(Pro8)-Oxytocin

Behavioral strategies that facilitate the maintenance of social bonds are critical for the preservation of high-quality social relationships. Central oxytocin (OT) activity modulates the behavioral features of socially monogamous relationships in a number of mammalian species (including marmoset monkeys), and plays a vital role in the behavioral maintenance of long-term social relationships. Two distinct variants of OT have been identified in some New World primates (including marmosets; Lee et al., 2011). The marmoset variant of the oxytocin ligand (Pro(8)-OT) is structurally distinct from the consensus mammalian variant of the oxytocin ligand (Leu(8)-OT), due to a proline substitution at the 8th amino-acid position. The goal of the present study was to determine if treating marmosets with Pro(8)-OT, relative to treatments with Leu(8)-OT, control saline, or an OT antagonist, had modulatory effects on the behavioral maintenance of long-term social relationships in marmosets. Treatment with the Pro(8) variant, but not the Leu(8) variant, of OT facilitated fidelity with a long-term partner by reducing time spent in close proximity with an opposite-sex stranger. However, this facilitative effect of Pro(8)-OT on proximity behavior manifested itself differently in male and female marmosets, such that females preferred to interact socially with their partner rather than a stranger when treated with Pro(8)-OT, while males spent less time in close proximity with both their partner and a stranger when treated with Pro(8)-OT. Furthermore, treatment with Pro(8)-OT, but not Leu(8)-OT, significantly delayed the expression of sexual solicitation behavior toward an opposite-sex stranger in both male and female marmosets, but had no effect on sociosexual behavior directed toward a long-term partner. These results suggest that the OT system is highly involved in reducing fidelity-threatening behaviors in well-established marmoset pairs, and that the effects were only produced by species-specific OT ligands.

Humans and various nonhuman primates respond negatively to inequity not in their favor (i.e., inequity aversion), when inequity between two individuals is introduced. Common marmosets, a highly prosocial species, further discriminated between human actors who reciprocated in social exchanges, and those who did not. Conversely, marmoset models of autism, induced via prenatal exposure to valproic acid (VPA marmosets), did not discriminate. Interestingly, previous studies of inequity aversion in marmosets have produced negative results, or were limited to males. Recent studies suggest that inequity aversion is highly influenced by the tasks employed. Here we show inequity aversion in both male and female marmosets using a novel task which required a relatively long duration of response. Marmosets were required to hold a spoon for 2 s to receive a reward. Marmosets successfully performed the task when they observed an unfamiliar conspecific partner obtaining the same reward (equity test). However, when they witnessed the partner receiving a more attractive reward for equal effort (inequity test), unexposed marmosets, which were not exposed to either valproic acid or saline during the fetal period refused to respond. This inequity aversion was not observed in unexposed marmosets when the partner was absent. In contrast, marmosets with fetal exposure to valproic acid (VPA marmosets) successfully executed the task irrespective of their partners' reward conditions. As prenatal exposure to valproic acid is a well-known procedure to induce autism spectrum disorder (ASD)-like behaviors in rodents, we propose that VPA marmosets failed to show inequity aversion due to weak social motivation or interest towards others.

Cooperatively-breeding and socially-monogamous primates, like marmosets and humans, exhibit high levels of social tolerance and prosociality toward others. Oxytocin (OXT) generally facilitates prosocial behavior, but there is growing recognition that OXT modulation of prosocial behavior is shaped by the context of social interactions and by other motivational states such as arousal or anxiety. To determine whether prosociality varies based on social context, we evaluated whether marmoset donors (Callithrix penicillata) preferentially rewarded pairmates versus opposite-sex strangers in a prosocial food-sharing task. To examine potential links among OXT, stress systems, and prosociality, we evaluated whether pretrial cortisol levels in marmosets altered the impact of OXT on prosocial responses. Marmosets exhibited spontaneous prosociality toward others, but they did so preferentially toward strangers compared to their pairmates. When donor marmosets were treated with marmoset-specific Pro(8)-OXT, they exhibited reduced prosociality toward strangers compared to marmosets treated with saline or consensus-mammalian Leu(8)-OXT. When pretrial cortisol levels were lower, marmosets exhibited higher prosociality toward strangers. These findings demonstrate that while marmosets show spontaneous prosocial responses toward others, they do so preferentially toward opposite-sex strangers. Cooperative breeding may be associated with the expression of prosociality, but the existence of a pair-bond between marmoset partners appears to be neither necessary nor sufficient for the expression of spontaneous prosocial responses. Furthermore, high prosociality toward strangers is significantly reduced in marmosets treated with Pro(8)-OXT, suggesting that OXT does not universally enhance prosociality, but, rather OXT modulation of prosocial behavior varies depending on social context.