The Troubles of Being Female: Investigating the relationship between social status and stress level in a population of adult female yellow baboons in Amboseli, Kenya

The famous Whitehall studies of the social determinants of health suggest that low social status has a negative effect on a person’s health due to a high level of chronic stress. Glucocorticoid hormone (GC) is released in the body as a direct response to stress, and is involved in important anti-inflammation and immunosuppression reactions that allow quick responses to stressful stimuli. For this reason, persistent and high levels of GC resulting from prolonged stress are thought to confer lower fitness by inhibiting immunity, producing a lower quality of life, and causing shorter longevity. There are many other examples of low dominance rank conferring lower fitness in animals, but this effect is speciesdependent because subordinate animals can retain benefits as well. Thus, it is scientifically interesting to determine the direction of this effect in primate species. This study aimed to discover the relationship between social rank and stress in the adult female population of yellow baboons in the Amboseli Basin of Kenya. A previous study of male Amboseli baboons found that, with one exception, there is a negative relationship between high male rank and stress level. This information helped lead to the hypothesis that there is a negative correlation between high female rank and stress level. To test this hypothesis, the analysis used longitudinal data from over 12,000 samples collected over a 13-year period (2000-2013) from 191 adult females. Generalized Linear Mixed Models (GLMMs) were used to predict the effect of social rank on stress, as measured by fecal glucocorticoid concentrations. Other factors such as age, reproductive status, and environmental conditions were entered into the models as fixed effects and individual female identities were entered as a random factor. The results showed no significant relationship between a female’s numerical rank and her stress level. Instead, it was determined that a female’s proportional, or relative, rank affects her stress such that lower ranking individuals have higher stress. Though the final results supported the initial hypothesis, the insignificance of numerical rank was surprising. Introduction: The study of wild primate populations gives us insight that the study of laboratory model populations cannot. Because primate behavior and physiological systems closely resemble our own, understanding them provides us with an evolutionary perspective on our own current condition. The Amboseli Baboon Research Project has dedicated over 40 years of work to a specific primate species, the yellow baboon, Papio cynocephalus ("Amboseli Baboon Research Project" 2015). Through the years, the project continues to amass longitudinal data on every aspect of the baboons under study—from birthdates, to consortships, to gastrointestinal biome compositions, and beyond. Included in this wealth of data are steroid hormone measures, including testosterone, progestin, estrogen, and glucocorticoids (a type of cortisol). These hormonal records allow us to investigate the way in which yellow baboons physiologically respond to their environment (otherwise described as behavioral endocrinology). My investigation specifically studied the interaction between an adult female baboon’s cortisol levels and her social status. The importance of finding predictors for stress lies in the psychological and physiological consequences that stress can have on an organism. These consequences, in turn, can ultimately confer lower fitness on this organism. There is already a strong literature that supports dominance rank as a predictor of stress, and Sapolsky may be the most preeminent researcher contributing to this literature. His review found correlations between low rank and low fitness in many animals including rats, mice, and baboons (Sapolsky, 2004). This pattern was initially thought to describe all subordinate animals (Holst, 1998), but this view was complicated by evidence of some species where higher-ranking organisms did not necessarily have lower stress levels than lower-ranking ones. Creel (2001) surveyed a wide range of cooperatively breeding species including birds, primates, dogs, mongooses, wolves, and rodents. He found that for free-range species, dominants rarely had significantly lower stress levels than subordinates (Creel, 2001). Abbott et al. (2003) found inconsistencies between which group, dominant or subordinate, had higher stress across primates species. Their study included macaques, olive baboons, and squirrel monkeys, among others (Abbott et al., 2003). The reason for these discrepancies involves the concept of allostatic load. Allostatic load is the maintenance of homeostasis in an organism; it is mediated by hormones, mainly glucocorticoid (GC), and includes the entirety of the physical burden and thus energy demand on an individual’s body (Goymann & Wingfield, 2004). The social composition of a species group can affect the allostatic load of its’ individuals, so it may be that there are costs to higher rank. There is a direct example of this found in Amboseli male baboons. The alpha male is as stressed as his lower ranked counterparts, but the beta has decreased stress (Gesquiere et al., 2011). It is equally likely that, as both Sapolsky (2004) and Abbott et al. (2003) discuss, subordinate groups have available coping strategies that assuage the otherwise detrimental strains of stress—such as social support or outlets for frustration. Taken together, these complications mean that it is necessary to determine the significance of rank on stress and fitness for an individual species before drawing any conclusions. Further, ranking systems can be sex-specific within species, (Sapolsky, 2005). One could take female-dominant hyenas as an example of this. But a more pertinent example is Amboseli yellow baboons, whose rank and sex differences will be discussed in detail shortly. This calls for an even higher degree of discernment in studies that characterize the relationships between rank, stress, and fitness. Stress consequences on the body, particularly those as a direct result of GCs, related hormones and feedback reactions, are fully characterized in the literature (Sapolsky et al., 2000). But communicating the full extent of their results goes beyond the scope of this introduction. Because my intent is to make the reader aware of the far-reaching costs of elevated stress levels, I will summarize a few key points of a review prepared by Sapolsky (2004) in order to emphasize the palpable effects of stress, which overlay the actions of GC. It should be noted that Sapolsky (2004) assumes a general underlying trend of low rank indicating high stress. Ø There is a direct relationship between the psychological stressors and the physical consequences of low rank. Psychological stressors of a subordinate (or any stressed individual) involve: o Lack of predictability in their environments. A rat who does not know when an electric shock will come will be much more stressed than a rat receiving the same shock but does know that it’s coming. o Lack of a sense of control. A rat that is trained in punishment avoidance and can try to avoid a shock is less stressed when the shock hits because he thinks he has the ability to evade it. o Lack of outlets for aggression. Distractions and the ability to displace aggression onto another individual reduces the stress response. o Personal interpretation of the stressor. For example, a rat that receives painful shocks that decrease in the amount of pain at every round will interpret the series of events more positively than a rat receiving increasingly painful shocks. The first rat will have lower stress levels than