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Sex differences in the reward value of familiar mates in prairie voles

The rewarding properties of social interactions facilitate relationship formation and maintenance. Prairie voles are one of the few laboratory species that form selective relationships, manifested as “partner preferences” for familiar partners versus strangers. While both sexes exhibit strong partner preferences, this similarity in outward behavior likely results from sex‐specific neurobiological mechanisms. We recently demonstrated that in operant trials, females worked hardest for access to familiar conspecifics of either sex, while males worked equally hard for access to any female, indicating a sex difference in social motivation. As tests were performed with one social target at a time, males might have experienced a ceiling effect, and familiar females might be more relatively rewarding in a choice scenario. Here we performed an operant social choice task in which voles lever‐pressed to gain temporary access to either the chamber containing their mate or one containing a novel opposite‐sex vole. Females worked hardest to access their mate, while males pressed at similar rates for either female. Individual male behavior was heterogeneous, congruent with multiple mating strategies in the wild. Voles exhibited preferences for favorable over unfavorable environments in a non‐social operant task, indicating that lack of social preference does not reflect lack of discrimination. Natural variation in oxytocin receptor genotype at the intronic single nucleotide polymorphism NT213739 was associated with oxytocin receptor density, and predicted individual variation in stranger‐directed aggressive behavior. These findings suggest that convergent preference behavior in male and female voles results from sex‐divergent pathways, particularly in the realm of social motivation.

likely results from sex-specific neurobiological mechanisms. We recently demonstrated that in operant trials, females worked hardest for access to familiar conspecifics of either sex, while males worked equally hard for access to any female, indicating a sex difference in social motivation. As tests were performed with one social target at a time, males might have experienced a ceiling effect, and familiar females might be more relatively rewarding in a choice scenario. Here we performed an operant social choice task in which voles lever-pressed to gain temporary access to either the chamber containing their mate or one containing a novel opposite-sex vole. Females worked hardest to access their mate, while males pressed at similar rates for either female. Individual male behavior was heterogeneous, congruent with multiple mating strategies in the wild. Voles exhibited preferences for favorable over unfavorable environments in a non-social operant task, indicating that lack of social preference does not reflect lack of discrimination. Natural variation in oxytocin receptor genotype at the intronic single nucleotide polymorphism NT213739 was associated with oxytocin receptor density, and predicted individual variation in stranger-directed aggressive behavior. These findings suggest that convergent preference behavior in male and female voles results from sex-divergent pathways, particularly in the realm of social motivation.

K E Y W O R D S
operant conditioning, Oxtr, partner preference, prairie vole, sex differences, social motivation, social reward

