The oxytocin (OT) hormone pathway is involved in numerous physiological 
processes, and one of its receptor genes (OXTR) has been implicated in pair 
bonding behavior in mammalian lineages. This observation is important for 
understanding social monogamy in primates, which occurs in only a small subset 
of taxa, including Azara's owl monkey (Aotus azarae). To examine the potential 
relationship between social monogamy and OXTR variation, we sequenced its 5' 
regulatory (4936bp) and coding (1167bp) regions in 25 owl monkeys from the 
Argentinean Gran Chaco, and examined OXTR sequences from 1092 humans from the 
1000 Genomes Project. We also assessed interspecific variation of OXTR in 25 
primate and rodent species that represent a set of phylogenetically and 
behaviorally disparate taxa. Our analysis revealed substantial variation in the 
putative 5' regulatory region of OXTR, with marked structural differences across 
primate taxa, particularly for humans and chimpanzees, which exhibited unique 
patterns of large motifs of dinucleotide A+T repeats upstream of the OXTR 5' 
UTR. In addition, we observed a large number of amino acid substitutions in the 
OXTR CDS region among New World primate taxa that distinguish them from Old 
World primates. Furthermore, primate taxa traditionally defined as socially 
monogamous (e.g., gibbons, owl monkeys, titi monkeys, and saki monkeys) all 
exhibited different amino acid motifs for their respective OXTR protein coding 
sequences. These findings support the notion that monogamy has evolved 
independently in Old World and New World primates, and that it has done so 
through different molecular mechanisms, not exclusively through the oxytocin 
pathway.