Morphological, Genetic, and Behavioral Comparisons of Two Prairie Vole Populations in the Field and Laboratory

Dorsal CA1 lesions of the hippocampus impact mating tactics in prairie voles by shifting non-monogamous males' use of space to resemble monogamous males

Alternative mating tactics within mating systems are characterized by discrete patterns of spatio-temporal overlap with same-and opposite-sex conspecifics and mating-relevant outcomes. Socially monogamous "residents" maintain relatively small home range sizes, have territories that almost exclusively overlap with their mating partners, and are more likely to produce offspring than non-bonded "wandering" conspecifics. Because mating tactics appear to be so closely tied to patterns of space use, differences in spatial cognitive abilities might differentially impact individual males' decisions to adopt a particular mating tactic and/or how efficient they are within their chosen mating tactic. Yet few studies have considered how the hippocampus, a brain region important for encoding cognitive maps and for processing contextual information, might impact how individuals adopt mating tactics or the spatio-temporal behaviors closely associated with them. We assessed the impact of lesions to the dorsal CA1 (dCA1) region of the hippocampus on male prairie vole space use, reproductive success, and mating tactics in semi-natural outdoor field conditions. Interestingly, dCA1 lesions did not impact the proportion of males that adopted resident or wandering mating tactics, and dCA1 lesions did not impact a male's ability to form a pair bond in the lab. In contrast, we found that lesioning the dCA1 shifted the home range size of reproductively successful and unsuccessful males. Furthermore, we found that patterns of space use among residents were unaffected by dCA1 lesions, whereas wanderers with dCA1 lesions showed pronounced reductions of their space use habits and resembled non-lesioned residents. Collectively, our study supports the hypothesis that wanderer male prairie voles rely on dCA1-mediated spatial cognition to navigate their world in a way that resident males do not. Such differences might have implications for how individuals efficiently attract and defend mates, obtain resources, defend territories, and outcompete rivals.

Still little evidence sex differences in spatial navigation are evolutionary adaptations

A putative male advantage in wayfinding ability is the most widely documented sex difference in human cognition and has also been observed in other animals. The common interpretation, the sex-specific adaptation hypothesis, posits that this male advantage evolved as an adaptive response to sex differences in home range size. A previous study a decade ago tested this hypothesis by comparing sex differences in home range size and spatial ability among 11 species and found no relationship. However, the study was limited by the small sample size, the lack of species with a larger female home range and the lack of non-Western human data. The present study represents an update that addresses all of these limitations, including data from 10 more species and from human subsistence cultures. Consistent with the previous result, we found little evidence that sex differences in spatial navigation and home range size are related. We conclude that sex differences in spatial ability are more likely due to experiential factors and/or unselected biological side effects, rather than functional outcomes of natural selection.

Effects of social and environmental contexts on multi-male mating and mixed paternity in socially monogamous female prairie voles

With whom and how often to mate are fundamental questions that impact individual reproductive success and the mating system. Relatively few studies have investigated female mating tactics compared with males. Here, we asked how differential access to mates influences the occurrence of mixed paternity and overall reproductive success in socially monogamous female prairie voles (Microtus ochrogaster). We created male- and female-biased sex ratios of prairie voles living in semi-natural outdoor enclosures. We ran paternity analyses to determine the identity and number of mating partners females had and the number of offspring produced. We found that 57.1% of females had litters fathered by two or more males when males outnumbered females, and 87.5% of females had litters with more than one father when females outnumbered males. However, the percentage of mixed paternity and the total number of embryos were not statistically different between social contexts. We determined that female fecundity (i.e. number of embryos) correlated with the number of male fathers in each litter across social contexts. Although our study did not support the hypothesis that social context directly influences female mating decisions, it did suggest that female multi-male mating might lead to increased fertilization success under semi-natural conditions.

Prairie vole offspring only prefer mothers over fathers when mothers are a unique resource, yet fathers are the primary source of variation in parental care

In biparental species, each parent represents a semi-independent source of variable caregiving. The nature of care may differ between parents, and the type of care offspring seek is likely to change across development. We asked if caregiving differed between prairie vole (Microtus ochrogaster) mothers and fathers, and which parent pups prefer over development. We categorized parents as high- or low-contact based on daily recordings of grooming and brooding behavior. Pups were tested for their preferences between parents on postnatal days 10, 14, 18, 22, and 26. We expected individual parents would vary in the amount of care they gave, with pups preferring the parent that gives the most care and/or that most meets their needs at each developmental timepoint. Mothers spent more time in contact with pups than fathers. Mothers were consistent in the amount of care they gave, whereas fathers were variable caregivers. Pups never preferred fathers over mothers, but only demonstrated a preference for mothers before weaning. Lastly, the amount of contact did not influence pup preferences. Our data indicate that mothers are consistent sources of caregiving relative to fathers, and pups show little evidence of attachment to a specific parent beyond meeting its own immediate needs.

