Potential Mates Research Articles

Generalization Increases the Adaptive Value of Mate Choice Copying When Immediate Copying Is Costly

When assessing potential mates is costly, there will be selection for copying others. Mate choice copying, which is the increased chance of mating with another individual after observing them mating with someone else (i.e., individual-based copying), has been documented in several species, including humans. It is touted as an example of the influence of culture on animal and human mating, but alone it is unlikely to lead the cultural evolution of mating. Rather, it requires mate choice copying with generalization (i.e., trait-based copying), which is the effect of immediate observations on future mating. This sort of mate choice copying has been documented in six species, including humans. Here, I extend an existing game theory model of mate choice copying to include generalization (Extension 1), a cost to immediate copying (Extension 2), and both previous extensions (Extension 3). The results show that Extensions 1 and 2 decrease the adaptive value of mate choice copying when compared to the original model. Extension 3 suggests that adding generalization to mate choice copying with a cost to immediate copying is a more likely evolutionary trajectory than adding it to mate choice copying without this cost. These results have implications for illuminating the emergence of the cultural evolution of mating preferences.

Urban sensory conditions alter rival interactions and mate choice in urban and forest túngara frogs

Abstract Sexual communication often takes place in networks with multiple competing signalers being simultaneously assessed by mate choosers. Altered sensory conditions, such as noise and light pollution, can affect communication by altering signal production and perception. While evidence of sensory pollution affecting sexual signaling is widespread, few studies assess impacts on sexual signaling during rival interactions as well as mate choice, let alone whether urban and non-urban populations have diverged in their response. Here, we investigate the effects of urban sensory conditions on sexual communication in urban and forest túngara frogs (Engystomops pustulosus). We recorded dyadic vocal rival interactions and assessed mate choice with and without noise and light pollution in the lab. We show that urban sensory conditions can directly impact the intensity of rival interactions, differences between rivals and mate choice, though changes were often in opposite directions for frogs of urban and forest origins. Moreover, we demonstrate that urban-induced changes in rival interactions can also indirectly affect how females choose between potential mates. Our study reveals origin-dependent direct and indirect effects of noise and light pollution, and suggests local adaptation of sexual communication in urban populations.

Condition‐Dependent Female Aggression and Its Effects on Mating Success and Sexual Cannibalism in Jumping Spiders

ABSTRACTPre‐copulatory sexual cannibalism is an extreme form of aggression toward potential mates. In this study, we examined condition‐dependent female aggression and its effects on mating success and frequency of pre‐copulatory sexual cannibalism in the jumping spider Phidippus audax Hentz 1845. We examined two of the leading hypotheses of why sexual cannibalism may occur, (1) female mate choice and (2) adaptive female foraging. We separated 137 adult virgin female P. audax into three feeding treatments: (1) well‐fed spiders, (2) 14‐day food deprived, and (3) 28‐day food deprived. We recorded weight loss (14‐ and 28‐day treatments) and survival rates. We conducted mating trials to measure the proportion of non‐aggressive and aggressive male and female behaviors, male mating success, and pre‐copulatory sexual cannibalism. Females deprived of food had higher weight loss and lower survival rates than well‐fed females. In addition, food deprived female spiders exhibited higher proportions of aggressive behaviors and pre‐copulatory sexual cannibalism, and lower mating success compared to well‐fed spiders. We found that as male size increased females were less likely to cannibalize males, but we found no effect of male body condition on whether a male mated with or was cannibalized by a female. Our results are consistent with the adaptive foraging hypothesis and corroborates prior research on pre‐copulatory sexual cannibalism.

Sperm depletion and mate choice of the crab spider, Ebrechtella tricuspidata (Fahricius)

