"Monogamy" in Primates: Variability, Trends, and Synthesis: Introduction to special issue on Primate Monogamy

Binding Affinity, Selectivity, and Pharmacokinetics of the Oxytocin Receptor Antagonist L-368,899 in the Coyote (Canis latrans)

L-368,899 is a selective small-molecule oxytocin receptor (OXTR) antagonist originally developed in the 1990s to prevent preterm labor. Although its utility for that purpose was limited, L-368,899 is now one of the most commonly used drugs in animal research for the selective blockade of neural OXTR after peripheral delivery. A growing number of rodent and primate studies have used L-368,899 to evaluate whether certain behaviors are oxytocin dependent. These studies have improved our understanding of oxytocin's function in the brains of rodents and monkeys, but very little work has been done in other mammals, and only a single paper in macaques has provided any evidence that L-368,899 can be detected in the CNS after peripheral delivery. The current study sought to extend those findings in a novel species: coyotes ( Canis latrans ). Coyotes are ubiquitous North American canids that form long-term monogamous pair-bonds. Although monogamy is rare in rodents and primates, all wild canid species studied to date exhibit social monogamy. Coyotes are therefore an excellent model organism for the study of oxytocin and social bonds. Our goal was to determine whether L-368,899 is a viable candidate for future use in behavioral studies in coyotes. We used captive coyotes at the USDA National Wildlife Research Center's Predator Research Facility to evaluate the pharmacokinetics of L-368,899 in blood and CSF during a 90-min time course after intramuscular injection. We then characterized the binding affinity and selectivity of L-368,899 to coyote OXTR and the structurally similar vasopressin 1a receptor. We found that L-368,899 peaked in CSF at 15 to 30 min after intramuscular injection and slowly accumulated in blood. L-368,899 was 40 times more selective for OXTR than vasopressin 1a receptors and bound to the coyote OXTR with an affinity of 12 nM. These features of L-368,899 support its utility in future studies to probe the oxytocin system of coyotes.

A Limiting Factor of Sex Attractants of Bactrocera dorsalis (Diptera: Tephritidae), Verified under Laboratory Conditions

At present, sexual attractants mainly control insect populations by killing males. However, the effect of sex attractants may be limited by the mating ability of the attracted insects. The Oriental fruit fly, Bactrocera dorsalis (Hendel), has a strong reproductive capacity; it brings great losses to agricultural production, which can be controlled by methods using sex attractant methyl eugenol that mainly attracts males. Therefore, we studied the multiple and continuous (as well as consecutive) mating ability of B. dorsalis through behavioral experiments. The results show that male B. dorsalis can mate 11 times on average, with females mating only 1.93 times, and that 10.81% of males mate more than 20 times. The reproductive capacity of male B. dorsalis decreased significantly after four to five instances of continuous mating. In different mating patterns, the reproductive fitness of polyandry is not the highest, rather, interval mating is the best. A limiting factor of the sex attractant effect was revealed in B. dorsalis through behavioral evidence.

Male genital organs of the black-crowned dwarf marmoset (Callibella humilis)

This study provides the first morphological description of the male genital organs of black-crowned dwarf marmoset (Callibella humilis). Internal and external genital organs were similar to other species of non-human primates. However, some shape and size differences were noted. The penile shaft was laterally flattened and comprised of two corpus cavernosus. These were split by a septum up to the level of the glans and then merged to form a single structure. The glans penis had small, keratinized spicules arising from epidermal or dermal projections. The small, fully ossified and well-mineralized penile bone, located at the distal end of the penis, consisted of a bone matrix surrounded by sparse osteoblasts and well-developed internal osteocytes. The penile urethra was lined with columnar pseudostratified epithelium, with areas of simple cuboidal epithelium. The testes were small relative to the body compared to other primate species. The findings of this work may support comparative studies of primate reproductive ecology.

