Doctoral Dissertation Improvement: Social Stress and Reproduction in Female Phayre's Leaf Monkeys (Trachypithecus phayrei)

Suny At Stony Brook, Stony Brook NY

Investigators

Abstract

Glucocorticoids (GCs) are adrenal hormones produced by the body in response to stress. Within social animals that form dominance hierarchies, it has been classically hypothesized that low-rankers have higher GCs because they experience stressors more frequently and have reduced access to social affiliation. Furthermore, elevated GCs are thought to impair reproduction, leading to individual and rank differences in fitness. Recent studies in the wild, however, have questioned both hypotheses, suggesting that high-rankers also experience stressors (e.g. aggression and social instability), and that reproduction is rarely affected by GCs. These results suggest that our current understanding of social stress and its associated consequences is inaccurate. More specifically, (1) the relationship between rank and GCs may be dependent on the specific correlates of rank (which may vary across species), rather than rank per se, and (2) chronically elevated GCs, at least at the level present in natural populations, may not lead to fitness consequences, at least from a reproductive perspective. The present study aims to refine our current understanding of the causes and consequences of social stress by evaluating stress, behavior, and reproduction in female Phayre's leaf monkeys (Trachypithecus phayrei). Because female Phayre's leaf monkeys are characterized by a dominance hierarchy in which rank is both unstable (changes frequently), and maintained individualistically, with little agonistic support from others, the stressors associated with high rank are hypothesized to outweigh those associated with low rank, leading to greater GCs amongst high-rankers. Furthermore, because high-ranking individuals (across species) commonly have improved reproductive success compared to low-rankers, the effects of GCs on reproduction in female Phayre's leaf monkeys are likely to be negligible. This study aims to accomplish three major goals: (1) to quantify the distribution of stressors and social affiliation in relation to rank, (2) to evaluate how this distribution explains GC patterns, and (3) to assess the effect of GCs on reproductive function. Focal animal observations and urine sampling will be conducted on 16 females in two groups (n=5, n =11) at Phu Khieo Wildlife Sanctuary, Thailand. Behaviors will be used to calculate rank, stressors, and social affiliation, and evaluated in relation to GCs measured from urine samples. Progesterone metabolites will be analyzed in cycling females to help address the effect of GCs on a variety of reproductive parameters such as number of cycles to conception and probability of conception. The broader impacts of the present study are two-fold. First, the study is internationally collaborative, and involves development and training of local Thai people in animal behavior and ecology. The knowledge imparted in this training experience advances the appreciation of nature and encourages conservation efforts in Thailand. Secondly, the results of the present study may have an important societal impact on our understanding of the causes and consequences of social stress in humans. This contribution is extremely valuable, as social stress has been identified as the causal factor for several forms of mental illness, as well as infertility. Therefore, furthering our understanding of the behavioral and reproductive correlates of GCs becomes critical for our efforts to improve human health.