Abstract Mounting evidence suggests that male sperm may be gradually depleted after consecutive matings, resulting in the decline of female reproductive output in insect species. It is predicted that females may employ adaptive strategies to avoid the sperm-depleted males, such as mating multiply with different males and/or discriminating against previously mated males (MMs). Similarly, males may exhibit adaptive behaviors toward females varying in mating status. However, in spiders with males lacking primary copulatory organs and their pedipalps modified to transfer sperm, there are few studies on male mating potential and previous mating experience on their subsequent mate choice. In this study, we used a polyandrous crab spider, Ebrechtella tricuspidata, a sit-and-wait predator with female-biased sex ratio as a model system to ascertain whether 1) male mating experiences influence female reproductive fitness; 2) females respond differently to males varying in mating status; and 3) males respond differently to females varying in mating status. Our results showed that female fecundity was independent of male mating experiences, but female fertility markedly declined with the increase of male previous mating experiences in the first eggsac. Counter to our predictions, females preferred to choose and mate with the larger males, regardless of their mating status. In contrast, male mating status influenced their mating preference toward females. Virgin males did not show any preference between virgin and mated females; however, MMs were more likely to prefer the virgin females over the mated ones. Overall, our results indicated that female choice depended primarily on male relative size rather than male mating status, whereas male choice depended on female mating status. Depending on the investigation of reproductive dynamics and mate choice in E. tricuspidata, the study will provide valuable insights into the adaptive behaviors of both males and females exhibiting toward mates varying in mating status.

Sex-specific expression of circadian rhythms enables allochronic speciation

Abstract Noctuid moths provide prime examples of species in various stages of allochronic speciation, where reproductive barriers are mediated by genetic divergence in daily or seasonal timing. Theory indicates that allochronic divergence might be one of the most plausible mechanisms of adaptive speciation, especially when timing is subject to divergent ecological selection. Here, we show that the validity of this theoretical expectation is entirely contingent on species characteristics of the mating system. Our analysis focuses on the moth Spodoptera frugiperda (Lepidoptera, Noctuidae), which occurs as two strains that differ in circadian reproductive activity. Unlike in generic models of assortative mating, where chronotypes diverge under mild assumptions, individual-based evolutionary simulations of the mating system and life cycle of S. frugiperda fail to recover allochronic diversification, even under conditions highly conducive to speciation. Instead, we observe that both chronotypes advance their activity schedule toward the early night, resulting in a rapid loss of allochronic variation. This outcome is caused by the fact that mating in S. frugiperda takes considerable time and potential mates are encountered sequentially, so that early males enjoy a systematic advantage. The undermining effect of male mate competition can be overcome when circadian genes evolve sex-specific expression, enabling early and late chronotypes to be maintained or even to diversify in sympatry. These results give new significance to sex differences in biological rhythms and suggest that species characteristics of the mating system and genetic architecture are key to understanding the scope for allochronic speciation across diverse species exhibiting variation in timing.

Age, sex, and temperature shape off-territory feeder use in black-capped chickadees.

Spatial ecology tends to focus on average movement patterns within animal groups; however, recent studies highlight the value of considering movement decisions both within and among individuals. We used a marked population of black-capped chickadees (Poecile atricapillus), to assess the causes and consequences of within- and among-individual differences in chickadee space use. Individuals that used feeders in addition to their most-visited "core feeder" were defined as engaging in off-territory feederuse. We found that females were more likely than males to visit off-territory feeders as ambient temperatures decrease and energetic demands increase. This may be due to sex-related differences in dominance, where males which are the dominant sex in chickadees, have priority access to feeders, while females must increase their foraging areas to meet higher energetic demand. We also found that independent of temperature, adult males were less likely than juvenile males and both juvenile and adult females to forage off-territory. We suggest that this may be due to age-specific benefits of space use in males, where un-paired juvenile males may increase feeder exploration to gain information about potential mates. Finally, we found that chickadees that had a higher propensity to visit off-territory feeders were less likely to survive to the next fall. Overall, our results suggest that dominance hierarchies and individual energetics impact within- and among-individual variation in off-territory feeder use. We provide suggestions for future studies to further investigate fitness-related consequences of within- and among-individual variation in space use.

Oxytocin receptor controls distinct components of pair bonding and development in prairie voles.

Oxytocin receptor (Oxtr) signaling influences complex social behaviors in diverse species, including social monogamy in prairie voles. How Oxtr regulates specific components of social attachment behaviors and the neural mechanisms mediating them remains unknown. Here, we examine prairie voles lacking Oxtr and demonstrate that pair bonding comprises distinct behavioral modules: the preference for a bonded partner, and the rejection of novel potential mates. Our longitudinal study of social attachment shows that Oxtr sex-specifically influences early interactions between novel partners facilitating the formation of partner preference. Additionally, Oxtr suppresses promiscuity towards novel potential mates following pair bonding, contributing to rejection. Oxtr function regulates coordinated patterns of gene expression in regions implicated in attachment behaviors and regulates the expression of oxytocin in the paraventricular nucleus of the hypothalamus, a principal source of oxytocin. Thus, Oxtr controls genetically separable components of pair bonding behaviors and coordinates development of the neural substrates of attachment.