The onset of puberty in colony-housed male and female titi monkeys (Plecturocebus cupreus): Possible effects of oxytocin treatment during peri-adolescent development

Oxytocin has been used to treat neurodevelopmental conditions in adolescent patients but possible effects on reproductive development have not been well investigated. The effects of daily intra-nasal oxytocin treatment (12-18 months of age) on puberty and fertility were studied in colony-housed, male and female titi monkeys (Plecturocebus cupreus). Body weight, urinary conjugated pregnanes and estrogens (defining cyclicity) in females, and androgens and sperm in urine of in males, were measured from 1 to 3 years of age to detect puberty. Serum testosterone was also measured in males at 13, 23 and 33 months of age and hemi-castration at 3 years of age enabled assessment of testicular morphometry and oxytocin receptor expression. An oxytocin treatment*time interaction suggested a minor, transient suppression in weight gain after treatment ended. Note that females weighed 10% less across all ages. Oxytocin-treated females exhibited early, spurious ovulations but neither regular cyclicity (≈30 months) nor pregnancies were affected by treatment. Oxytocin did not affect the pubertal increase in urinary androgen or the first appearance of sperm, which occurred as early as 15 months of age. Treatment did delay the puberty-associated rise in serum testosterone in males. All males were pubertal by 22 months and all females by 32 months of age. Although no major male or female fertility outcome was observed, oxytocin demonstrated some physiological effects through a delay of testosterone secretion in males, induction of precocious ovulation in females, and a suppression of general weight gain for the months following treatment.

Ecology impacts the decrease of Spirochaetes and Prevotella in the fecal gut microbiota of urban humans

Compared to the huge microbial diversity in most mammals, human gut microbiomes have lost diversity while becoming specialized for animal-based diets – especially compared to chimps, their genetically closest ancestors. The lowered microbial diversity within the gut of westernized populations has also been associated with different kinds of chronic inflammatory diseases in humans. To further deepen our knowledge on phylogenetic and ecologic impacts on human health and fitness, we established the herein presented biobank as well as its comprehensive microbiota analysis. In total, 368 stool samples from 38 different animal species, including Homo sapiens, belonging to four diverse mammalian orders were collected at seven different locations and analyzed by 16S rRNA gene amplicon sequencing. Comprehensive data analysis was performed to (i) determine the overall impact of host phylogeny vs. diet, location, and ecology and to (ii) examine the general pattern of fecal bacterial diversity across captive mammals and humans.

By using a controlled study design with captive mammals we could verify that host phylogeny is the most dominant driver of mammalian gut microbiota composition. However, the effect of ecology appears to be able to overcome host phylogeny and should therefore be studied in more detail in future studies. Most importantly, our study could observe a remarkable decrease of Spirochaetes and Prevotella in westernized humans and platyrrhines, which is probably not only due to diet, but also to the social behavior and structure in these communities.

Our study highlights the importance of phylogenetic relationship and ecology within the evolution of mammalian fecal microbiota composition. Particularly, the observed decrease of Spirochaetes and Prevotella in westernized communities might be associated to lifestyle dependent rapid evolutionary changes, potentially involved in the establishment of dysbiotic microbiomes, which promote the etiology of chronic diseases.

What's in a Moment: What Can Be Learned About Pair Bonding From Studying Moment-To-Moment Behavioral Synchrony Between Partners?

Our understanding of the behavioral and physiological mechanisms of monogamy largely comes from studies of behavioral interactions unique to pair-bonded individuals. By focusing on these highly marked behaviors, a remarkable conservation in the mechanisms underlying pair bonding has been revealed; however, we continue to know very little about the range of behavioral and neurobiological mechanisms that could explain the great diversity of pair-bonding phenotypes that exists both within and across species. In order to capture the dynamic nature of bonds over time and across contexts, we need specific, operationally-defined behavioral variables relevant across such a diversity of scenarios. Additionally, we need to be able to situate these behavioral variables within broader frameworks that allow us to interpret and compare patterns seen across species. Here I review what is known about behavioral synchrony with respect to pair bonding and discuss using synchrony as such a variable as well as a framework to expand on our understanding of pair bonding across timescales, contexts and species. First, I discuss the importance of behavioral synchrony and parental coordination for reproductive success in monogamous biparental bird species. Second, I highlight research documenting the critical importance of interpersonal coordination for human social relationships. Finally, I present recent work that experimentally bridges these lines of research by quantifying moment-to-moment behavioral synchrony during brief social interactions in zebra finch dyads. All together, these distinct perspectives support the notion that synchrony (1) is a shared premise for sociality across species, (2) is deeply shaped by social experiences, and (3) exists across timescales, behaviors, and levels of physiology. Conceptualizing pair bonding through the framework of behavioral synchrony is likely to facilitate a deeper understanding of the nuances of how social experiences and interactions impact the brain and behavior.