| INTRODUCTION
Prairie voles are socially monogamous rodents that often form lifelong opposite-sex pair bonds in natural environments, and both males and females exhibit selective preferences for familiar mates in the laboratory. [1][2][3] Despite similar partner preference behavior, there are several indications that mechanisms underlying bond formation in males and females may differ-including in the timing of bond formation, reliance on specific neuropeptide signaling pathways, and the role of behavioral reward. [4][5][6] We recently demonstrated that female prairie voles work harder to gain access to mates versus stranger males, whereas males work equally hard to access mates and unfamiliar females, 4 indicating high salience of any female. However, such testing was conducted with only one available social stimulus at a time, leaving open the possibility that males would still work harder for a known mate over an unfamiliar stranger if given an opportunity to do so. In the present study, we developed an operant choice setup to determine whether prairie voles would consistently learn to press for one stimulus versus another (non-social choice testing), then asked whether males and females would both exhibit preferences for familiar mates versus unfamiliar opposite-sex strangers in operant social choice tests.
We then related oxytocin receptor genotype data to affiliative and aggressive behaviors recorded during these tests.
The partner preference test (PPT), developed in the laboratory of Sue Carter, has become the standard method to assess preferences for familiar mates in voles. 3 During the PPT, the focal vole is placed in the middle of a three-chamber apparatus where they can freely explore and spend time near either a familiar vole (e.g., partner) or an unfamiliar vole (e.g., novel, opposite-sex subject/stranger), both of whom are tethered at opposite ends. 3,7 Partner preference is assessed by measuring how much time a focal vole spends huddling with each of the stimulus voles. While this method can indicate selective preference for familiar mates, such preferences could emerge for a variety of reasons, such as enhanced motivation to interact with a mate, increased tolerance of a mate, and/or aversion of unfamiliar "stranger" voles. Because subjects can freely interact with stimulus voles, data from a PPT cannot distinguish between these hypotheses.
Social motivation and reward signaling are important components of prairie vole mate relationships. Dopamine signaling is necessary for social bond formation and maintenance between mates, 8,9 although this does not appear to be the case for same-sex peer relationships. 10 Opioid signaling is also necessary for partnership maintenance, and interacts with dopamine signaling within the nucleus accumbens. 11,12 Behaviorally, prairie voles show a conditioned preference for cues associated with mates in socially conditioned place preference tests, 13,14 although unfamiliar opposite-sex conditioning has not been tested. These studies strongly suggest that social motivationreflecting a drive to seek a socially rewarding partner-is involved in prairie vole mate relationships.
Previous operant paradigms assessing social motivation in rodents have also relied on focal subjects having only one social option at a time (reviewed in Ref. 15). For example, rates of pressing for pup delivery differ in dams with different lesions, 16 and rates of nosepokes for access to aggress upon a subordinate mouse differ by drug treatment. 17 Different social rewards have been presented on different days, for example female mice tend to press more for access to a novel female versus male conspecific when presented on alternating days, 18 and only female prairie voles pressed more for familiar voles in similar tests. 4 Operant choice tests have typically been used to make comparisons between social and non-social rewards; for instance male Syrian hamsters will overcome a weighted door more often for access to a social chamber with a novel conspecific over an empty chamber. 19 Males of two different strains of mice preferentially lever press for food versus social rewards, 20 while rats press for social rewards versus food. 21 Social operant studies also routinely employ unfamiliar strangers as the social reward for lever pressing. To our knowledge, only one other study has measured social preference in rodents given simultaneous opportunity to access familiar versus unfamiliar conspecifics. 21 Female rats placed in a two-choice operant apparatus press more for access to a unfamiliar female conspecific (non-cagemate) over an familiar female conspecific (cagemate). 21 Rats' novelty preference in an operant social choice test is consistent with their behavior in non-operant social choice tests and peer partner preference tests, in which rats tend to either prefer novelty or lack social preferences, but do not display familiarity preferences. 22,23 In this study, we developed an operant choice setup to determine whether prairie vole males would work harder for familiar mates than unfamiliar conspecifics, as females were expected to. We assessed whether these voles would exhibit consistent preferences across days for one (clearly preferable) stimulus over another in a non-social choice paradigm ( Figure 1) to determine whether learned associations between a particular lever and a particular exposure would occur. We then directly assessed the role of familiarity in social motivation in female and male prairie voles by providing an opportunity for voles to "work" for access to familiar and unfamiliar opposite-sex conspecifics in a two-choice operant apparatus ( Figure 1). While prairie voles exhibited consistent preferences in the choice apparatus, there was a striking divergence in social motivation by sex; males pressed a similar amount for both partner and stranger chambers, whereas females preferentially work to access their mate. While males exhibited no overall preferences, individual males differed in their behavior, with some consistently preferring a familiar mate, others preferring a stranger, and yet others exhibiting a lack of preference. This greater heterogeneity among males than females may be related to alternative mating tactics among male prairie voles in the wild. Genotype at the intronic NT213739 single nucleotide polymorphism (SNP) in the oxytocin receptor gene (Oxtr) has recently been associated with individual variation in striatal oxytocin receptor density as well as partner preference formation in prairie voles. 24,25 We found genotypereceptor density correlations, along with a strong genotypeaggression relationship in males, underscoring oxytocin signaling's role in selective/antisocial aspects of pairbonding.

| Animals
Male and female prairie voles (Microtus ochrogaster) were bred locally at Smith College, as described in previous studies. 13,26 Prairie voles were bred in long day lengths (14 h light:10 h dark) and weaned at 21 days of age. At weaning, subjects were initially housed in a samesex pair with an age-matched individual, typically a littermate. Once voles were at least 45 days old, they began the training and testing protocol described below. A total of 16 focal animals (7 females; 9 males) completed training and testing with 16 opposite-sex partners (7 males; 9 females) and 30 opposite-sex strangers (14 females; 16 males). Missing data and exclusions from analysis are described in the statistical analysis section. All procedures were approved by the Institutional Animal Care and Use Committee of Smith College (ASAF #272) and were conducted in accordance with national guidelines.