Cryptic female choice in response to male pheromones in Drosophila melanogaster

Females control the paternity of their offspring by selectively mating with males they perceive to be of high quality. In species where females mate with multiple males in succession, females may bias offspring paternity by favoring the sperm of one male over another, a process known as cryptic female choice.1 While evidence of cryptic female choice exists in multiple taxa, the mechanisms underlying this process have remained difficult to unravel.2 Understanding cryptic female choice requires demonstration of a female-driven post-mating bias in sperm use and paternity and a causal link between this bias and male cues.3 In this study, we present evidence of cryptic female choice in female Drosophila melanogaster. Through experiments utilizing transgenic males expressing fluorescent sperm, we observed that exposure to attractive males between matings prompts females to expel the ejaculate of their initial mate more rapidly than in the presence of less attractive males. While doing so, females exhibit a bias in sperm storage against their first mate, thereby favoring the paternity of their subsequent mate. Our findings reveal that females adjust the timing of ejaculate expulsion in response to male pheromones in their environment, specifically heptanal and 11-cis-vaccenyl acetate, which are sensed by females through specific odorant receptors. We provide a cryptic female choice mechanism allowing a female to modulate the share of paternity of her first mate depending on the sensing of the quality of potential mates in her environment. These findings showcase that paternity can be influenced by events beyond copulation.

Behavioral Interactions and Mate Compatibility Influence the Reproductive Success of New England Cottontails (Sylvilagus transitionalis) in a Conservation Breeding Program

Wild populations of New England cottontails (NECs, Sylvilagus transitionalis) are declining and occupy a small proportion of their historic range. To conserve this species, wild-caught cottontails participate in a reintroduction breeding program. To increase the program’s productivity, this study described breeding behavior in pairings with (n = 3 pairings) and without (n = 9 pairings) reproductive success. Females were paired with two males consecutively and behaviors were recorded using 24 h continuous video footage. Activity peaked between 19:00 and 06:00. Copulation was only observed in successful pairings, and the gestation period was 31 days. Successful pairings had a significantly higher rate of follows (p < 0.001) and a lower rate of dashes (p < 0.0001) compared to unsuccessful pairings. Females had significantly higher rates of charge (p < 0.0005), chase (p < 0.0003), and dash (p < 0.0016) than males. Males had higher rates of follow (p < 0.005) than females. Males and females altered their behavior in the presence of different potential mates, suggesting that mate compatibility may influence breeding program productivity. This study significantly expands the understanding of NEC breeding behavior and is applicable to the management of this imperiled species living at low densities in fragmented landscapes.

Some for a Good Time, Some for a Long Time: Women’s Ratings of the Mate Value of Men with Different Addictions – An Experimental Study

Background: The present study aimed to explore women’s perception of men with different addictions in terms of their short- and long-term mate value. Objectives: 2,525 women (age range: 18–40, M = 28.35, SD = 6.39) were randomized to six conditions in a vignette-based experiment where a male of otherwise high mating value was described as suffering from either gambling, gaming, cannabis, anabolic androgenic steroid, and alcohol addiction or as not suffering from addiction (control). Results: Regarding long-term mate value of the target, the control target was rated higher than each of the targets. The gaming target was rated higher than the alcohol, cannabis, and gambling targets. Finally, the AAS target was rated as higher on long-term mate value than the alcohol and gambling addiction targets. Conclusions: Overall, women seem to perceive risk-taking in the face of uncertainty, reflected by gambling addiction, as an attractive behavioral tendency in men in terms of short-term mating. In contrast, potential long-term mates with gaming or chemical addictions are viewed more negatively, probably because it signals inadequate time and resources to be invested in a relationship.

Under the influence: exogenous testosterone influences men's cross-sex perceptions of sexual interest.

