Staying put or leaving home: endocrine, neuroendocrine and behavioral consequences in male African striped mice

African striped mice (Rhabdomys pumilio) as a neurobehavioral model for male parental care

Parental care is diversely demonstrated across the animal kingdom, such that active practitioners and repertoires of parental behavior vary dramatically between and within taxa. For mammals, maternal care is ubiquitous while paternal and alloparental care are rare. The African striped mouse, a rodent species in the family Muridae, demonstrates maternal, paternal, and alloparental care. Because socio-environmental factors can considerably influence the development of their social behavior, including that of paternal and alloparental care, African striped mice are considered socially flexible. Here, we highlight African striped mice as a new model for the neurobiological study of male parental care. We first provide essential background information on the species' natural ecological setting and reproductive behavior, as well as the species-relevant interaction between ecology and reproduction. We then introduce the nature of maternal, paternal, and alloparental care in the species. Lastly, we provide a review of existing developmental and neurobiological perspectives and highlight potential avenues for future research.

Social evolution in mammals

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Do alternative reproductive tactics predict problem-solving performance in African striped mice?

In changing environments, animals face unexpected problems to solve. Not all individuals in a population are equally able to solve new problems. It still remains unclear what factors (e.g. age and body condition) influence the propensity of problem solving. We investigated variation in problem-solving performance among males following alternative reproductive tactics (ARTs). We studied a free-ranging population of the African striped mouse (Rhabdomys pumilio). Adult male striped mice can employ 3 ARTs: (1) dominant group-living breeders, (2) philopatric living in their natal group, and (3) solitary-living roamers. ARTs in male striped mice reflect differences in competitiveness, sociality and physiology which could influence their problem-solving performance. We tested a total of 48 males in 2 years with two tasks: a string-pulling task to reach food and a door-opening task to reach the nest. Since male striped mice differ in personality traits independent of ARTs, we also measured activity, boldness and exploration. In addition, we assessed the association of body condition and age with problem solving. Problem solving was related the interaction of age and ARTs. The younger philopatrics had better performance in a food-extraction task whereas the older breeders were faster at solving the door-opening task. Individual differences in traits related to personality were significant correlates of problem-solving performance: pro-active mice (i.e. more active and explorative and bolder) performed better in both tasks. Finally, problem-solving performance was not consistent between the two tasks. Our study provides evidence of correlates of ARTs, age and personality on problem-solving abilities.

More than just mothers: The neurobiological and neuroendocrine underpinnings of allomaternal caregiving

In a minority of mammalian species, mothers depend on others to help raise their offspring. New research is investigating the neuroendocrine mechanisms supporting this allomaternal behavior. Several hormones have been implicated in allomaternal caregiving; however, the role of specific hormones is variable across species, perhaps because allomothering independently evolved multiple times. Brain regions involved in maternal behavior in non-human animals, such as the medial preoptic area, are also critically involved in allomaternal behavior. Allomaternal experience modulates hormonal systems, neural plasticity, and behavioral reactivity. In humans, fatherhood-induced decreases in testosterone and increases in oxytocin may support sensitive caregiving. Fathers and mothers activate similar neural systems when exposed to child stimuli, and this can be considered a global "parental caregiving" network. Finally, early work on caregiving by non-kin (e.g., foster parents) suggests reliance on similar mechanisms as biologically-related parents. This article is part of the 'Parental Brain and Behavior' Special Issue.

Changes in behavior and brain immediate early gene expression in male threespined sticklebacks as they become fathers

Motherhood is a period of intense behavioral and brain activity. However, we know less about the neural and molecular mechanisms associated with the demands of fatherhood. Here, we report the results of two experiments designed to track changes in behavior and brain activation associated with fatherhood in male threespined stickleback fish (Gasterosteus aculeatus), a species in which fathers are the sole providers of parental care. In experiment 1, we tested whether males' behavioral reactions to different social stimuli depends on parental status, i.e. whether they were providing parental care. Parental males visited their nest more in response to social stimuli compared to nonparental males. Rates of courtship behavior were high in non-parental males but low in parental males. In experiment 2, we used a quantitative in situ hybridization method to compare the expression of an immediate early gene (Egr-1) across the breeding cycle - from establishing a territory to caring for offspring. Egr-1 expression peaked when the activities associated with fatherhood were greatest (when they were providing care to fry), and then returned to baseline levels once offspring were independent. The medial dorsal telencephalon (basolateral amygdala), lateral part of dorsal telencephalon (hippocampus) and anterior tuberal nucleus (ventral medial hypothalamus) exhibited high levels of Egr-1 expression during the breeding cycle. These results help to define the neural circuitry associated with fatherhood in fishes, and are consistent with the hypothesis that fatherhood - like motherhood - is a period of intense behavioral and neural activity.

Compared to what: What can we say about nonapeptide function and social behavior without a frame of reference?

