Abstract
The aim of this study was to evaluate the role of environmental (dry versus wet season) and individual (sex, body mass and reproductive status) factors in the levels of faecal cortisol metabolites (FGCs) in Gracilinanus agilis faecal samples as an index of stress levels in this species; as well as its association with abundance of Eimeria spp, as an indicator of immunocompetence against parasites. Our study found that FGCFGCs are a reliable indicator of adrenal activity in G. agilis. We found that FGCFGCs increase considerably by environmental stressors like the dry season. Moreover, the observed positive association between FGCs and body mass is the result of the effect of season and reproduction in both variables. We also demonstrated that an increase in FGC levels among G. agilis during the dry season is associated with a rise in the probability of being infected by Eimeria spp. Hence, our finding supports the corticosteroid-fitness hypothesis, which predicts that increased glucocorticoids as a response to stressors usually results in decreased fitness of individuals, translated into low future survival and reproductive success, and higher parasite infection. To our knowledge, this is the first study that integrates environmental changes, hormone responses and parasite loads in a US marsupial in both empirical and experimental approaches.
Highlights
To cope with challenging stimuli, free-ranging animals use a variety of life-history and behavioural strategies to maintain survival and increase fitness
We demonstrated that an increase in FGC levels among G. agilis during the dry season is associated with a rise in the probability of being infected by Eimeria spp
We found that an increase in FGC levels among G. agilis during the dry season is associated with a rise in the probability of being infected by Eimeria spp., which was the most common and abundant parasite infecting G. agilis individual in the study site (Strona et al 2015)
Summary
To cope with challenging stimuli, free-ranging animals use a variety of life-history and behavioural strategies to maintain survival and increase fitness. GCs can be found free in the blood plasma or bound tightly to carrier proteins in the blood Once they exert their actions by binding to GC receptors in target tissues, they are transported to the liver where they are metabolised and excreted as water soluble conjugates in the urine or faeces (Meikle, 1989; Brownie, 1992; Möstl and Palme, 2002). The measurement of metabolites of GCs in faecal samples has been approved as a reliable way to assess the adrenal response during stress in animals (Möstl and Palme, 2002), allowing long-term studies of the physiological response in individuals (Palme et al, 2000; Romano et al, 2010) as well as in populations (Dloniak et al, 2004)
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