Sex Bias in Gut Microbiome Transmission in Newly Paired Marmosets (Callithrix jacchus)

Social behavior can alter the microbiome composition via transmission among social partners, but there have been few controlled experimental studies of gut microbiome transmission among social partners in primates. We collected longitudinal fecal samples from eight unrelated male-female pairs of marmoset monkeys prior to pairing and for 8 weeks following pairing. We then sequenced 16S rRNA to characterize the changes in the gut microbiome that resulted from the pairing. Marmoset pairs had a higher similarity in gut microbiome communities after pairing than before pairing. We discovered sex differences in the degrees of change in gut microbiome communities following pairing. Specifically, the gut microbiome communities in males exhibited greater dissimilarity from the prepairing stage (baseline) than the gut microbiome communities in females. Conversely, females showed a gradual stabilization in the rate of the gut microbiome community turnover. Importantly, we found that the male fecal samples harbored more female-source gut microbes after pairing, especially early in pairing (paired test, P < 0.05), possibly linked to sex bias in the frequencies of social behavior. From this controlled study, we report for the first time that pair-living primates undergo significant changes in gut microbiome during pairing and that females transmit more microbes to their partners than males do. The potential biases influencing which microbes are transmitted on the basis of sex and whether they are due to sex biases in other behavioral or physiological features need to be widely investigated in other nonhuman primates and humans in the future.IMPORTANCE In this controlled study, we collected longitudinal fecal samples from 16 male and female marmoset monkeys for 2 weeks prior to and for 8 weeks after pairing in male-female dyads. We report for the first time that marmoset monkeys undergo significant changes to the gut microbiome following pairing and that these changes are sex-biased; i.e., females transmit more microbes to their social partners than males do. Marmosets exhibit pair bonding behavior such as spatial proximity, physical contact, and grooming, and sex biases in these behavioral patterns may contribute to the observed sex bias in social transmission of gut microbiomes.

The evolution of pair-living, sexual monogamy, and cooperative infant care: Insights from research on wild owl monkeys, titis, sakis, and tamarins

"Monogamy" and pair bonding have long been of interest to anthropologists and primatologists. Their study contributes to our knowledge of human evolutionary biology and social evolution without the cultural trappings associated with studying human societies directly. Here, we first provide an overview of theoretical considerations, followed by an evaluation of recent comparative studies of the evolution of "social monogamy"; we are left with serious doubts about the conclusions of these studies that stem from the often poor quality of the data used and an overreliance on secondary sources without vetting the data therein. We then describe our field research program on four "monogamous" platyrrhines (owl monkeys, titis, sakis, and tamarins), evaluate how well our data support various hypotheses proposed to explain "monogamy," and compare our data to those reported on the same genera in comparative studies. Overall, we found a distressing lack of agreement between the data used in comparative studies and data from the literature for the taxa that we work with. In the final section, we propose areas of research that deserve more attention. We stress the need for more high-quality natural history data, and we urge researchers to be cautious about the uncritical use of variables of uncertain internal validity. Overall, it is imperative that biological anthropologists establish and follow clear criteria for comparing and combining results from published studies and that researchers, reviewers, and editors alike comply with these standards to improve the transparency, reproducibility, and interpretability of causal inferences made in comparative studies.

Variation in the density of oxytocin receptors in the brain as mechanism of adaptation to specific social and reproductive strategies

Most species have predominant forms of social and reproductive behavior driven by many years of selection pressures and evolution. For example, rodent species can live in small or large groups, behave more tolerant or aggressively toward conspecifics (including newborns), and form or not bonds with other members of the group (including sexual partners). Any of those behavioral adaptations could result in good fitness for the species, but could also require compromises such as sharing resources, greater parental investment, increased risk of predation, etc. We propose that the oxytocin (OXT) system, among others neuroendocrine peptides, is at the basis of a neural mechanism that adapts and predisposes species to a particular social and reproductive form of living. In this review we will show evidence that the variability in the density of receptors for OXT (OXTR) in the nucleus accumbens (NAc) and the lateral septum (LS) predisposes species to adopt at least 4 different social and reproductive strategies in rodents. Large or medium size groups with lower conspecific spacing (preferred separation distance maintained by adult conspecifics), and high levels of promiscuity are characterized by low levels of OXTR in the NAc and LS (e.g. Ratus norvegicus, Ctenomys sociabilis, Scotinomys teguina, Cavia porcellus); small size groups with higher conspecific spacing and low levels of promiscuity are characterized by high OXTR in the NAc and the LS (e.g. Peromyscus californicus); large or medium groups with lower conspecific spacing and low levels of promiscuity characterized by high levels of OXTR in the NAc but low levels in the LS (e.g. Microtus ochrogaster, Heterocephalus glaber, Microtus kikuchii); and small or medium size groups with higher conspecific spacing and high levels of promiscuity characterized by low levels of OXTR in the NAc and high OXTR in the LS (e.g. Mus musculus, Ctenomys haigi, Peromyscus maniculatus, Microtus pennsylvanicus, Microtus montanus). Careful analysis of the distribution of OXTR, and other peptides receptors, in the brain can contribute to understand its function but also to predict reproductive and social strategies of species.