Social Games and Genic Selection Drive Mammalian Mating System Evolution and Speciation

Mating system theory based on economics of resource defense has been applied to describe social system diversity across taxa. Such models are generally successful but fail to account for stable mating systems across different environments or shifts in mating system without a change in ecological conditions. We propose an alternative approach to resource defense theory based on frequency-dependent competition among genetically determined alternative behavioral strategies characterizing many social systems (polygyny, monogamy, sneak). We modeled payoffs for competition, neighborhood choice, and paternal care to determine evolutionary transitions among mating systems. Our model predicts four stable outcomes driven by the balance between cooperative and agonistic behaviors: promiscuity (two or three strategies), polygyny, and monogamy. Phylogenetic analysis of 288 rodent species supports assumptions of our model and is consistent with patterns of evolutionarily stable states and mating system transitions. Support for model assumptions include that monogamy and polygyny evolve from promiscuity and that paternal care and monogamy are coadapted in rodents. As predicted by our model, monogamy and polygyny occur in sister taxa among rodents more often than by chance. Transitions to monogamy also favor higher speciation rates in subsequent lineages, relative to polygynous sister lineages. Taken together, our results suggest that genetically based neighborhood choice behavior and paternal care can drive transitions in mating system evolution. While our genic mating system theory could complement resource-based theory, it can explain mating system transitions regardless of resource distribution and provides alternative explanations, such as evolutionary inertia, when resource ecology and mating systems do not match.

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.

The Monogamy Paradox: What Do Love and Sex Have to Do With It?

Genetic monogamy is rare-at least at the level of a species-and monogamy can exist in the absence of sexual fidelity. Rather than focusing on mating exclusivity, it has become common to use the term "social monogamy" to describe a cluster of social features, including the capacity for selective and lasting social bonds, central to what humans call "love." Socially monogamous mammals often exhibit selective aggression toward strangers and form extended families. These features of social monogamy in mammals are supported by patterns of hormonal function originating in the neurobiology of maternity, including oxytocin, as well as a more primitive vasopressin pathway. Another key feature of social monogamy is reduced sexual dimorphism. Processes associated with sexual differentiation offer clues to the mysteries surrounding the evolution of monogamy. Although there is consistency in the necessary ingredients, it is likely that there is no single recipe for social monogamy. As reviewed here, genes for steroids and peptides and their receptors are variable and are subject to epigenetic regulation across the lifespan permitting individual, gender and species variations and providing substrates for evolution. Reduced sensitivity to gonadal androgens, and a concurrent increased reliance on vasopressin (for selective defense) and oxytocin (for selective affiliation) may have offered pathways to the emergence of social monogamy.

Individual Differences in Social Behavior and Cortical Vasopressin Receptor: Genetics, Epigenetics, and Evolution

Social behavior is among the most complex and variable of traits. Despite its diversity, we know little about how genetic and developmental factors interact to shape natural variation in social behavior. This review surveys recent work on individual differences in the expression of the vasopressin 1a receptor (V1aR), a major regulator of social behavior, in the neocortex of the socially monogamous prairie vole. V1aR exhibits profound variation in the retrosplenial cortex (RSC), a region critical to spatial and contextual memory. RSC-V1aR abundance is associated with patterns of male space-use and sexual fidelity in the field: males with high RSC-V1aR show high spatial and sexual fidelity to partners, while low RSC-V1aR males are significantly more likely to mate outside the pair-bond. Individual differences in RSC-V1aR are predicted by a set of linked single nucleotide polymorphisms within the avpr1a locus. These alternative alleles have been actively maintained by selection, suggesting that the brain differences represent a balanced polymorphism. Lastly, the alleles occur within regulatory sequences, and result in differential sensitivity to environmental perturbation. Together the data provide insight into how genetic, epigenetic and evolutionary forces interact to shape the social brain.

