Stress and immunity at the invasion front: a comparison across cane toad (Rhinella marina) populations

At an invasion front, energetic and physiological trade-offs may differ from those at the range-core as a result of selection for enhanced dispersal, combined with a low density of conspecifics (which reduces pathogen transmission and competition for food). We measured traits related to energy stores and immunity in wild cane toads (Rhinella marina) across a 750-km transect from their invasion front in tropical Australia, back into sites colonized 21 years earlier. Several traits were found to vary with population age; some linearly and others in a curvilinear manner. The relative size of spleens and fat bodies was highest in the oldest and newest populations, where rates of lungworm infection were lowest. Toads from older populations produced more corticosterone in response to a standardized stressor, and had higher lymphocyte counts (but lower basophil counts). The amount of skin swelling elicited by phytohaemagglutinin injection did not vary geographically, although recruitment of leukocytes to the injected tissue was higher in toads from long-colonized areas. Because this was a field-based study, we cannot differentiate the effects of population age, toad density or pathogen pressure on our measures of stress and immune responses, nor can we distinguish whether the causation involves hard-wired adaptive processes or phenotypically plastic responses. Nonetheless, our data demonstrate substantial variation in immune systems among toads at varying distances from an invasion front, showing that a biological invasion imposes strong pressures on physiological systems of the invader.