A comparative study of litter size and sex composition in a large dataset of callitrichine monkeys

In many birds and mammals, the size and sex composition of litters can have important downstream effects for individual offspring. Primates are model organisms for questions of cooperation and conflict, but the factors shaping interactions among same-age siblings have been less-studied in primates because most species bear single young. However, callitrichines (marmosets, tamarins, and lion tamarins) frequently bear litters of two or more, thereby providing the opportunity to ask whether variation in the size and sex composition of litters affects development, survival, and reproduction. To investigate these questions, we compiled a large dataset of nine species of callitrichines (n = 27,080 individuals; Callithrix geoffroyi, Callithrix jacchus, Cebuella pygmaea, Saguinus imperator, Saguinus oedipus, Leontopithecus chrysomelas, Leontopithecus chrysopygus, Leontopithecus rosalia, and Callimico goeldii) from zoo and laboratory populations spanning 80 years (1938-2018). Through this comparative approach, we found several lines of evidence that litter size and sex composition may impact fitness. Singletons have higher survivorship than litter-born peers and they significantly outperform litter-born individuals on two measures of reproductive performance. Further, for some species, individuals born in a mixed-sex litter outperform isosexually-born individuals (i.e., those born in all-male or all-female litters), suggesting that same-sex competition may limit reproductive performance. We also document several interesting demographic trends. All but one species (C. pygmaea) has a male-biased birth sex ratio with higher survivorship from birth to sexual maturity among females (although this was significant in only two species). Isosexual litters occurred at the expected frequency (with one exception: C. pygmaea), unlike other animals, where isosexual litters are typically overrepresented. Taken together, our results indicate a modest negative effect of same-age sibling competition on reproductive output in captive callitrichines. This study also serves to illustrate the value of zoo and laboratory records for biological inquiry.

Dopamine receptor manipulation does not alter patterns of partner preference in long-term marmoset pairs

The relationship between socially monogamous mates is dynamic and regulated by neurobiological influences. Research in rodent models has indicated a key role for the neurotransmitter dopamine (DA) and its receptors (DAR) in mediating the formation and maintenance of monogamous bonds. DAR activation was pharmacologically manipulated in marmosets housed in long-term pairs. Marmosets exposed to DAR manipulation were tested in a partner preference test under two social conditions: one in which their mate could visually observe their interactions with an opposite-sex individual, and one in which their pair mate could not visually observe these interactions. Marmosets displayed a spatial preference for the mate compared to an unfamiliar conspecific, however, they displayed a sexual preference for an unfamiliar conspecific over their mate. D1R manipulation had no impact on marmoset partner preference. However, activation of D2Rs reduced the time marmosets spent in contact with either stimulus animal, indicating a decrease in social interest, but did not reduce time spent in proximity to the stimulus animals nor number of sexual solicitations. Additionally, social context (visibility of the mate) did not influence marmoset behavior. These findings suggest that D2Rs may be involved in regulating generalized, but not partner-specific, social interest in marmoset monkeys.

