Corticosterone–immune interactions during captive stress in invading Australian cane toads (Rhinella marina)

Vertebrates cope with physiological challenges using two major mechanisms: the immune system and the hypothalamic pituitary–adrenal axis (e.g., the glucocorticoid stress response). Because the two systems are tightly integrated, we need simultaneous studies of both systems, in a range of species, to understand how vertebrates respond to novel challenges. To clarify how glucocorticoids modulate the amphibian immune system, we measured three immune parameters and plasma corticosterone (CORT), before and after inflicting a stressor (capture and captive confinement) on introduced cane toads (Rhinella marina) near their invasion front in Australia. Stress increased CORT levels, decreased complement lysis capacity, increased leukocyte oxidative burst, and did not change heterologous erythrocyte agglutination. The strength of the CORT response was positively correlated with leukocyte oxidative burst, and morphological features associated with invasiveness in cane toads (relative leg length) were correlated with stress responsiveness. No immune parameter that we measured was affected by a toad's infection by a parasitic nematode (Rhabdias pseudosphaerocephala), but the CORT response was muted in infected versus uninfected toads. These results illustrate the complex immune–stress interactions in wild populations of a non-traditional model vertebrate species, and describe immune adaptations of an important invasive species. ►We examined baseline and acute stress corticosterone and immunity in cane toads. ►As expected, corticosterone increased after captive handling. ►One measure of immunity increased, one decreased, and one was unchanged after stress. ►Immune changes appeared consistent with adaptive acute stress immunomodulation. ►Features associated with invasiveness correlated with the glucocorticoid response.