Oxytocin facilitates fidelity in well-established marmoset pairs by reducing sociosexual behavior toward opposite-sex strangers

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 (Pro⁸-OT) is structurally distinct from the consensus mammalian variant of the oxytocin ligand (Leu⁸-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 Pro⁸-OT, relative to treatments with Leu⁸-OT, control saline, or an OT antagonist, had modulatory effects on the behavioral maintenance of long-term social relationships in marmosets. Treatment with the Pro⁸ variant, but not the Leu⁸ 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 Pro⁸-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 Pro⁸-OT, while males spent less time in close proximity with both their partner and a stranger when treated with Pro⁸-OT. Furthermore, treatment with Pro⁸-OT, but not Leu⁸-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.