stressors and can shape responses to stress. However, the effects of stress on social relationships in different contexts are not well understood. Meadow voles provide an opportunity to study these effects on peer relationships outside of a reproductive context. In winter months, female meadow voles cohabit with peers of both sexes, and social huddling is facilitated by exposure to short, winter-like day lengths in the lab. We investigated the role of stress and corticosterone (cort) levels in social behavior in short day-housed female meadow voles. A brief forced swim elevated cort levels, and we assessed the effects of this stressor on new and established relationships between females. In pairs formed following exposure to swim stress, the stressor significantly reduced the fraction of huddling time subjects spent with a familiar partner. Swim stress did not affect partner preferences in pairs established prior to the stressor. Finally, we examined fecal glucocorticoid metabolite levels via immunoassay in voles housed under short day (10h light) versus long day (14 h light) conditions and detected higher glucocorticoid levels in long day-housed voles. These findings support a role for stress regulation in the formation of social relationships in female meadow voles, and are consistent with a potential role for seasonal variation in cort in the behavioral transition from solitary to social. Together they highlight the importance of stress and possibly glucocorticoid signaling for social behavior.

Corticotropin-releasing factor receptor densities vary with photoperiod and sociality

Life in social groups relies on prosocial behaviors as well as on reduction of antisocial behaviors such as aggression and territoriality. The mechanisms supporting variation in behaviors that give rise to group living (sociality) are largely unknown. Female meadow voles exhibit natural seasonal variation in sociality: females are aggressive and territorial in summer, while in winter they share burrows and nest in mixed-sex groups. This behavioral shift is paralleled in the lab by day length-dependent variation in partner preference formation and social huddling. We exploit natural variation in meadow vole sociality in order to examine changes in neural pathways that coincide with environmental and behavioral variations. Mounting evidence suggests that the corticotropin-releasing factor system, encompassing multiple peptides and two receptor subtypes (CRF1 and CRF2), may play an important role in regulating social behaviors. We report day-length dependent variation in CRF1 and CRF2 receptor binding in female meadow voles, and relate these findings to previously collected oxytocin receptor (OTR) binding data and behavioral data for the same individuals. CRF1 receptor binding was greater in summer-like long day lengths (LD), particularly in the hippocampus, while CRF2 receptor binding was greater in winter-like short day lengths (SD) in the cingulate cortex and hippocampus. OTR varied with day length in the bed nucleus of the stria terminalis, nucleus accumbens, and hippocampus. SD voles huddled more extensively than LD voles, and greater huddling time was associated with more CRF1 receptor binding and less CRF2 receptor binding in subregions of the lateral septum. CRF2 receptor associations with behavior mirrored those of OTR in the lateral septum. Finally, estradiol treatment affected density of CRF receptors in multiple brain regions. CRF receptors and their ligands are promising candidates for enhancing understanding of the regulation of non-sexual social behavior between group living peers.

Sexually selected traits: a fundamental framework for studies on behavioral epigenetics

Emerging evidence suggests that epigenetic-based mechanisms contribute to various aspects of sex differences in brain and behavior. The major obstacle in establishing and fully understanding this linkage is identifying the traits that are most susceptible to epigenetic modification. We have proposed that sexual selection provides a conceptual framework for identifying such traits. These are traits involved in intrasexual competition for mates and intersexual choice of mating partners and generally entail a combination of male-male competition and female choice. These behaviors are programmed during early embryonic and postnatal development, particularly during the transition from the juvenile to adult periods, by exposure of the brain to steroid hormones, including estradiol and testosterone. We evaluate the evidence that endocrine-disrupting compounds, including bisphenol A, can interfere with the vital epigenetic and gene expression pathways and with the elaboration of sexually selected traits with epigenetic mechanisms presumably governing the expression of these traits. Finally, we review the evidence to suggest that these steroid hormones can induce a variety of epigenetic changes in the brain, including the extent of DNA methylation, histone protein alterations, and even alterations of noncoding RNA, and that many of the changes differ between males and females. Although much previous attention has focused on primary sex differences in reproductive behaviors, such as male mounting and female lordosis, we outline why secondary sex differences related to competition and mate choice might also trace their origins back to steroid-induced epigenetic programming in disparate regions of the brain.