| Experimental timeline
Voles passed through three phases of the study: training, non-social choice operant testing, and social choice operant testing ( Figure 1).
During phase 1/food training, focal voles were trained to press a lever for a pellet of food using a single-lever apparatus. Lever pressing behavior was shaped using manual reinforcement, and gradually transitioned to automated reward delivery (described below in Food training, Section 2.6).
After a focal vole learned the association and motor skill, they transitioned to phase 2/non-social choice to establish whether voles could learn a stable association between two levers and their rewards, and exhibit a preference. Voles were placed in a two-lever apparatus with chambers on either side of the central chamber. In place of a food reward, lever pressing now resulted in access to two different chambers: one "good" (positive valence environment), and one "bad" (negative valence environment) chamber, described in detail below.
After completing food training ($12 days on average) and the nonsocial choice "good/bad" protocol (5 days), focal voles were placed in a fresh cage with an infertile but hormonally intact opposite-sex vole.
24 h after pairing, initial bond strength was assessed using a partner preference test (PPT). 48 h following the PPT, voles began phase 3/social choice testing. Focal voles lever pressed for access to two different chambers containing their partner (familiar) and a stranger (unfamiliar) opposite-sex vole. The social choice phase was carried out across 8 days. The day after the final social choice operant session, focal voles were administered a second PPT to assess late-stage bond strength, and finally underwent 4 days of testing with their partner in a single social chamber (non-choice) social operant test. All testing occurred during the light phase. At the conclusion of the study, voles were sacrificed and liver samples were collected for Oxtr genotype analysis. Oxtr genotype was also collected from liver samples from our prior social operant study. 4 Genotype data were analyzed across the two studies, and compared to oxytocin receptor autoradiography data from the first study.
F I G U R E 1 Testing apparatuses and timeline. Top panel: Testing apparatus illustration by phase. Left: In phase 1, voles underwent food training within a single-chamber apparatus. In phases 2 and 3 (middle and right, respectively), voles were placed in a three-chamber apparatus, wherein the middle operant chamber contained levers that contingently opened a door on either side to a choice chamber containing an environmental ("good" or "bad"; non-social choice) or social opportunity (partner or stranger vole; social choice). Bottom panel: Experimental timeline. Apparatus diagrams are shown above the phases with which they were associated

| Gonadectomies and hormonal replacement
Prior to the formation of opposite sex pairs, partners of focal voles were rendered infertile in order to prevent reproduction, as pregnancy can affect the strength of partner relationships. 27 Female partners of male focal voles underwent tubal ligation to preserve an intact hormonal state. Bilateral incisions were made on the dorsal skin and muscle wall of the voles to access the ovaries. Two knots were placed below each ovary at the top of the uterine horn, sparing the vasculature. The wound was closed using a sterile suture.
Male partners of female focal voles were castrated, then implanted with testosterone (T) capsules to maintain circulating T levels. Testes were accessed by a midline incision, and a tie was placed over the testicular artery to cut off the blood supply. Testes Voles received 0.05 mg/kg buprenorphine and 1.0 mg/kg metacam subcutaneously prior to surgery, and again the following day.
Female strangers were not surgically altered, as female partners remained hormonally intact following tubal ligation. Male strangers were also intact, as prior testing indicated no difference in female effort to access hormonally intact strangers versus castrated + T implanted strangers ( Figure S1).

| Testing schedules
Voles were trained using different operant training schedules as detailed below: fixed-ratio (FR), and progressive-ratio (PR). FR schedules required a subject to lever press a fixed number of times before a reward was provided (food pellet, chamber access, etc.). For PR, the number of lever presses needed to gain a reward increased proportionally to the number of rewards received. For example, in a PR-1 schedule, one lever press initially yields one reward; however, the vole then needed to press two times to receive the next reward, 3 for the following reward, and so on. Specific schedules are shown in Figure 1 and described in the methods for each testing phase below.

| Food training
In order to motivate voles to learn to associate a lever with reward delivery, voles were placed on food restriction and given food pellets

