Simultaneous investigation of urinary and faecal glucocorticoid metabolite concentrations reveals short- versus long-term drivers of HPA-axis activity in a wild primate (Papio ursinus)

Hormonal responses to mating competition in male Tonkean macaques

Glucocorticoid and androgen hormones play a prominent role in male reproductive effort. Their production usually increases in non-human primates during mating competition, which may include rivalry for access to receptive females, struggles for high dominance rank, or social pressure on low-ranking individuals. It is generally assumed that glucocorticoids and androgens are associated with mating challenges rather than dominance status, but the involvement of multiple factors makes it difficult to disentangle the two. In this regard, Tonkean macaques provide a suitable model because they are characterized by relaxed dominance and year-round breeding, meaning that there is typically no more than one receptive female in a group, and thus first-ranking males can easily monopolize her. We studied two captive groups of Tonkean macaques over an 80-month period, recording the reproductive status of females, collecting urine from males and sampling behaviors in both sexes. Male urinary hormone concentrations could be affected by increased competition caused by the mating period, the number of males and the degree of female attractiveness. The highest increases in androgens were recorded in males performing female mate-guarding. Despite the importance of dominance status in determining which males can mate, we found no significant effect of male rank on glucocorticoids and only a marginal effect on androgens during mate-guarding. Both types of hormones were more directly involved in the mating effort of males than in their dominance status. Our results show that their function can be understood in light of the particular competitive needs generated by the species-specific social system.

Linking glucocorticoid variations to monthly and daily behavior in a wild endangered neotropical primate

Identifying the factors swaying physiological stress levels in wild animals can help depict how they cope with environmental and social stressors, shedding light on their feeding ecology, behavioral plasticity, and adaptability. Here, we used noninvasive methods to explore the link between glucocorticoid levels and behavior in an endangered neotropical primate facing habitat fragmentation pressure, the black lion tamarin (Leontopithecus chrysopygus). We investigated monthly and day-to-day glucocorticoid variations independently to attempt to disentangle the complex nature of the adrenocortical activity. Between May 2019 to March 2020, we followed two groups of black lion tamarins in two different areas, a continuous forest and a small fragment, and gathered behavioral data (over 95 days in total; 8.6 ± 3.9 days/month) and fecal samples (N_samples  = 468; 4.93 ± 3.5 samples/day) simultaneously. Preliminary analyses enabled us to identify circadian variations linked to the biological rhythm, which were taken into account in subsequent models. Monthly analyses revealed that black lion tamarin fecal glucocorticoid metabolite levels vary according to changes in activity budget associated with the fruit consumption, movement, and resting time of the groups. At a day-to-day level, while intergroup encounters led to increases in fecal glucocorticoid metabolite concentrations, we found that changes in food intake or activity level did not trigger physiological stress responses. These findings suggest that diet and ranging patterns, driven by food availability and distribution, influence physiological stress at a seasonal scale, while acute stressors such as interspecific competition trigger short-term stress responses. Exploring fecal glucocorticoid metabolite variations over different timescales can help uncover the predictive and reactive facets of physiological stress in wild species. Moreover, having a comprehensive understanding of the physiological state of species is a valuable conservation tool for evaluating how they cope in changing environments.

Quantifying allo-grooming in wild chacma baboons (Papio ursinus) using tri-axial acceleration data and machine learning

Quantification of activity budgets is pivotal for understanding how animals respond to changes in their environment. Social grooming is a key activity that underpins various social processes with consequences for health and fitness. Traditional methods use direct (focal) observations to calculate grooming rates, providing systematic but sparse data. Accelerometers, in contrast, can quantify activity budgets continuously but have not been used to quantify social grooming. We test whether grooming can be accurately identified using machine learning (random forest model) trained on labelled acceleration data from wild chacma baboons (Papio ursinus). We successfully identified giving and receiving grooming with high precision (81% and 91%) and recall (87% and 79%). Giving grooming was associated with a distinct rhythmical signal along the surge axis. Receiving grooming had similar acceleration signals to resting, and thus was more difficult to assign. We applied our machine learning model to n = 680 collar data days from n = 12 baboons and found that grooming rates obtained from accelerometers were significantly and positively correlated with direct observation rates for giving but not receiving grooming. The ability to collect continuous grooming data in wild populations will allow researchers to re-examine and expand upon long-standing questions regarding the formation and function of grooming bonds.