Spatial Navigation Strategies in Peromyscus: a Comparative Study

A male advantage in spatial abilities is predicted to evolve in species where males rely on expansion of home territory to locate dispersed mates during the breeding season. We sought to examine mechanistic underpinnings of this evolved trait by comparing spatial navigational abilities in two species of Peromyscus that employ widely different reproductive strategies. Males and females from outbred stocks of deer mice (P. maniculatus bairdii) in which males engage in territorial expansion and mate search and California mice (P. californicus insignis), in which males do not, were administered tasks that assessed spatial learning and memory, and activity and exploratory behaviours. The maze employed for these studies included four spatial cues that could be used to aid in locating 1 of 12 potential escape holes. As predicted, male deer mice outperformed conspecific females and California mice males in maze performance and memory, and this difference appeared to be due to extent to which animals used spatial cues to guide maze navigation. Consistent with territorial expansion as a component of competition for mates, male deer mice were more active and engaged in more exploratory and less anxiety-related behaviours than conspecific females and California mice males. The results have implications for understanding and studying the cognitive and behavioural mechanisms that have evolved through male-male competition that involves territorial expansion and mate search.

Sex differences in spatial cognition in an invertebrate: the cuttlefish

Evidence of sex differences in spatial cognition have been reported in a wide range of vertebrate species. Several evolutionary hypotheses have been proposed to explain these differences. The one best supported is the range size hypothesis that links spatial ability to range size. Our study aimed to determine whether male cuttlefish (Sepia officinalis; cephalopod mollusc) range over a larger area than females and whether this difference is associated with a cognitive dimorphism in orientation abilities. First, we assessed the distance travelled by sexually immature and mature cuttlefish of both sexes when placed in an open field (test 1). Second, cuttlefish were trained to solve a spatial task in a T-maze, and the spatial strategy preferentially used (right/left turn or visual cues) was determined (test 2). Our results showed that sexually mature males travelled a longer distance in test 1, and were more likely to use visual cues to orient in test 2, compared with the other three groups. This paper demonstrates for the first time a cognitive dimorphism between sexes in an invertebrate. The data conform to the predictions of the range size hypothesis. Comparative studies with other invertebrate species might lead to a better understanding of the evolution of cognitive dimorphism.

Plasma testosterone levels are related to various aspects of locomotor activity in wild-caught male meadow voles (Microtus pennsylvanicus)

The relationship between plasma testosterone levels and locomotor activity in wild-caught sexually mature male meadow voles (Microtus pennsylvanicus) was assessed in the laboratory. Several aspects of locomotor activity were monitored for 1 h on two consecutive days using the automated Digiscan activity monitoring system. Plasma testosterone levels were determined immediately following the second day of activity monitoring. Significant Pearson correlations were obtained between plasma testosterone levels and total distance traveled [r(10) = 0.55, p < 0.05] and amount of time spent in movement [r(10) - 0.55, p < 0.05] on the second day. The wild voles showed a reduction in activity levels from the first to the second day of activity monitoring, which is indicative of habituation to a novel environment. This study provides direct evidence for a significant correlation between laboratory measures of behavioral activity and plasma testosterone levels in a wild-caught rodent. These findings indicate that previous assessments of hormone-behavior relationships in laboratory-bred rodents are consistent with the relationship between hormones and behavior in wild rodents.

Sexually dimorphic spatial learning varies seasonally in two populations of deer mice

Spatial learning in photoperiodically induced breeding (reproductive) and non-breeding (non-reproductive) adult male and female deer mice (Peromyscus maniculatus) was examined in a Morris water-maze task. Sexually mature, adult male and female deer mice that were derived from either a mainland population (P. m. artemisiae) or an island population (P. m. angustus) were required to learn the spatial position of a hidden, submerged platform in a water maze. Deer mice were tested either during the breeding season (summer; long day photoperiod) or during the non-breeding season (winter; short day photoperiod) with a total of six blocks of four trials conducted in a single day. Retention was tested with two probe trials which occurred one and three days after acquisition. During the breeding season male spatial task acquisition was superior to female spatial task acquisition for both populations. In contrast, during the non-breeding season there were no significant sex differences in spatial acquisition for either population. This change in sexually dimorphic spatial learning was due to female spatial-performance decreasing from non-breeding season to the breeding season and male spatial-performance increasing over the same period. Both populations displayed similar seasonal variations in sexually dimorphic water-maze task performance. There were, however, overall population differences in water-maze task performance that were related to the ecology of the mice, with the insular mice displaying shorter latencies to reach the hidden platform than did the mainland deer mice.(ABSTRACT TRUNCATED AT 250 WORDS)