| Operant non-social choice
Once subjects learned how to lever press, they were transitioned to a three-chamber apparatus for the non-social choice phase ("good/ bad"). The goal for this environmental preference test was to verify that voles could learn the association of each lever with its chamber and demonstrate preference for a rewarding chamber over an aversive one through lever presses. Voles underwent daily non-social choice testing for 6 days.
For the non-social choice tests, the middle chamber contained two levers on opposite sides of the chamber. Each lever was adjacent to a separate door that led to a tube connecting the middle operant (lever pressing) chamber to two other "choice" chambers. Good and bad sides were randomly assigned and counterbalanced across subjects. The good or bad side for each vole remained constant throughout testing (e.g., if the good chamber was on the left side for day 1, it remained on the left side throughout habituation and actual testing days). On the "good" side, voles were presented with a layer of soiled bedding from their home cage, fresh produce (spinach, carrots, and apples), Cheerios, a Shepherd Shack, and a novel object that changed daily (e.g., a metal sports whistle). On the "bad" side, voles were presented with a shallow tray of water (n = 12 [7 females; 5 males]). The first four males underwent testing in a "good"/"neutral" paradigm (neutral = empty chamber) before the water trays were printed. These subjects had a similar testing experience but were not included in the main analysis.

| Testing (days 3-6)
The first 2 days of testing were identical to the latter 20 min of habituation day 2, with the entire testing period totaling 30 min. For the last 2 days of testing, the testing schedule was increased to a FR-4.

| Partner preference testing
Partner preference tests were conducted at "early" and "late" stages: 24 h after pairing with an opposite-sex mate, and at the conclusion of social operant testing, 14 days after pairing. PPTs were carried out as previously described using a three-chambered apparatus connected by tubes (each chamber measured 17 cm Â 28 cm Â 12.5 cm). 13,26,29 Opposite-sex "partner" and "stranger" voles were tethered on opposing sides of the three-chamber apparatus. The untethered focal vole was then placed in the middle chamber and was freely able to explore the entire apparatus for 3 h. PPT sessions were video recorded for subsequent offline behavioral analysis. Time in each chamber was scored, as was the time a focal vole spent in physical contact with another subject ("huddling"). The number of aggressive bouts was also noted for each session.

| Operant social choice
The same apparatus was used for social choice as for non-social choice testing. Before a test session was initiated, the focal vole's partner was tethered to one of the choice sides, and a novel oppositesex vole (the "stranger") was tethered to the other choice side. A different (novel) stranger vole was used each for each social choice testing session. As with the choice environment testing, partner/stranger sides were consistent across the entire social choice testing phase.
Partner and stranger sides were counterbalanced across subjects such that approximately half of all subjects had the partner on the same side as the former "good" side from the previous phase (non-social choice), whereas the other subjects had the partner on the "bad" side.
Novel visual cues were also used for operant social choice training and testing. All social choice testing sessions were video recorded for off-line analysis.  As in another prairie vole colony, the C/C genotype was rare 24 ; thus C/C and C/T individuals were pooled as C carriers. Samples from both studies were genotyped together. Genotype data from study 1 are related to oxytocin receptor binding data obtained in that study (see methods in Ref. 4), and genotype data from both studies are analyzed relative to aggressive behavior for all individuals for whom both genotype and aggression data were available (n = 41). For the social choice phase (n = 14; 5 females, 9 males), huddling preference was defined as % time huddling (min)/total access time (minutes with that social target's door raised). Partner preferences were analyzed by 2 Â 2 ANOVA (huddling [partner; stranger] Â sex).

| Statistical analysis
After screening for sex differences, social choice data were separately analyzed by sex using a 2 Â 2 ANOVA (chamber [partner; stranger] Â schedule [d1-4/FR; d5-8/PR]). Individual paired t-tests were run across social choice data for lever presses Â chamber (partner; stranger) to determine chamber preference. Aggressive bouts data were analyzed using a 2 Â 2 ANOVA (average bouts across the 8 days of social choice Â chamber). For aggression correlation analyses, aggressive bouts were scaled relative to the number of door openings (aggressive bouts/total access opportunities). Two female subjects were excluded from analysis for the social choice phase due to low operant activity (<20 lever presses in total across all 8 days of the social phase).

Comparisons of behavior by Oxtr genotype were conducted by
Welch's t-test assuming unequal variances. Oxytocin receptor binding data by genotype was compared in four brain regions by 2-way ANOVA (genotype*brain region) followed by within-group comparisons adjusted using the False Discovery Rate procedure of Benjamini, Krieger, and Yekutieli. 31 Statistical analyses were performed using RStudio version 1.4.1103 32 running R version 3.5.2 33 using the following packages: tidyverse 34 ; and stats (aov; TukeyHSD [adjusted for multiple comparisons]). 33 3 | RESULTS