Our understanding of behavior and mechanism is undermined by the absence of a frame of reference because relationships between individuals and species are without context. We highlight a need to be more comparative, using nonapeptide (vasopressin and oxytocin) modulation of social behavior as an example. We reconsider the use of model organisms and the term 'social' in this context, contrasting two popular models for nonapeptide regulation of social behavior. We then propose that a frame of reference should be established by studying mechanisms of behavior across taxa along the same continua. If we are to ever establish a unifying theory of behavior, we must transcend individual examples and determine the relative relationships of behavior and mechanism among and between species.

Alternative reproductive tactics in female striped mice: Solitary breeders have lower corticosterone levels than communal breeders

Alternative reproductive tactics (ARTs), where members of the same sex and population show distinct reproductive phenotypes governed by decision-rules, have been well-documented in males of many species, but are less well understood in females. The relative plasticity hypothesis (RPH) predicts that switches between plastic ARTs are mediated by changes in steroid hormones. This has received much support in males, but little is known about the endocrine control of female ARTs. Here, using a free-living population of African striped mice (Rhabdomys pumilio) over five breeding seasons, we tested whether females following different tactics differed in corticosterone and testosterone levels, as reported for male striped mice using ARTs, and in progesterone and oestrogen, which are important in female reproduction. Female striped mice employ three ARTs: communal breeders give birth in a shared nest and provide alloparental care, returners leave the group temporarily to give birth, and solitary breeders leave to give birth and do not return. We expected communal breeders and returners to have higher corticosterone, owing to the social stress of group-living, and lower testosterone than solitary breeders, which must defend territories alone. Solitary breeders had lower corticosterone than returners and communal breeders, as predicted, but testosterone and progesterone did not differ between ARTs. Oestrogen levels were higher in returners (measured before leaving the group) than in communal and solitary breeders, consistent with a modulatory role. Our study demonstrates hormonal differences between females following (or about to follow) different tactics, and provides the first support for the RPH in females.

Oxytocin and vasopressin systems in genetic syndromes and neurodevelopmental disorders

Oxytocin (OT) and arginine vasopressin (AVP) are two small, related neuropeptide hormones found in many mammalian species, including humans. Dysregulation of these neuropeptides have been associated with changes in behavior, especially social interactions. We review how the OT and AVP systems have been investigated in Autism Spectrum Disorder (ASD), Prader-Willi Syndrome (PWS), Williams Syndrome (WS) and Fragile X syndrome (FXS). All of these neurodevelopmental disorders (NDD) are marked by social deficits. While PWS, WS and FXS have identified genetic mutations, ASD stems from multiple genes with complex interactions. Animal models of NDD are invaluable for studying the role and relatedness of OT and AVP in the developing brain. We present data from a FXS mouse model affecting the fragile X mental retardation 1 (Fmr1) gene, resulting in decreased OT and AVP staining cells in some brain regions. Reviewing the research about OT and AVP in these NDD suggests that altered OT pathways may be downstream from different etiological factors and perturbations in development. This has implications for ongoing studies of the therapeutic application of OT in NDD. This article is part of a Special Issue entitled Oxytocin and Social Behav.

Arginine vasopressin plasma levels change seasonally in African striped mice but do not differ between alternative reproductive tactics

Arginine vasopressin (AVP) is an important hormone for osmoregulation, while as a neuropeptide in the brain it plays an important role in the regulation of social behaviors. Dry habitats are often the home of obligately sociable species such as meerkats and Damaraland mole-rats, leading to the hypothesis that high plasma AVP levels needed for osmoregulation might be associated with the regulation of social behavior. We tested this in a facultative sociable species, the African striped mouse (Rhabdomys pumilio). During the moist breeding season, both solitary- and group-living reproductive tactics occur in this species, which is obligatory sociable in the dry season. We collected 196 plasma samples from striped mice following different reproductive tactics both during the moist and the dry season. Solitary mice did not have lower AVP levels than sociable mice, rejecting the hypothesis that peripheral AVP is involved in the regulation of alternative reproductive tactics. However, we found significantly higher AVP levels during the dry season, with AVP levels correlated with the abundance of food plants, the main source of water for striped mice. Plasma AVP levels were not correlated with testosterone or corticosterone levels. Our study underlines the important role that AVP plays in osmoregulation, particularly for a free ranging mammal living under harsh arid conditions.

