Developmental changes in responsiveness to parents and unfamiliar adults in a monogamous monkey (Callicebus moloch)

Expression of bond-related behaviors affects titi monkey responsiveness to oxytocin and vasopressin treatments

Social bonds influence physiology and behavior, which can shape how individuals respond to physical and affective challenges. Coppery titi monkey (Plecturocebus cupreus) offspring form selective bonds with their fathers, making them ideal for investigating how father-daughter bonds influence juveniles' responses to oxytocin (OT) and arginine-vasopressin (AVP) manipulations. We quantified the expression of father-daughter bond-related behaviors in females (n = 10) and gave acute intranasal treatments of saline, low/medium/high OT, low/high AVP, or an OT receptor antagonist (OTA) to subjects prior to a parent preference test. While females spent more time in proximity to their parents than strangers, we found a large degree of individual variation. Females with greater expression of bonding behaviors responded to OT treatments in a dose-dependent manner. Subjects also spent less time in proximity to strangers when treated with High OT (p = 0.003) and Low OT (p = 0.007), but more time when treated with High AVP (p = 0.007), Low AVP (p = 0.009), and OTA (p = 0.001). Findings from the present study suggest that variation in the expression of bond-related behaviors may alter responsiveness to OT and AVP, increasing engagement with unfamiliar social others. This enhanced sociality with strangers may promote the formation of pair bonds with partners.

A meta-analysis of sex differences in animal personality: no evidence for the greater male variability hypothesis

The notion that men are more variable than women has become embedded into scientific thinking. For mental traits like personality, greater male variability has been partly attributed to biology, underpinned by claims that there is generally greater variation among males than females in non-human animals due to stronger sexual selection on males. However, evidence for greater male variability is limited to morphological traits, and there is little information regarding sex differences in personality-like behaviours for non-human animals. Here, we meta-analysed sex differences in means and variances for over 2100 effects (204 studies) from 220 species (covering five broad taxonomic groups) across five personality traits: boldness, aggression, activity, sociality and exploration. We also tested if sexual size dimorphism, a proxy for sex-specific sexual selection, explains variation in the magnitude of sex differences in personality. We found no significant differences in personality between the sexes. In addition, sexual size dimorphism did not explain variation in the magnitude of the observed sex differences in the mean or variance in personality for any taxonomic group. In sum, we find no evidence for widespread sex differences in variability in non-human animal personality.

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.

Effects of chronic intranasal oxytocin on behavior and cerebral glucose uptake in juvenile titi monkeys

Intranasal oxytocin (IN OXT) has been proposed as a treatment for autism spectrum disorder (ASD); however, little is known about the effects of long-term exposure. This is the first study in a non-human primate species to examine how developmental exposure to chronic IN OXT affects juvenile's interactions with family members, social preference for parents versus strangers, anxiety-like behavior, and cerebral glucose metabolism. Titi monkeys are socially monogamous and biparental; their family bonds share important characteristics with human family bonds. Fourteen males and 15 females were treated intranasally with saline (n = 14) or 0.8 IU/kg OXT (n = 15), daily from 12 to 18 months of age. Compared to SAL-treated animals, OXT-treated animals of both sexes spent significantly more time grooming other family members (F₁ = 8.97, p = 0.006). Overall, OXT-treated subjects were more social (F₁ = 8.35, p = 0.005) during preference tests. OXT-treated females displayed an enhanced preference for their parents (t = 2.265, p = 0.026). OXT-treated males had a blunted preference for their parents and an increase in the time spent near unfamiliar pairs (F1 = 10.89, p = 0.001). During anxiety tests, OXT-treated males refused to complete the task more often than SAL-treated males and had longer latencies (p < 0.0001). Neuroimaging studies revealed that OXT-treated animals had higher glucose uptake across the social salience network as a whole after one month of treatment (F_1,9 = 1.07, p = 0.042). Our results suggest moderate prosocial effects of chronic IN OXT, that did not depend on anxiolytic properties. We also found important sex differences that should be considered in a translational context.