| Non-social (environmental) preferences
Non-social operant choice tests were used to establish whether voles would consistently learn to associate specific levers with different stimuli, and lever press more for the preferred stimulus. A two-way ANOVA (sex [male; female] * chamber [good; bad]) revealed that voles lever pressed more for access to the "good" chamber (F [1,10] = 18.627, p = 0.002; Figure 2). There was no significant effect Lever presses (mean 4d) ** ** F I G U R E 2 Prairie voles can distinguish between negative ("bad") and positive ("good) valence environments, and demonstrate preference for the latter. Males and females alike selectively lever pressed for access to the "good" environment over the "bad" one during the non-social choice phase. Dots represent the mean number of lever presses over 4-days of testing. Bars represent group means. Asterisks indicate significant environment preference within sex. **p < 0.01 of sex (p = 0.392) nor a sex * chamber interaction (p = 0.548). This indicates that both male and female voles are able to reliably distinguish between chambers and the corresponding levers that provides access. Thus, any lack of preference would not indicate lack of learning ability in the test.

| Lever pressing
We first examined whether relative lever pressing (P pressing -S pressing) differed by sex. Females pressed relatively more for their partner than did males (t [11.9] = À2.26, p = 0.04). Within-sex, females lever-pressed significantly more for access to the chamber containing their partner over a stranger vole (F [1,4] = 15.37, p = 0.017; Figure 3A), with no effect of schedule (F [1,4]  As there was notable individual variation in lever pressing preference, particularly among males, we followed up group analyses with individual analyses. When daily pressing of individual subjects was examined across the social choice phase, three of five females significantly or marginally (p < 0.1) preferred their partners, while the remaining two pressed more for their partner on average, but with no significant difference ( Figure 3B). Among males, three of nine significantly or marginally preferred their partners, one significantly preferred the stranger female, and the remaining five exhibited no consistent preferences ( Figure 3C).

| Huddling
In addition to analyzing lever pressing, we also explored the amount of time focal voles spent huddling with their partner or a stranger.
During PPT1, both males and females significantly preferred huddling with partner over the stranger ( Figure

| Oxtr genotype, oxytocin receptor density, and social behavior
Oxtr genotype at the intronic locus NT213739 has been associated with both oxytocin receptor binding density within the nucleus accumbens, and preference behaviors. 24,37 We asked whether oxytocin receptor genotype was associated with oxytocin receptor binding density and social behavior in animals from the present study as well as samples collected from a prior social operant study recently con- was compared to oxytocin receptor densities obtained for females in study 1. C carriers exhibited substantially higher OTR binding in specific brain regions ( Figure 5A; two-way ANOVA; effect of genotype: p < 0.002, effect of brain region: p < 0.0234, genotype*region interaction: p < 0.0008). As in prior studies, C carriers exhibited higher binding in the nucleus accumbens (p < 0.0002, adjusted for multiple comparisons). In addition, the lateral septum (LS) was newly identified as a region with significant genotype-dependent binding density (p < 0.0005 LS, adjusted for multiple comparisons).
Oxtr genotype was also associated with behavior. Strangerdirected aggression was consistent across groups, and could thus be Lever pressing effort was also compared to Oxtr genotype across data sets in comparable groups. Within study 1, female lever pressing was comparable in female-female and female-male pairs. Across these groups, female C carriers exhibited a trend towards greater lever pressing for the partner/total (p = 0.056). This is consistent with both higher NAcc OTR in C carriers, as well as the positive correlation between NAcc OTR and partner lever pressing reported in study 1. Lever pressing effort was also compared in the groups that were matched across study 1 and the present study: opposite-sex housed males and females pressing on a PR-1 schedule for their partner in a single social chamber (this was the main operant apparatus used in study 1, and was also used at the conclusion of the present study). No relationships between lever pressing effort and genotype were found in male or female focal individuals across these data sets.

| DISCUSSION
Social contact with mates is behaviorally rewarding for prairie voles, and there have been some indications of sex differences in reward value. 4,13,14 The present study extends these findings, demonstrating that even when faced with a direct choice between a partner and a stranger, males do not consistently work harder to access their partner, unlike female prairie voles. This sex difference reveals a striking disconnect between social motivation and partner preference, as both Oxtr genotype predicted variation in aggressive behavior, with male C carriers exhibiting reduced stranger-directed aggression relative to T/T homozygotes and behaviorally, for instance engaging in biparental care and forming partner preferences for a pair-bonded mate. Such behavioral similarity between the sexes may, however, arise from sex-specific mechanisms that compensate for differences in gonadal hormone exposure. 5,40 These so-called "latent" sex differences (also referred to as mechanistic, convergent, or divergent sex differences) often manifest as small but repeatable sex differences in behavior that can mask larger sex differences that may be revealed once underlying mechanisms are probed (Ref. 41; for sex-specific pain processing as an example, see Ref. 42).
Early in the study of prairie vole pair-bonding, oxytocin was deemed more important for partnership formation in females than in males, and vasopressinergic pathways became the focus of studies on male prairie voles. 43 In subsequent years, most studies have focused on neuropeptide, dopaminergic, and opioid signaling pathways in one sex at a time, although studies in one sex and then the other reveal potentially similar roles of some of these pathways. 11,44 To identify or quantify sex differences, it is critical to include males and females in the same study. While this practice is increasingly common, analysis of subjects by sex is still far from the norm. 45

