ABSTRACT
Previous research has shown heart rate to be substantially higher in squirrel monkeys (Saimiri sciureus) than titi monkeys (Callicebus moloch). In order to evaluate whether species differences in heart rate can be accounted for by contrasting patterns of autonomic activity, heart rate in response to novel test conditions was compared using standard pharmacological agents that selectively block the sympathetic (propranolol) or parasympathetic (atropine) components of the autonomic nervous system. Squirrel monkeys were found to exhibit greater sympathetic response to novelty than titi monkeys. In contrast, sympathetic activity in titi monkeys, but not squirrel monkeys, was quickly counteracted by a strong parasympathetic response. Intrinsic heart rates, estimated by blocking both parasympathetic and sympathetic input to the heart, were within the ranges of values predicted by body weight. Heart rate for titi monkeys stabilizes at intrinsic heart rate, whereas heart rate for squirrel monkeys is maintained well above intrinsic heart rate in a novel environment. The contrast between species in autonomic balance is consistent with and probably contributes to each species characteristic mode of interacting with their social and non-social environment.