Oxytocin structure and function in New World monkeys: from pharmacology to behavior

Oxytocin (OT) is a hypothalamic nonapeptide that mediates a host of physiological and behavioral processes including reproductive physiology and social attachments. While the OT sequence structure is highly conserved among mammals, New World monkeys (NWMs) represent an unusual "hot spot" in OT structure variability among mammals. At least 6 distinct OT ligand variants among NWMs exist, yet it is currently unclear whether these evolved structural changes result in meaningful functional consequences. NWMs offer a new area to explore how these modifications to OT and its canonical G-protein coupled OT receptor (OTR) may mediate specific cellular, physiological and behavioral outcomes. In this review, we highlight relationships between OT ligand and OTR structural variability, specifically examining coevolution between OT ligands, OTRs, and physiological and behavioral phenotypes across NWMs. We consider whether these evolved modifications to the OT structure alter pharmacological profiles at human and marmoset OTRs, including changes to receptor binding, intracellular signaling and receptor internalization. Finally, we evaluate whether exogenous manipulation using OT variants in marmoset monkeys differentially enhance or impair behavioral processes involved in social relationships between pairmates, opposite-sex strangers, and parents and their offspring. Overall, it appears that changes to OT ligands in NWMs result in important changes ranging from cellular signaling to broad measures of social behavior.

Role of oxytocin in parental behaviour

Both animal and human studies have provided conclusive evidence that oxytocin (OXT) acts in the brain (eg, medial preoptic area, ventral tegmental area, nucleus accumbens) to promote parental behaviour under different reproductive and physiological conditions. OXT appears to accelerate and strengthen the neural process that makes newborns attractive or rewarding. Furthermore, OXT reduces stress/anxiety and might improve mood and well being, resulting in indirect benefits for parents. However, OXT also plays a role in the development of species reproductive and social strategies, making some species or individuals more prone to display caring activities in nonreproductive contexts. There are important differences in the development of the OXT system and its regulation by gonadal hormones that can make individuals or species very different. Those intra- and interspecific differences in the OXT system have been associated with differences in parental behaviour. For example, differences in OXT levels in body fluids and genetic variants for the OXT and OXT receptor genes have been associated with variability in parental mood and behaviour in humans. Thus, OXT has received much attention as a potential therapeutic agent for affective, emotional and behavioural problems. Despite many preliminary studies indicating promising findings, several unknown aspects of the OXT system remain to be addressed before we can achieve a complete understanding of its function in the brain. The enormous interest that this area of study has attracted in the last decade will likely continually contribute to advancing our understanding of the role of OXT in parental behaviour and other behavioural and physiological functions.

Why help? Relationship quality, not strategic grooming predicts infant-care in group-living marmosets

Cooperatively breeding common marmosets raise their infants with the help of other adult group members, but individual care-taking contribution can vary considerably. We tested four hypotheses that may explain this variation within marmoset family groups. The pay-for-help hypothesis argues that allogrooming is used strategically by parents to pay helpers for helping. The pay-for-infant-access hypothesis claims that helpers use allogrooming as payment for infant-access. The intrinsic predisposition hypothesis suggests that more affiliative individuals are also more motivated for infant-care, and the relationship quality hypothesis that individuals involved in highly affiliative relationships with main caregivers contribute more to infant-care. To test these hypotheses, we followed five marmoset family groups over a total of eight reproductive cycles, and quantified affiliative behavior, infant-carrying, and food sharing over six to 12 weeks around infant-birth. We found no evidence for either the pay-for-help or pay-for-infant-access hypotheses nor did intrinsic prosocial predisposition determine individual infant-care. Mutual dyadic affiliation, however, was positively linked to infant-carrying and food sharing in female and male breeders and in male helpers. This suggests that cooperation during infant-care is mediated by relationship quality rather than strategic grooming in marmosets. Overall, these results may also contribute to a better understanding of cooperation in humans.

Social Monogamy in Nonhuman Primates: Phylogeny, Phenotype, and Physiology

Monogamy as a social system has been both a scientific puzzle and a sociocultural issue for decades. In this review, we examine social monogamy from a comparative perspective with a focus on primates, our closest genetic relatives. We break down monogamy into component elements, including pair-bonding and partner preference, mate guarding or jealousy, social attachment, and biparental care. Our survey of primates shows that not all features are present in species classified as socially monogamous, in the same way that human monogamous relationships may not include all elements-a perspective we refer to as "monogamy à la carte." Our review includes a survey of the neurobiological correlates of social monogamy in primates, exploring unique or common pathways for the elemental components of monogamy. This compilation reveals that the components of monogamy are modulated by a suite of androgenic steroids, glucocorticoid hormones, the nonapeptide hormones oxytocin and vasopressin, and other neurotransmitter systems (e.g., dopamine and opioids). We propose that efforts to understand the biological underpinnings of complex human and animal sociosexual relationships will be well served by exploring individual phenotypic traits, as opposed to pursuing these questions with the assumption that monogamy is a unitary trait or a species-specific characteristic.

