Contrasting responses to intruders and to involuntary separation by monogamous and polygynous New World monkeys.

The strength and quality of the relationship between heterosexual pairmates were compared in two species of New World monkeys, the polygynous squirrel monkey (Saimiri) and the monogamous titi monkey (Callicebus). This was accomplished by measuring behavioral and adrenocortical responses to intruders and to involuntary separation from the pairmate. The species diverged sharply in their responses to these conditions. For titi monkeys, the presence of an intruder resulted in a marked increase in behavioral signs of agitation, which were more prominent in the subject of the same sex as the intruder. Most male titi monkeys showed an increase in plasma cortisol in the presence of a male intruder; this pattern was not seen in their response to a female intruder. Female titi monkeys showed no consistent pattern of adrenocortical response to intruders of either sex. For squirrel monkeys, the presence of an intruder resulted in increased frequency of displays in male subjects, but not in female subjects. Both sexes showed a reliable reduction in plasma cortisol, below basal levels, in the presence of a female intruder, whereas no effect on cortisol was produced by a male intruder. Titi monkeys, but not squirrel monkeys, responded to separation with a significant increase in plasma cortisol and in behavioral indices of agitation or distress.