Infant titi monkey behavior in the open field test and the effect of early adversity

The open field test is commonly used to measure anxiety-related behavior and exploration in rodents. Here, we used it as a standardized novel environment in which to evaluate the behavioral response of infant titi monkeys (Callicebus cupreus), to determine the effect of presence of individual family members, and to assess how adverse early experience alters infant behavior. Infants were tested in the open field for 5 days at ages 4 and 6 months in four successive 5 min trials on each day. A transport cage, which was situated on one side of the open field, was either empty (non-social control) or contained the father, mother, or sibling. Infant locomotor, vocalization, and exploratory behavior were quantified. Results indicated that age, sex, social condition, and early experience all had significant effects on infant behavior. Specifically, infants were generally more exploratory at 6 months and male infants were more exploratory than females. Infants distinguished between social and non-social conditions but made few behavioral distinctions between the attachment figure and other individuals. Infants which had adverse early life experience demonstrated greater emotional and physical independence, suggesting that early adversity led to resiliency in the novel environment.

Do wild titi monkeys show empathy?

Abstract. We observed a putative case of empathy among wild black-fronted titi monkeys (Callicebus nigrifrons) from two different groups (D and R). In over 10 years of behavioural observations of five habituated groups of this species, only low levels of inter-group tolerance have been observed. However, on one day, we encountered the adult male from group D limping (poor hind limb motor coordination) as he travelled alone along the ground. Interestingly, we observed that members of group R did not express any agonistic behaviour towards this neighbouring male and apparently allowed this disabled individual to follow them in the forest for over 5 h. They stayed low in the forest (< 2 m above the ground) and < 10 m horizontally from the individual, and remained in visual contact with him. At the end of the day, this male from group D slept in the sleeping site of group R and was groomed by the adult female of group R. Such tolerance between members of different groups has never been previously observed in this species. Furthermore, group R exposed themselves to increased predation risk by staying close to the ground for protracted periods. The behaviour of group R could be interpreted by as a putative case of empathic responding in this species.

Relationship between age and activity varies by sex in titi monkeys (Callicebus cupreus)

Motor activity increases during development and typically peaks before reproductive maturity. Males may become more active than do females, and this sex difference in activity may be a mechanism contributing to sex-biased dispersal. The generality of the link between sex differences in activity and sex differences in dispersal would be strengthened with evidence of greater female, as compared to male, activity during development in a species with a female bias in dispersal. Titi monkeys are a good model for such an investigation because subadult females leave their natal group at an earlier age than do subadult males. This study examines whether developing female titi monkeys become more active with age than do developing male titi monkeys. Locomotor activity of 13 female and 8 male captive titi monkeys was measured as they matured from initial ambulatory independence (6 months of age) to reproductive maturity (24 months). Increased activity of females was associated most strongly with increasing age. Increased activity of males, however, was associated most strongly with repeated testing. Compared to maturing males, greater activity of maturing females may underlie the earlier dispersal of female titi monkeys that has been reported.

Social preferences of developing guinea pigs (Cavia porcellus) from the preweaning to the periadolescent periods

Preference tests in a novel environment (Experiment 1) and unobtrusive observations in a specialized living environment (Experiment 2) examined the attractiveness of various classes of conspecifics for maturing guinea pigs (Cavia porcellus). It was found that (a). the young continued to remain near the mother well beyond weaning; (b). there was increased time spent with unrelated adult females, but not males, after weaning; (c). male and female offspring behaved similarly; and (d). littermates spent considerable time with each other. These results provide no evidence that guinea pigs approaching sexual maturity begin to associate preferentially with novel animals or potential breeding partners. Choices were largely predictable from earlier findings of the ability of various classes of conspecifics to reduce hypothalamic-pituitary-adrenal activity of the young.