Stress and immunity at the invasion front: a comparison across cane toad (Rhinella marina) populations GREGORY P. BROWN 1 *, CRYSTAL KELEHEAR 1,2 , CATHERINE M. SHILTON 3 , BENJAMIN L. PHILLIPS 4 and RICK SHINE 1 1 School of Biological Sciences, University of Sydney, Sydney, NSW, 2006, Australia 2 Smithsonian Tropical Research Institute, Apartado 0843-03092, Balboa, Ancon, Republic of Panama 3 Berrimah Veterinary Laboratories, Northern Territory, Department of Primary Industries and Fisheries, Berrimah, NT, 0828, Australia 4 School of Biosciences, University of Melbourne, Melbourne, VIC, 3010, Australia Received 29 April 2015; revised 17 June 2015; accepted for publication 17 June 2015 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. © 2015 The Linnean Society of London, Biological Journal of the Linnean Society, 2015, 00, 000–000. ADDITIONAL KEYWORDS: amphibian – Bufo marinus – fat bodies – immunocompetence – immune response – Rhabdias pseudosphaerocephala – spleen size. INTRODUCTION Eco-evolutionary conditions on invasion fronts differ dramatically from those experienced by non-expand- ing populations (Phillips, Brown & Shine, 2010a; Zalewski & Bartoszewicz, 2012; Therry et al., 2014b). Perhaps most fundamentally, populations on the expanding range edge are (by definition) at a lower density than those in the core of the range. This low density has clear ecological implications. Because competition with conspecifics is lessened, individuals on the range edge have greater resources available to them and so may be fitter than counterparts in the range core (Brown, Kelehear & Shine, 2013). Second, range-front individuals are likely to suffer less para- sitism than range-core individuals because of serial founder events at the invasion front (particularly if uninfected individuals are better dispersers) and also (possibly) lowered transmission rates at low host den- sity (Knolle, 1989; Phillips et al., 2010c; Kelehear, Brown & Shine, 2012). The strong density gradient on the invasion front also has evolutionary implications: invasion front populations will tend to be r-selected rather than K-selected (Phillips, Brown & Shine, 2010b), and several evolutionary forces will combine to create upward pressure on dispersal rates (Travis *Corresponding author. E-mail: gregory.brown@sydney.edu.au 1 © 2015 The Linnean Society of London, Biological Journal of the Linnean Society, 2015, , – Biological Journal of the Linnean Society, 2015,  , – . With 6 figures.