The sexual misperception bias is a cognitive bias in which men tend to overestimate sexual interest from women, potentially shaped by evolutionary mating strategies. Testosterone, often linked to mating behaviors, might play a role in sustaining sexual overperceptions. To explore this possibility, we conducted a placebo-controlled study with 190 heterosexual men, administering either 11 mg of testosterone or a placebo. Participants interacted with an attractive female confederate, while naïve raters assessed the confederate's affiliative behaviors. Our findings suggest that exogenous testosterone did not broadly impact sexual overperception. However, we found that affiliative behavior from the confederate was positively correlated with perceived sexual interest among testosterone-treated, but not placebo-treated men. In addition, we found that this effect among testosterone-treated men was contingent on their self-perceived attractiveness. Specifically, the confederate's affiliative behaviors were positively correlated with perceived sexual interest, but only for testosterone-treated men with average or above average self-perceived attractiveness. Furthermore, our data revealed that men's tendency to project their own short-term and long-term mating interests increases as a function of self-perceived attractiveness, and this coupling is enhanced by testosterone for long-term interest. Taken together, these results suggest that testosterone may potentiate existing biases, particularly when sexual motivation is high, and bias perceptions of friendly behavior when engaging in cross-sex mindreading. This study adds to the understanding of the neuroendocrine bases of social cognition, suggesting that testosterone can affect men's perceptions of potential mates.

Mosquitoes integrate visual and acoustic cues to mediate conspecific interactions in swarms

Male mosquitoes form aerial aggregations, known as swarms, to attract females and maximize their chances of finding a mate. Within these swarms, individuals must be able to recognize potential mates and navigate the social environment to successfully intercept a mating partner. Prior research has almost exclusively focused on the role of acoustic cues in mediating the male mosquito's ability to recognize and pursue females. However, the role of other sensory modalities in this behavior has not been explored. Moreover, how males avoid collisions with one another in the swarm while pursuing females remains poorly understood. In this study, we combined free-flight and tethered-flight simulator experiments to demonstrate that swarming Anopheles coluzzii mosquitoes integrate visual and acoustic information to track conspecifics and avoid collisions. Our tethered experiments revealed that acoustic stimuli gated mosquito steering responses to visual objects simulating nearby mosquitoes, especially in males that exhibited a strong response toward visual objects in the presence of female flight tones. Additionally, we observed that visual cues alone could trigger changes in mosquitoes' wingbeat amplitude and frequency. These findings were corroborated by our free-flight experiments, which revealed that Anopheles coluzzii modulate their thrust-based flight responses to nearby conspecifics in a similar manner to tethered animals, potentially allowing for collision avoidance within swarms. Together, these results demonstrate that both males and females integrate multiple sensory inputs to mediate swarming behavior, and for males, the change in flight kinematics in response to multimodal cues might allow them to simultaneously track females while avoiding collisions.

Female pumas exhibit behavioral plasticity through partitioning temporal activity at communication hubs based on life stage

AbstractIntraspecific communication among carnivores can be complex and risky, and many solitary carnivores (including pumas; Puma concolor) primarily communicate with potential mates and competitors indirectly via scent marking at communication hubs. We used multiple temporal analyses to understand if pumas of three classes (males, solo females, and females traveling with dependent kittens) varied in their temporal use of communication hubs to reduce risk while gathering information. We hypothesized that males would visit scrapes at typical times (nighttime), but that female behavioral strategies might vary based on whether they had dependent kittens or not. We found that pumas of different classes varied in their temporal patterns: male pumas concentrated their temporal activity at night (which is typical for pumas), solo females also tended to visit during the nighttime (although slightly earlier on average than males), while females with kittens were more active in the daytime. Our findings highlight the importance of communication hubs for intersexual communication in pumas and the flexibility that is inherent in puma behavioral ecology. Females adjusted their temporal behavior and visitation based on their reproductive status and life history stage, likely to assess dominant territorial males while also limiting their risk. Our study provides a mechanistic view of how communication through scent marking allows both long‐term and spatially separated communication to occur and that may allow pumas at different life stages with different risk tolerances to communicate with each other.

