Cortisol is a hormone related to stress levels of many organisms, including amphibians. In a study recently published in the journal PLoS ONE we studied the relationship between the release of this hormone and Batrachochytrium dendrobatidis infection in Common Midwife toad larvae.
The results of our study show that cortisol levels of the animals increased as we climbed in altitude, and that this increase occurs at a higher rate in animals infected with the pathogenic fungus. Therefore, populations at higher altitudes are subject to higher stress levels, perhaps due to the extreme temperature changes they experience, but also infection with the pathogenic fungus is an additional source of stress.
Furthermore, in a laboratory experiment with the larvae of the Mallorcan Midwife toad, we noted that levels of this hormone are higher when they are infected with the hypervirulent strain of the fungus (which has been introduced in the Guadarrama Mountains and in much of the world) than when infected with the South African strain of the fungus (which is found in Mallorca).
Finally, we also recorded how the levels of cortisol are related to the behaviour of amphibians in the wild. Animals, infected by fungus and hence with high levels of the hormone, were less healthy. Probably the high energy costs associated with stress contributes to higher mortality of infected newly metamorphosed individuals, which must also start looking for food and escape predators in their new land environment.
You can download the article here.
Levels of cortisol in infected populations (in red) and uninfected (green) of Common Midwife toad related to altitude.