Effects of social defeat on paternal behavior and pair bonding behavior in male California mice (Peromyscus californicus)

Male parental care is an important social behavior for several mammalian species. Psychosocial stress is usually found to inhibit maternal behavior, but effects on paternal behavior have been less consistent. We tested the effects of social defeat stress on pair bond formation and paternal behavior in the monogamous California mouse (Peromyscus californicus). Social defeat reduced time spent in a chamber with a stranger female during a partner preference test conducted 24h after pairing, but increased latency to the first litter. In 10min partner preference tests conducted after the birth of pups, both control and stressed males exhibited selective aggression towards stranger females. Unlike prairie voles, side by side contact was not observed in either partner preference test. Stressed male California mice engaged in more paternal behavior than controls and had reduced anxiety-like responses in the open-field test. Defeat stress enhanced prodynorphin and KOR expression in the medial preoptic area (MPOA) but not PVN. Increased KOR signaling has been linked to increased selective aggression in prairie voles. Together the results show that defeat stress enhances behaviors related to parental care and pair bonding in male California mice.

Imaging, Behavior and Endocrine Analysis of "Jealousy" in a Monogamous Primate

Understanding the neurobiology of social bonding in non-human primates is a critical step in understanding the evolution of monogamy, as well as understanding the neural substrates for emotion and behavior. Coppery titi monkeys (Callicebus cupreus) form strong pair bonds, characterized by selective preference for their pair mate, mate-guarding, physiological and behavioral agitation upon separation, and social buffering. Mate-guarding, or the "maintenance" phase of pair bonding, is relatively under-studied in primates. In the current study, we used functional imaging to examine how male titi monkeys viewing their pair mate in close proximity to a stranger male would change regional cerebral glucose metabolism. We predicted that this situation would challenge the pair bond and induce "jealousy" in the males. Animals were injected with [¹⁸F]-fluorodeoxyglucose (FDG), returned to their cage for 30 min of conscious uptake, placed under anesthesia, and then scanned for 1 hour on a microPET P4 scanner. During the FDG uptake, males (n=8) had a view of either their female pair mate next to a stranger male ("jealousy" condition) or a stranger female next to a stranger male (control condition). Blood and cerebrospinal fluid samples were collected and assayed for testosterone, cortisol, oxytocin, and vasopressin. Positron emission tomography (PET) was co-registered with structural magnetic resonance imaging (MRI), and region of interest analysis was carried out. Bayesian multivariate multilevel analyses found that the right lateral septum (Pr(b>0)=93%), left posterior cingulate cortex (Pr(b>0)=99%), and left anterior cingulate (Pr(b>0)=96%) showed higher FDG uptake in the jealousy condition compared to the control condition, while the right medial amygdala (Pr(b>0)=85%) showed lower FDG uptake. Plasma testosterone and cortisol concentrations were higher during the jealousy condition. During the jealousy condition, duration of time spent looking across at the pair mate next to a stranger male was associated with higher plasma cortisol concentrations. The lateral septum has been shown to be involved in mate-guarding and mating-induced aggression in monogamous rodents, while the cingulate cortex has been linked to territoriality. These neural and physiological changes may underpin the emotion of jealousy, which can act in a monogamous species to preserve the long-term integrity of the pair.

Pair bond formation leads to a sustained increase in global cerebral glucose metabolism in monogamous male titi monkeys (Callicebus cupreus)

Social bonds, especially attachment relationships, are crucial to our health and happiness. However, what we know about the neural substrates of these bonds is almost exclusively limited to rodent models and correlational experiments in humans. Here, we used socially monogamous non-human primates, titi monkeys (Callicebus cupreus) to experimentally examine changes in regional and global cerebral glucose metabolism (GCGM) during the formation and maintenance of pair bonds. Baseline positron emission tomography (PET) scans were taken of thirteen unpaired male titi monkeys. Seven males were then experimentally paired with females, scanned and compared, after one week, to six age-matched control males. Five of the six control males were then also paired and scanned after one week. Scans were repeated on all males after four months of pairing. PET scans were coregistered with structural magnetic resonance imaging (MRI), and region of interest (ROI) analysis was carried out. A primary finding was that paired males showed a significant increase in [¹⁸F]-fluorodeoxyglucose (FDG) uptake in whole brain following one week of pairing, which is maintained out to four months. Dopaminergic, "motivational" areas and those involved in social behavior showed the greatest change in glucose uptake. In contrast, control areas changed only marginally more than GCGM. These findings confirm the large effects of social bonds on GCGM. They also suggest that more studies should examine how social manipulations affect whole-brain FDG uptake, as opposed to assuming that it does not change across condition.

