Testosterone immunomodulation in free-living and captive Rhinella icterica male toads

Abstract

Testosterone (T) regulates immune function, with both immunostimulatory and immunosuppressive effects on several vertebrates. We investigated the covariation between plasma T and corticosterone (CORT) levels and immunity (plasma bacterial killing ability (BKA), and neutrophil to lymphocyte ratio (NLR)) in free-living Rhinella icterica male toads inside and outside the reproductive season. We found an overall positive correlation between steroids and immune traits, with toads during the reproductive season displaying increased T, CORT and BKA. We also investigated the T transdermal application effects on T, CORT, phagocytosis of blood cells, BKA and NLR in captive toads. Toads were treated with T (1, 10 or 100 µg) or vehicle (sesame oil) for eight consecutive days. Animals were bled on the first and eighth days of treatment. Increased plasma T was observed on the first and last day of T-treatment, while increased BKA was observed following all T doses on the last day, with a positive correlation between T and BKA. Plasma CORT, NLR and phagocytosis increased on the last day for all T-treated and vehicle groups. Overall, we demonstrated a positive covariation between T and immune traits in the field and T-induced augmented BKA in captive toads, indicating a T immunoenhancing effect in R. icterica males. This article is part of the theme issue ‘Amphibian immunity: stress, disease and ecoimmunology’.

Introduction to the special issue Amphibian immunity: stress, disease and ecoimmunology

“…Considering the challenges invasive species face as they expand into novel environments with substantial seasonal changes in food availability, energy restrictions might shift their immune function toward more economical and general avenues of combating pathogens [ 112 ]. Another paper of this issue, [ 113 ] investigates a native Brazilian toad species ( Rhinella icterica ), to establish a positive correlation between testosterone, corticosterone and immune function in the field, and a positive correlation between testosterone and immune function in captive male toads, suggesting these steroids show an immunostimulatory effect during the reproduction in anurans [ 113 ].…”