Pearson, B. L., Reeder, D. M., Judge, P. G. 2015. Crowding increases salivary cortisol but not self-directed behavior in captive baboons. American Journal of Primatology 77, 462-67.
Reduced space can lead to crowding in social animals. Crowding increases the risk of agonistic interactions that, in turn, may require additional physiological defensive coping mechanisms affecting health. To determine the stress induced from increased social density in a group of nineteen baboons living in an indoor/outdoor enclosure, saliva cortisol levels and rates of anxiety-related behavior were analyzed across two unique crowding episodes. Initially, mean salivary cortisol levels when animals were restricted to their indoor quarters were compared to those when they also had access to their larger outdoor enclosure. Then, mean cortisol levels were compared before, during, and after two distinct crowding periods of long and short duration. Crowding resulted in significantly elevated cortisol during crowding periods compared to non-crowded periods. Cortisol levels returned to baseline following two crowding episodes contrasting in their length and ambient climate conditions. These cortisol elevations indicate greater metabolic costs of maintaining homeostasis under social stress resulting from reduced space. Self-directed behavior, conversely, was not reliably elevated during crowding. Results suggest that the potential for negative social interactions, and/or the uncertainty associated with social threat can cause physiological stress responses detected by salivary cortisol. Self-directed behavioral measures of stress may constitute inadequate indicators of social stress in colony-housed monkeys or represent subjective emotional arousal unrelated to hypothalamic-pituitary adrenal axis activation.