Interactive effects of intrasexual competitiveness, same-sex competition, and physical attractiveness on temporal discounting

Studies have shown that men discount the future and prefer immediate-but-smaller over delayed-but-larger rewards when exposed to mating opportunities (e.g., attractive opposite-sex targets) or threats (e.g., same-sex competition) whereas women's discounting in response to similar cues appears mixed, suggesting that mating-motivated discounting is primarily a male phenomenon. Importantly, this line of research has not yet examined the role of individual difference variables as well as how the attractiveness of potential mates and perceptions of competition jointly influence discounting rates. We conducted a novel test of the effect of trait intrasexual competitiveness (ISC) using dating profiles varying on target attractiveness and same-sex competition to observe their interactive effects on participants' discounting. Results showed that when targets were attractive, higher ISC was associated with steeper discounting for both men and women, and this association was stronger when competition was high rather than low. ISC still predicted discounting when targets were low in attractiveness but competition was high; high ISC did not predict discounting only in the low attractiveness and low competition condition. These findings reveal ISC as a factor that leads women to discount as much as men, and that high-ISC individuals may be more responsive to competition than to target attractiveness.

Inbreeding avoidance, competition and natal dispersal in a pair-living, genetically monogamous mammal, Azara's owl monkey (Aotus azarae).

Natal dispersal is an important life-history stage influencing individual fitness, social dynamics of groups and population structure. Understanding factors influencing dispersal is essential for evaluating explanations for the evolution and maintenance of social organization, including parental care and mating systems. The social and mating systems of Azara's owl monkeys (Aotus azarae) are infrequent among mammals; these primates are pair-living, serially and genetically monogamous and both sexes directly care for offspring. To evaluate the role that competition and inbreeding avoidance play in shaping dispersal patterns, we used 25 years of demographic and genetic data to examine how variation in timing of natal dispersal is related to social (adult replacements, step-parents, births and group size) and ecological factors (seasonal abundance of resources) in a wild population of A. azarae in Formosa, Argentina. We found that all males and females dispersed from their natal groups, but subadults delayed dispersal when a step-parent of the opposite sex joined the group, indicating that they may perceive these step-parents as potential mates. Dispersal was more probable when resource conditions were better, regardless of age. Overall, agonistic conflict over food and potential mates with adults in the natal group, as well as inbreeding avoidance, contribute to regulating dispersal.

Time and place affect the acoustic structure of frog advertisement calls

Abstract Acoustic communication signals are important for species recognition and mate attraction across numerous taxa. For instance, most of the thousands of species of frogs have a species-specific advertisement call that females use to localize and discriminate among potential mates. Thus, the acoustic structure of the advertisement call is critical for reproductive success. The acoustic structure of calls will generally diverge over evolutionary time and can be influenced by the calls of sympatric species. While many studies have shown the influence of geography on contemporary call variation in populations of frogs, no study has compared the acoustic structure of frog calls across many species to ask whether we can detect an influence of divergence time and overall geographic overlap on the differences in acoustic structure of species-typical calls that we observe now. To this end, we compared acoustic features of the calls of 225 species of frogs within 4 families. Furthermore, we used a behavioral assay from 1 species of frog to determine which acoustic features to prioritize in our large-scale analyses. We found evidence that both phylogeny (time) and geography (place) relate to advertisement call acoustics albeit with large variation in these relationships across the 4 families in the analysis. Overall, these results suggest that, despite the many ecological and evolutionary forces that influence call structure, the broad forces of time and place can shape aspects of advertisement call acoustics.

The use of anthropogenic areas helps explain male brown bear movement rates and distance travelled during the mating season

AbstractDuring the reproductive period, mating strategies are a significant driver of adaptations in animal behaviour. For instance, for polygamous species, greater movement rates during the mating season may be advantageous due to the increased probability of encountering several potential mates. The brown bear Ursus arctos is a solitary carnivore that lives at low densities, with a polygamous mating system and an extended mating season of nearly 3 months. Here, we hypothesized that male brown bears may show changes in movement patterns and space‐use behaviour during their mating season. Using long‐term (2002–2013) telemetry data from the Finnish Karelia male population (n = 24 individuals; n = 10 688 GPS locations), we first analysed daily movement metrics, that is, speed, net and total distance with respect to the period (mating vs. post‐mating) and several environmental predictors. Then, we conducted a step‐selection analysis for each of these periods. Throughout the year, male bears selected forested/shrub habitats and increased movement rates near main roads. During the mating season, reproductive needs seem to trigger roaming behaviour in adult males to maximize encounter rates with potential receptive females. However, all movement metrics increased within areas of high human activity, suggesting a bear response to a higher risk perception while using those areas. During the post‐mating period, overlapping with the bear hyperphagia and the hunting season, males selected anthropogenic areas farther from main roads and trails, suggesting a trade‐off between foraging opportunities and risk avoidance.