Growing up in the family or growing up alone influences behavior and hormones, but not arginine vasopressin receptor 1a expression in male African striped mice

In many species males display alternative reproductive tactics (ARTs). While males of different tactics differ behaviorally in the field, it is often not known whether these behavioral differences would also occur under standardized laboratory conditions, nor how ARTs are regulated by the brain. In the present study we kept male African striped mice (Rhabdomys pumilio) in captivity either in family groups or solitary, to mimic ARTs observed in the field. This allowed us to study these males behaviorally under standardized conditions, to replicate physiological findings from the field, and to study the expression of the arginine vasopressin 1a receptor (AVPR1a) in their brains. Changes in either peptide release or receptor expression (or both) might regulate ARTs with differential timelines, with peptide secretion being faster than receptor expression. As observed in the field, family living males had higher corticosterone but lower testosterone levels than singly housed males. Surprisingly, singly housed males were less aggressive while at the same time having higher testosterone levels. We found no differences in AVPR1a expression. In a previous study it was shown that singly housed males have higher levels of AVP stored in their brain, which potentially could be secreted when the social situation changes, for example to establish social bonds. Our study on AVPR1a suggests the hypothesis that, given that the receptor distribution and expression of singly housed males do not differ from that of family-living males, the brains of singly-housed males have a similar capacity to be responsive to AVP when given the chance to interact socially.

Experimental increase of testosterone increases boldness and decreases anxiety in male African striped mouse helpers

Males of many species can adjust their behaviors to environmental conditions by changing reproductive tactics. Testosterone surges in adult breeding males typically inhibit the expression of paternal care while facilitating the expression of aggression during environmental changes. Similarly, in non-breeding philopatric males of cooperatively breeding species, up-regulation of testosterone may inhibit alloparental care while facilitating dispersal, i.e. males might become bolder and more explorative. We tested this hypothesis in philopatric male African striped mice, Rhabdomys pumilio. Striped mouse males can either remain in their natal groups providing alloparental care or they can disperse seeking mating opportunities. Compared to philopatric males, dispersed males typically show higher testosterone levels and lower corticosterone levels, and more aggression toward pups and same sex conspecifics. We experimentally increased the testosterone levels of the philopatric males kept in their family groups when pups were present. Testosterone-treated males did not differ significantly from control males in alloparental care and in aggression toward same-sex conspecifics. Compared to the control males, testosterone treated males were bolder, more active, and less anxious; they also showed lower corticosterone levels. The philopatric males were sensitive to our testosterone treatment for dispersal- and anxiety-like behavior but insensitive for social behaviors. Our results suggest a role of testosterone in dispersal.

Life in groups: the roles of oxytocin in mammalian sociality

In recent decades, scientific understanding of the many roles of oxytocin (OT) in social behavior has advanced tremendously. The focus of this research has been on maternal attachments and reproductive pair-bonds, and much less is known about the substrates of sociality outside of reproductive contexts. It is now apparent that OT influences many aspects of social behavior including recognition, trust, empathy, and other components of the behavioral repertoire of social species. This review provides a comparative perspective on the contributions of OT to life in mammalian social groups. We provide background on the functions of OT in maternal attachments and the early social environment, and give an overview of the role of OT circuitry in support of different mating systems. We then introduce peer relationships in group-living rodents as a means for studying the importance of OT in non-reproductive affiliative behaviors. We review species differences in oxytocin receptor (OTR) distributions in solitary and group-living species of South American tuco-tucos and in African mole-rats, as well as singing mice. We discuss variation in OTR levels with seasonal changes in social behavior in female meadow voles, and the effects of OT manipulations on peer huddling behavior. Finally, we discuss avenues of promise for future investigation, and relate current findings to research in humans and non-human primates. There is growing evidence that OT is involved in social selectivity, including increases in aggression toward social outgroups and decreased huddling with unfamiliar individuals, which may support existing social structures or relationships at the expense of others. OT’s effects reach beyond maternal attachment and pair bonds to play a role in affiliative behavior underlying “friendships”, organization of broad social structures, and maintenance of established social relationships with individuals or groups.

Endocrinology of sociality: comparisons between sociable and solitary individuals within the same population of African striped mice

The social organization of species ranges from solitary-living to complex social groups. While the evolutionary reasons of group-living are well studied, the physiological mechanisms underlying alternative social systems are poorly understood. By studying group-living and solitary individuals of the same species, we can determine hormonal correlates of sociality without the problem of confounding phylogenetic factors. The African striped mouse (Rhabdomys pumilio) is a socially flexible species, which can be solitary or alternatively form complex family groups, depending on population density and the extent of reproductive competition. We predicted group-living striped mice to show signs of reproductive suppression and social stress, resulting in higher corticosterone but lower testosterone levels when compared to solitary-living individuals. To determine whether differences in social organization correlated with hormonal differences, we collected blood samples from free-living striped mice during four breeding seasons when we experimentally induced solitary-living in philopatric individuals by locally reducing population density. Striped mice that were group-living did not change their corticosterone or estosterone levels during the study, indicating that there was no temporal effect during the breeding season. Striped mice of both sexes had significantly lower corticosterone levels after switching from group- to solitary-living. Solitary males - but not solitary females - had higher testosterone levels than group-living conspecifics. Our results suggest that group-living results in physiological stress and can induce reproductive suppression, at least in philopatric males. The switch to solitary-living may thus be a tactic to avoid reproductive competition within groups, and is associated with decreased stress hormone levels and onset of independent reproduction.