| Heterogeneity in male behavior
While all voles exhibited strong partner preferences, there was extensive individual variation in lever pressing preferences for the partner versus stranger. Males, in particular, displayed distinct but consistent classes of social preferences: some pressed significantly more for the partner, others pressed more for the stranger, whereas others pressed similar amounts for both conspecifics. Females were less variable, with some females exhibiting significant preferences for their partner versus a stranger over the 8 days testing interval, and others not exhibiting significant preferences, but all females pressed more for their partner than for the stranger on average.
Increased heterogeneity in social motivation in males in consistent with inter-male variation in mating strategies in the field. While most prairie voles are socially monogamous (and exhibit a "resident" strategy for mate partnerships), a notable percentage of both male and female prairie voles (25%-40%) are non-monogamous (sometimes referred to as "wanderers"). 38 Some of the variation in mating tactics may be due to environmental conditions. 38,50 Our finding extends this distribution of individual variability in mating tactics and quantifies it using an operant paradigm.
While behavior in males varied considerably between individuals, within individuals (males and females) there was a high degree of consistency in lever pressing responses, including across choice and nonchoice social paradigms. Partner-directed lever pressing in the social choice (2 chamber) operant test was strongly correlated with pressing in the single-social-chamber version run at the conclusion of this study, indicating both paradigms are effective at measuring differences in social motivation. This is underscored by the consistency of sex-specific patterns of pressing in the present study and our initial social operant study, 4 and individual voles tended to exhibit consistent preferences for partners, strangers, or neither across days. Partner huddling was also correlated across the two partner preference tests. In contrast, operant responses were unrelated to PPT huddling and chamber times, demonstrating that these tasks get at different aspects of social behavior.

| Oxtr polymorphism and social behavior
Prior work suggests that a SNP in the intron of the prairie vole Oxtr gene NT213739 contributes to individual differences in striatal oxytocin receptor protein levels and attachment behavior. 24,37 Despite the relatively low presence of C carriers in our colony, there was a very strong effect of Oxtr genotype on aggression in male prairie voles, with C allele carriers exhibiting significantly less aggression than T/T homozygous individuals. This relationship between Oxtr genotype and aggression underscores the connection between oxytocin and some "antisocial" social behaviors. In particular, while oxytocin promotes ingroup social behaviors, partner selectivity for familiar individuals is accompanied by increased aggression towards strangers. Partnership formation in prairie voles occurs alongside increases in strangerdirected aggression, 9,43,51,52 and increased outgroup discrimination has also been described in humans. 53 Oxytocin receptor density was strongly influenced by genotype in the nucleus accumbens and lateral septum. Variation in oxytocin receptor binding density by NT204321 genotype replicated differences in the nucleus accumbens described in two prior reports, 24,25 and identified a relationship in the lateral septum that had not previously been reported. Interestingly, genotype and housing were each associated with neural receptor density differences in different/nonoverlapping brain regions. 4 In summary, by developing direct measures of partner-directed motivation in a choice context, we quantified a distinct behavioral component implicated in pair bonding. In doing so, we identified sex differences in the role of reward in preference behaviors, based on heterogeneity in male social motivation for familiar versus unfamiliar females. This advance is critical for subsequent investigation of the neural and genetic systems that contribute to pair bond motivation in males and females, as well as for parsing a well-delineated example of "latent" sex differences.

ACKNOWLEDGMENTS
We are grateful to Jessie Chen, Juliane Donahue Bombosch, Maddie

DATA AVAILABILITY STATEMENT
The data that support the findings of this study are available in the Open Science Framework under the project title "Sex differences in the reward value of familiar mates in prairie voles" at https://osf.io/ wmjfs/.