Heart rate and social status among male cynomolgus monkeys (Macaca fascicularis) housed in disrupted social groupings

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Abstract
The relationship between social status and telemetrically recorded heart rate (HR) was studied among 29 adult male cynomolgus monkeys (Macaca fascicularis) which had been maintained in six social groups (n = four or five) for 26 months. The membership of each group was reorganized periodically (20 times in 26 months) by shifting monkeys from group to group in a way that maximized each animal's exposure to social strangers. The social behavior of all animals was monitored, via focal sampling, twice per week; social status was based on the outcome of agonistic encounters and was determined every 2 weeks. For four nights on each of two occasions (months 13 and 25 of the experiment), the HRs of all animals were sampled 12 times per hour, 30 seconds per sample between 1800 h and 0900 h. A more limited nocturnal HR sampling was performed during the first month of the experiment. The behavioral observations revealed that, despite the repeated social reorganization, some monkeys were habitually dominant in their groups while others were consistently subordinate. Further, those monkeys falling above the median in dominance status had significantly lower HRs than their subordinate counterparts (104 vs. 117 beats per minuts; P = 0.01). The effect was observed irrespective of the month in which observations were made, and was corroborated by more limited observations made at the beginning of the experiment. Finally, those few monkeys which rose or fell in rank were characterized by inverse changes in HR, supporting the hypothesis that behavioral factors influence HR (rather than visa versa).