The neurobiological causes and effects of alloparenting

Alloparenting, defined as care provided by individuals other than parents, is a universal behavior among humans that has shaped our evolutionary history and remains important in contemporary society. Dysfunctions in alloparenting can have serious and sometimes fatal consequences for vulnerable infants and children. In spite of the importance of alloparenting, they still have much to learn regarding the underlying neurobiological systems governing its expression. Here, they review how a lack of alloparental behavior among traditional laboratory species has led to a blind spot in our understanding of this critical facet of human social behavior and the relevant neurobiology. Based on what is known, they draw from model systems ranging from voles to meerkats to primates to describe a conserved set of neuroendocrine mechanisms supporting the expression of alloparental care. In this review we describe the neurobiological and behavioral prerequisites, ontogeny, and consequences of alloparental care. Lastly, they identified several outstanding topics in the area of alloparental care that deserve further research efforts to better advance human health and wellbeing. © 2016 Wiley Periodicals, Inc. Develop Neurobiol 77: 214-232, 2017.

A preference to bond? Male prairie voles form pair bonds even in the presence of multiple receptive females

Pair bonds are the cornerstone of a monogamous relationship. When individuals of the same species engage in monogamy and promiscuity (i.e. alternative reproductive tactics) it can be difficult to determine which tactic confers greater fitness, as measures of fitness can be difficult to ascertain. However, in these circumstances, whether animals preferentially establish pair bonds can reveal decisions that presumably reflect the animals' assessment of how to best maximize reproductive success. In nature, the majority of prairie voles, Microtus ochrogaster, establishes pair bonds and engages in social monogamy while a minority of individuals remains single and presumably mates promiscuously. The existence of these two tactics raises the interesting question: do bonded male prairie voles choose to 'settle' (for just one partner) or are they preferentially 'settling down'? To determine which of these two tactics is preferred, we provided single male prairie voles simultaneous access to two sexually receptive females for 24 h and then subsequently tested males in 'partner preference tests' with each female independently contrasted with a novel female. We aimed to determine whether males would form a pair bond with one, both or none of the original females. We found that males formed pair bonds with one of the two females. We also investigated male- and female-initiated aggression and found that during the bonding process males were more aggressive with females that they did not ultimately form a bond with. In the partner preference tests, males showed more aggression towards unfamiliar females than towards familiar females. Mismatches in male- and female-initiated aggression suggest that aggressive interactions may be perpetuated more by males than by females. Taken together, our results demonstrate that under conditions that are ideal for forgoing bonding and engaging in multiple matings, males choose to establish a pair bond, suggesting that selective pressures may have facilitated bonding by males.

Sexual fidelity trade-offs promote regulatory variation in the prairie vole brain

Individual variation in social behavior seems ubiquitous, but we know little about how it relates to brain diversity. Among monogamous prairie voles, levels of vasopressin receptor (encoded by the gene avpr1a) in brain regions related to spatial memory predict male space use and sexual fidelity in the field. We find that trade-offs between the benefits of male fidelity and infidelity are reflected in patterns of territorial intrusion, offspring paternity, avpr1a expression, and the evolutionary fitness of alternative avpr1a alleles. DNA variation at the avpr1a locus includes polymorphisms that reliably predict the epigenetic status and neural expression of avpr1a, and patterns of DNA diversity demonstrate that avpr1a regulatory variation has been favored by selection. In prairie voles, trade-offs in the fitness consequences of social behaviors seem to promote neuronal and molecular diversity.

Social recognition in paired but not single male prairie voles

Social recognition is an integral component of behavior that underlies many much larger behavioral suites. For example, monogamous pair bonding is relatively meaningless if an individual cannot recall with whom the bond was with. Prairie voles (Microtus ochrogaster) are a socially monogamous rodent, well known for their long-term pair bonds between males and females. Although previous work has shown that bonded males reliably spend more time with their pair-mate over an unfamiliar female, recent work has demonstrated that single male prairie voles do not discriminate between females. This discrepancy raises the important question: do paired males distinguish between non-mate females? We asked whether pair bonding alters the expression of social recognition in male voles by comparing social recognition of single and pair bonded males using the habituation/dishabituation paradigm. We found that pair bonded, but not single male prairie voles exhibit social recognition of (non-mate) females, suggesting a shift in cognitive behavior after pair bond formation. This difference is not due to differences in motivation to engage in social exchanges, as males attempted to contact unfamiliar females at similar levels. Based on these data, we speculate that the stage of life (single or bonded) influences the relevance of attending to social information of same- and opposite-sex conspecifics.