Hormonal and cycle phase predictors of within-women shifts in self-perceived attractiveness: Tests of alternative functional models

Prior research has produced mixed findings regarding whether women feel more attractive during the fertile phase of the menstrual cycle. Here, we analyzed cycle phase and hormonal predictors of women's self-perceived attractiveness (SPA) assessed within a daily diary study. Forty-three women indicated their SPA, sexual desire, and interest in their own partners or other potential mates each day across 1–2 menstrual cycles; saliva samples collected on corresponding days were assayed for estradiol, progesterone, and testosterone; and photos of the women taken at weekly intervals were rated for attractiveness. Contrary to some prior studies, we did not find a significant increase in SPA within the estimated fertile window (i.e., cycle days when conception is possible). However, within-cycle fluctuations in progesterone were significantly negatively associated with shifts in SPA, with a visible nadir in SPA in the mid-luteal phase. Women's sexual desire and SPA were positively associated, and the two variables fluctuated in very similar ways across the cycle. Third-party ratings of women's photos provided no evidence that women's SPA simply tracked actual changes in their visible attractiveness. Finally, for partnered women, changes in SPA correlated with shifts in attraction to own partners at least as strongly as it did with shifts in fantasy about extra-pair partners. Our findings provide preliminary evidence for the idea that SPA is a component of women's sexual motivation that may change in ways similar to other hormonally regulated shifts in motivational priorities. Additional large-scale studies are necessary to test replication of these preliminary findings.

Hide and seek: chemical cues drive site preference among potential mates and intraguild competitors

Abstract Chemical perception is essential among arthropods for mate recognition, prey search, and predator avoidance, especially for solitary predators which are often aggressive. Such mechanisms may be intensified in environments like the Caatinga, a seasonally dry tropical forest in Brazil, characterized by low habitat complexity and high seasonal variation. Thus, we investigated chemical perception between two scorpion species from this environment, involved in intraguild competition. Experiments assessed their response to chemical cues from prey, predators, and potential mates. We use Y-mazes to test the time spent by the scorpions between Y-arms with or without a given substrate-borne chemical cue (site preference) and the number of active individuals during trials (presence of activity). Scorpions’ activity was not influenced by chemical stimuli, although they clearly exhibit site preferences. The smaller predators avoided sites with the larger species’ chemical cues, while the larger predators preferred sites with the smaller species’ cues. Additionally, both species trailed female chemical cues. These findings suggest a dual-oriented arms race where prey and predator modulate their behaviour to avoid and hunt heterospecifics, respectively. This study provides the first evidence of a scorpion using chemical cues to detect a heterospecific scorpion and highlights the importance of this trait in arachnids.

Oxytocin receptor function regulates neural signatures of pair bonding and fidelity in the nucleus accumbens.

The formation of enduring relationships dramatically influences future behavior, promoting affiliation between familiar individuals. How such attachments are encoded to elicit and reinforce specific social behaviors in distinct ethological contexts remains unknown. Signaling via the oxytocin receptor (Oxtr) in the nucleus accumbens (NAc) facilitates social reward as well as pair bond formation between mates in socially monogamous prairie voles 1-9 . How Oxtr function influences activity in the NAc during pair bonding to promote affiliative behavior with partners and rejection of other potential mates has not been determined. Using longitudinal in vivo fiber photometry in wild-type prairie voles and those lacking Oxtr, we demonstrate that Oxtr function sex-specifically regulates pair bonding behaviors and associated activity in the NAc. Oxtr function influences prosocial behavior in females in a state-dependent manner. Females lacking Oxtr demonstrate reduced prosocial behaviors and lower activity in the NAc during initial chemosensory investigation of novel males. Upon pair bonding, affiliative behavior with partners and neural activity in the NAc during these interactions increase, but these changes do not require Oxtr function. Conversely, males lacking Oxtr display increased prosocial investigation of novel females. Using the altered patterns of behavior and activity in the NAc of males lacking Oxtr during their first interactions with a female, we can predict their future preference for a partner or stranger days later. These results demonstrate that Oxtr function sex-specifically influences the early development of pair bonds by modulating prosociality and the neural processing of sensory cues and social interactions with novel individuals, unmasking underlying sex differences in the neural pathways regulating the formation of long-term relationships.