Long-term-stability of relationship structure in family groups of common marmosets, and its link to proactive prosociality

Cooperatively breeding, group-living common marmosets show differentiated relationships, where more strongly bonded dyads within a group engage more in affiliative interactions than less strongly bonded ones. Intriguingly, recent results suggest that strong bonds do not only occur between breeding partners but between individuals from any sex or status, and that strong-bond partners exhibit correlated oxytocin fluctuations (dyadic oxytocin synchrony, OTS) over a period of six weeks. To date, it is unclear whether such relationships are stable over time and whether they are also reflected in higher partner-specific proactive prosociality. To assess the long-term stability of the relationship structure of common marmoset family groups, we investigated whether hormonal and behavioral markers of group structure (dyadic OTS, dyadic affiliation, and individual group integration) in common marmoset families remained stable over a period of six months. We collected baseline urinary OT and social behavior of 36 dyads from three family groups in a non-reproductive period (period A), and again six months later, around the birth of new infants (period B). Patterns of dyadic OTS, dyadic affiliation, and individual group integration were consistent between the two study periods. Oxytocin data from a fourth group (10 dyads), collected in two non-reproductive periods separated by a period of more than five years, could replicate this finding. Furthermore, OTS was also correlated with proactive prosociality that was assessed experimentally for 38 dyads during an earlier study. These results suggest that differentiated relationships are stable over time, even between group members other than the breeding pair, and that more strongly bonded partners also show higher levels of proactive prosociality. Future studies are necessary to identify whether these relationships have an adaptive function, perhaps with regard to positive consequences on cooperativeness.

Infertility, impotence, and emasculation--psychosocial contexts for abandoning reproduction

From a Darwinian perspective we live to reproduce, but in various situations genetic males elect not to reproduce by choosing medical treatments leading to infertility, impotence, and, in the extreme, emasculation. For many men, infertility can be psychologically distressing. However, for certain genetic males, being infertile may improve their quality of life. Examples include (1) men who seek vasectomy, (2) individuals with Gender Dysphoria (e.g., transwomen, and modern day voluntary eunuchs), (3) most gay men, and (4) men treated for testicular and prostate cancer. Men who desire vasectomy typically have a Darwinian fitness W >1 at the time of their vasectomies; i.e., after they have their desired number of offspring or consider themselves past an age for parenting newborns. In contrast, prostate and testicular cancer patients, along with individuals with extreme Gender Dysphoria, do not necessarily seek to be sterile, but accept it as an unavoidable consequence of the treatment for their condition undertaken for survival (in case of cancer patients) or to achieve a better quality of life (for those with Gender Dysphoria). Most gay men do not father children, but they may play an avuncular role, providing for their siblings’ offspring's welfare, thus improving their inclusive fitness through kin selection. In a strictly Darwinian model, the primary motivation for all individuals is to reproduce, but there are many situations for men to remove themselves from the breeding populations because they have achieved a fitness W ≥1, or have stronger medical or psychological needs that preclude remaining fertile.

Neuropeptide diversity and the regulation of social behavior in New World primates

Oxytocin (OT) and vasopressin (AVP) are important hypothalamic neuropeptides that regulate peripheral physiology, and have emerged as important modulators of brain function, particularly in the social realm. OT structure and the genes that ultimately determine structure are highly conserved among diverse eutherian mammals, but recent discoveries have identified surprising variability in OT and peptide structure in New World monkeys (NWM), with five new OT variants identified to date. This review explores these new findings in light of comparative OT/AVP ligand evolution, documents coevolutionary changes in the oxytocin and vasopressin receptors (OTR and V1aR), and highlights the distribution of neuropeptidergic neurons and receptors in the primate brain. Finally, the behavioral consequences of OT and AVP in regulating NWM sociality are summarized, demonstrating important neuromodulatory effects of these compounds and OT ligand-specific influences in certain social domains.