Intraspecific variation in social organization by genetic variation, developmental plasticity, social flexibility or entirely extrinsic factors

Previously, it was widely believed that each species has a specific social organization, but we know now that many species show intraspecific variation in their social organization. Four different processes can lead to intraspecific variation in social organization: (i) genetic variation between individuals owing to local adaptation (between populations) or evolutionarily stable strategies within populations; (ii) developmental plasticity evolved in long-term (more than one generation) unpredictable and short-term (one generation) predictable environments, which is mediated by organizational physiological effects during early ontogeny; (iii) social flexibility evolved in highly unpredictable environments, which is mediated by activational physiological effects in adults; (iv) entirely extrinsic factors such as the death of a dominant breeder. Variation in social behaviour occurs between individuals in the case of genetic variation and developmental plasticity, but within individuals in the case of social flexibility. It is important to study intraspecific variation in social organization to understand the social systems of species because it reveals the mechanisms by which species can adapt to changing environments, offers a useful tool to study the ultimate and proximate causes of sociality, and is an interesting phenomenon by itself that needs scientific explanation.

Female alternative mating tactics, reproductive success and nonapeptide receptor expression in the social decision-making network

The decision to mate may be one of the most important decisions that animals make. For monogamous species, this decision can carry the added weight of limiting future mating opportunities. The mechanisms that govern these decisions have presumably been shaped by evolution in ways that optimize decision-making processes. In particular, a so-called social decision-making network (SDM) has been proposed, which integrates brain structures comprising the 'social behavior network' with a neural system associated with reward. Here, we investigate the neural phenotypic differences in the SDM for oxytocin and vasopressin receptors (OTR, V1aR) of female socially monogamous prairie voles living in naturalistic conditions. We focus on these receptors because they are profoundly involved in mammalian social behavior. We found that V1aR in the bed nucleus of the stria terminalis, medial amygdala and ventral pallidum, and OTR in the nucleus accumbens and hippocampus significantly differed between pregnant and non-pregnant females. Most of these areas are more closely related to the reward component of the SDM. V1aR in the ventral pallidum was also greater in paired than in single females. Finally, reproductive success within mating tactics was related to receptor density in brain structures across the SDM, particularly those serving as the interface between the social behavior network and the reward system. Our data support the hypothesis that neural phenotype for neuromodulatory nonapeptide receptors within the SDM relates to natural behavior associated with reproductive decisions.

Oxytocin receptor density is associated with male mating tactics and social monogamy

Despite its well-described role in female affiliation, the influence of oxytocin on male pairbonding is largely unknown. However, recent human studies indicate that this nonapeptide has a potent influence on male behaviors commonly associated with monogamy. Here we investigated the distribution of oxytocin receptors (OTR) throughout the forebrain of the socially monogamous male prairie vole (Microtus ochrogaster). Because males vary in both sexual and spatial fidelity, we explored the extent to which OTR predicted monogamous or non-monogamous patterns of space use, mating success and sexual fidelity in free-living males. We found that monogamous males expressed higher OTR density in the nucleus accumbens than non-monogamous males, a result that mirrors species differences in voles with different mating systems. OTR density in the posterior portion of the insula predicted mating success. Finally, OTR in the hippocampus and septohippocampal nucleus, which are nuclei associated with spatial memory, predicted patterns of space use and reproductive success within mating tactics. Our data highlight the importance of oxytocin receptor in neural structures associated with pairbonding and socio-spatial memory in male mating tactics. The role of memory in mating systems is often neglected, despite the fact that mating tactics impose an inherently spatial challenge for animals. Identifying mechanisms responsible for relating information about the social world with mechanisms mediating pairbonding and mating tactics is crucial to fully appreciate the suite of factors driving mating systems. This article is part of a Special Issue entitled Oxytocin, Vasopressin, and Social Behavior.

Reflections on Don Juan and on the utility of the unhappy love affair

Based on an unusual clinical experience of a teenage boy in child psychotherapy, two conclusions are proposed: (1) that the extremely unhappy, early love affairs that occur in most men's lives serve a valuable function in helping them separate from their mothers sufficiently to be able to realistically relate to appropriate marriage partners, and (2) that some Don Juans start new relationships in order to break them off, rather than the reverse.

