The mother's load of stress: Examining correlations and consequences of maternal stress in wild Colobus vellerosus monkeys

Abstract

Individuals rarely, if ever, share equitable and optimal access to available energy in life, and thus anthropologists have a long-standing interest in exploring intraspecific and temporal variation in life history traits by inquiring why, how, and to what extent individuals express phenotypic flexibility in making energetic trade-offs. Along these lines, one of the primary evolutionary problems many primate mothers face is marathoning long-term reproductive investments in singleton offspring while simultaneously occupying social and physical environments that are complex and frequently changing. Thus, weathering these environmental disruptions requires flexible and carefully calibrated energetic trade-offs. One means for moderating energetic investments is the circulation of glucocorticoids (GCs), which are pleiotropic hormones responsible for orchestrating internal energetic priorities. The overarching goal of this project is to examine whether nonhuman primate mothers respond to dynamic social environments with energetic trade-offs facilitated by glucocorticoids, and if so, to what extent elevations in baseline GCs alter the expression of maternal behaviors or the timing of offspring life histories. In investigating this question, we collected paired behavioral data and hormone (fecal glucocorticoid metabolite, FCM) samples from 15 adult females and three adult males belonging to a wild population of Colobus vellerosus (black-and-white colobus monkeys) in central Ghana over the course of 18 weeks in 2018 and 2019. Broadly, this dissertation is broken into three thematic elements: (1) developing a baseline understanding of FCMs in this population; (2) examining social stress associations with mothers’ behavioral and endocrine coping mechanisms; and (3) exploring altered maternal behavioral expression and potential developmental outcomes when FCMs are high. More specifically, in chapter two I provide a biological validation of our hormone collection, extraction, and enzyme immunoassay methodologies, a critical first step when initiating FCM research in any new population. Following this, in chapter three I assess whether females respond to cues of decreased male quality or heightened infanticide risk by increasing social monitoring and/or initiating anticipatory glucocorticoid responses. Lastly, in chapter four, I explore whether colobus mothers alter expression of maternal behaviors (and thus reproductive investments) during periods of heightened FCMs, and what, if any, consequences this may have for their developing infants. These results support that GCs, combined with behavioral coping mechanisms, are an active mediator of energetic trade-offs in wild colobus monkeys, as these monkeys experienced FCM variation in response to acute stressor events (such as wounding of self or resident alpha male) and parturition, as well as altered weekly rates of antagonistic displays by alpha males. Further, in our study, during weeks with higher-than-expected FCMs levels, colobus mothers flexibly altered expression of maternal behaviors by spending relatively less time carrying and holding infants and reducing rates of shared proximity with their infants. Our preliminary analyses suggest that exposure to elevated maternal GCs in early life (either through direct hormone or indirect behavioral forms) have developmental consequences for offspring, with infants slowing development when exposed to higher GCs or compromised maternal care. Together, these results support in filling an existing research gap connecting maternal GC circulation and adaptive consequences for mothers and offspring in wild primate populations.