The endangered black lion tamarin, (Leontopithecus chrysopygus), lives in mostly degraded and highly fragmented landscape in the state of Sao Paulo, Brazil. Olivier Kaisin is a PhD student who wants to know whether declining environmental conditions are causing increased stress to the tamarins. Researchers often use glucocorticoid (GC) levels as a measure of physiological stress, as many animals, including primates, produce and release GCs in response to stress. Many researchers have argued that prolonged elevation of GC levels has a negative impact on individual survival or reproduction, but it is not clear whether this is true for most primates. Given that 60% of primate species are currently threatened with extinction, it would be nice to know whether conservation biologists could use GC levels to identify populations that are at risk.
One of the unadvertised features of graduate programs is that students need to learn about their study system before doing research. In this spirit, before beginning his tamarin study, Kaisin (working with several other researchers) did a meta-analysis of all studies (published until 2020) that compared cortisol levels in primates from disturbed vs. undisturbed habitats to see if the type of disturbance influenced GC levels. Disturbance types included hunting, tourism, habitat loss, ongoing logging, habitat degradation and other human activities. Habitat loss was a reduction in forest fragment size to less than 500 hectares. Habitat degradation resulted from logging in the past 20 years that led to changes in forest structure and diversity, but did not substantially reduce the size of the forest habitat. Other human activities did not fit into the five disturbance types, and included activities such as mining, urbanization and access to rubbish.
The graph below shows the effects of the different disturbance types. “Hedges g” is a test statistic used in meta-analyses to look for effects of different variables. The midpoint of the bar (or the diamond in the case of the overall effect) is the mean value of Hedges g, while the endpoints of each bar (or diamond) indicate the 95% confidence interval. If the entire interval does not overlap 0, then we can conclude that there is a statistically significant effect of that variable. Based on this analysis, both hunting and habitat loss were associated with significant increases in glucocorticoid levels in primates, contributing to a significant overall increase in glucocorticoid levels in response to disturbance.
As Kaisin and his colleagues point out, six of the studies actually showed a significant decrease in GC levels in association with disturbance. For example, howler monkeys had reduced GC levels in response to ongoing logging. The researchers interpret this surprising GC decrease on the elimination of large predators from the logged forest, which substantially reduces howler monkey stress levels. As a second example, in Madagascar, an invasive tree species in the degraded site provided important fruits for red-bellied lemurs, leading to well-fed lemurs with reduced GC levels. Unfortunately, these confounding variables cannot be easily controlled, so researchers need to consider each study on a case-by-case basis. Some families of primates were more influenced by stress than others. In particular, hominids (great apes) and atelids (New World monkeys such as howler, spider and woolly monkeys) both showed significantly greater GC levels in association with stress. Three families showed smaller increases while three other families of primates were basically unaffected.
The researchers emphasize that many more studies are needed in order to understand when we should expect stress to elevate GC levels in primates. For example, only one of the studies looked at stress effects on Asian primates. Future studies in endocrinological primatology should relate how prolonged stress influences fitness – including survival, growth and development and reproductive success. In turn, this would allow the conservation community to understand the relationship between stress and future population viability.
note: the paper that describes this research is from the journal Conservation Biology. The reference is Kaisin, O., Fuzessy, L., Poncin, P., Brotcorne, F. and Culot, L., 2021. A meta‐analysis of anthropogenic impacts on physiological stress in wild primates. Conservation Biology, 35(1), pp.101-114. Thanks to the Society for Conservation Biology for allowing me to use figures from the paper. Copyright © 2021 by the Society for Conservation Biology. All rights reserved.