Stress hormones provide a cloudy picture of environmental quality and fitnessSubmitted by editor on 5 July 2016.Get the paper!
Wildlife are facing many stressors spanning climate change, loss and alteration of habitats, and pollutants. In an effort to determine whether organisms can cope with an altered environment, many ecologists and conservation biologists have begun to turn to measures of physiology. One of the most popular physiological measurements are the stress hormones corticosterone and cortisol. The hope is that monitoring hormone levels will provide an early warning signal of when wildlife are negatively impacted by environmental change, allowing managers and practitioners ample time to put conservation efforts in place.
It is often assumed that increasing stress hormone levels will indicate that an organism is finding its habitat more challenging to reproduce or survive in. However, because these hormones have roles outside of the acute stress response, they may not always provide a clear snapshot of how individual animals may be fairing in their environment. Understanding whether stress hormones will be useful monitoring tools therefore requires validation of a number of assumptions. First, hormone levels should allow researchers to distinguish between environments of different quality. Second, differences in hormone levels should provide information on the health or success (i.e., reproductive success or survival) of individuals.
In our paper, we investigated these two assumptions in tree swallows, which are part of the aerial insectivore guild of birds that is declining more quickly than any other group of birds in North America. The tree swallows in our population nested in two types of habitats that differed in food availability, distance to roadways, distance to water, and many other features. While female tree swallows invested (i.e., laid) different numbers of eggs between the two types of habitats, their stress hormone levels (obtained from blood sampling) did not reflect this. Further, we recorded differences in how often parents fed their chicks, female body condition, and current and future stress hormone levels of female parents without any influences on reproductive success (mass of chicks or number of chicks raised) or survival. Overall, stress hormone levels in this population of breeding tree swallows did not reflect environmental quality or condition/success.
We took an integrative approach to assess how useful stress hormones could be for inferring habitat quality and fitness, and few studies have been able to measure both environmental quality and the reproductive success and survival of their organisms of interest. By measuring aspects of the structural habitat, food resources (flying insects), behaviour, physiology, and multiple components of success, we were able to conclude that stress hormones may be an uninformative measure at some time periods, or in some populations overall. Our results serve as a caution, adding to a growing body of research showing that the application of plasma measures of stress hormones in ecological and conservation settings may sometimes be considerably more cloudy than we might hope.
Christine L. Madliger and Oliver P. Love