Female prairie vole mate-choice is affected by the males' birth litter composition

Experimental testing and retrospective examination of breeding records were used to examine the influence of sex composition and/or size of males' birth litters on female mate-choice. Sexually naïve female prairie voles (Microtus ochrogaster) avoided males derived from all-male litters, but showed no preference for, or aversion to, males from single-male litters or from more typical mixed-sex litters. Examination of the pregnancy status of females after two weeks of pairing with a male allowed us to estimate the probabilites of a pups' intrauterine position relative to siblings for various litter sizes. The typical prairie vole pup derived from a mixed-sex litter comprised of 4.4 pups, and had a 13% chance of being isolated from siblings in utero and a 22% chance of being between siblings in utero. Pups from single-sex litters tended to be larger at weaning than did pups from mixed-sex litters; however, male size did not influence female choice behavior. These results suggest that some aspect of the perinatal experience of prairie vole pups from single sex litters can influence social interactions later in life.

Asynchronous breeding in the socially monogamous prairie vole

At least two hypotheses have been proposed for why animals should breed synchronously: (1) to swamp predators and reduce the probability of offspring being killed and (2) to promote monogamy when paternal care is important for offspring survival. Thus prey and monogamous species are likely candidates for synchronous breeding. We conducted an experiment to determine if a prey species, the monogamously breeding prairie vole ( Microtus ochrogaster (Wagner, 1842)), breeds synchronously. We conducted eight replicates in which we placed six nulliparous females and six adult males in semi-natural enclosures for 18–21 days to determine if they bred synchronously. The time of conception ranged from 2 to 18 days within replicates with no indication of breeding synchrony within any of the populations. Thus neither predator avoidance nor paternal care models were supported for prairie voles. We conclude that rodents in general are not good models for breeding synchrony and that females use alternative mating tactics to enhance their lifetime reproductive success. We discuss our results in the context of the prairie vole mating system.

Variation in neural V1aR predicts sexual fidelity and space use among male prairie voles in semi-natural settings

Although prairie voles (Microtus ochrogaster) are socially monogamous, males vary in both sexual and spatial fidelity. Most males form pairbonds, cohabit with one female, and defend territories. Wandering males, in contrast, have expansive home ranges that overlap many males and females. In the laboratory, pairing is regulated by arginine vasopressin and its predominant CNS receptor, vasopressin 1a receptor (V1aR). We investigated individual differences in forebrain V1aR expression of male prairie voles in mixed-sex seminatural enclosures. Individual differences in V1aR were compared with space use measured by radio telemetry and paternity determined with microsatellite markers. Animals engaging in extra-pair fertilizations (EPFs) as either wanderers or paired residents overlapped significantly more in same- and opposite-sex home ranges. Surprisingly, neither social fidelity measured by space use nor sexual fidelity measured by paternity was associated with V1aR expression in the ventral pallidum (VPall) or lateral septum, areas causally related to pairbond formation. In contrast, V1aR expression in the posterior cingulate/retrosplenial cortex (PCing) and laterodorsal thalamus (LDThal), areas implicated in spatial memory, strongly covaried with space use and paternity. Animals engaging in EPFs either as wanderers or paired residents exhibited low levels of LDThal and PCing V1aR expression. Individual differences in brain and behavior parallel differences between prairie voles and promiscuous congeners. The concordance among space use, paternity, and V1aR in spatial circuits suggests a common link between the mechanisms of spatial behaviors and success at EPF. The combined data demonstrate how organismal biology can inform our understanding of individual and species differences in behavioral mechanisms.

Anogenital distance predicts female choice and male potency in prairie voles

Anogenital distance (AGD) in rodents is a useful indicator of masculinization or feminization due to prenatal hormonal effects. If such cues convey useful information about both 'maleness' and more importantly mate quality, then females may select males based on this cue or other cues related to it. We tested this hypothesis by asking if female prairie voles (Microtus ochrogaster) preferred males with relatively longer AGDs and if AGD correlated with fitness enhancing characteristics such as sperm count, sperm size, and gonad size. Not only did preferred males have significantly longer AGD and larger testes than nonpreferred males, but AGD was directly related to the testes size, seminal vesicle size, and the number of sperm stored. We re-evaluated data collected in semi-natural field enclosures and discovered that males that were members of a pairbond had longer AGD than single males. This later result, taken under semi-natural conditions, was consistent with results we obtained in the laboratory. Taken together these data indicate that AGD serves as a useful cue of male potency and that females preferentially associate with males that demonstrate this masculinized phenotype. Moreover, these data imply that females may select mates based on their potential to effectively fertilize ova, a potentially important trait for a species that forms life-long pairbonds.