ORIGINAL PAPER Reduced investment in immune function in invasion-front populations of the cane toad (Rhinella marina) in Australia David Llewellyn • Michael B. Thompson • Gregory P. Brown • Benjamin L. Phillips • Richard Shine Received: 16 June 2011 / Accepted: 16 November 2011 Ó Springer Science+Business Media B.V. 2011 Abstract In an invasive species, selection for increased rates of dispersal at the expanding range front may favor the evolution of reduced investment into any trait that does not contribute to more rapid dispersal. Thus, populations at the invasion front may exhibit reduced investment into the immune system. To test this prediction, cane toads (Rhinella marina) from parents collected from populations across the toads’ invasion history in tropical Australia were raised in a standard environment. When their immune systems were challenged by injection of bacterial lipopolysaccharide, the toads’ metabolic rates rose by up to 40%. The magnitude of elevation in metabolic rate was lower in toads derived from the invasion front than in those from long-established populations. Our results support the hypothesis that an animal’s investment in immune defenses can be modified by selective forces that arise in the course of a biological invasion. Keywords Bufo marinus  Immunocompetence  Invasive species  Life-history trade-offs  Lipopolysaccharide  Metabolic rate Introduction For many organisms, the major threats to individual viability do not come from the external (abiotic) environment, predators or competitors. Instead, an individual’s fitness is influenced most strongly by interactions with pathogens and parasites (Daszak et al. 2000; Cunningham et al. 2003; Wobeser 2006). Prior to the development of antibiotics, for example, a high proportion of mortality and morbidity in humans, domestic animals and agricultural plants (the three sets of taxa for which most is known) was attributable to bacterial infection and parasitism (Luckey 1959; Goldberg and Luckey 1959; Diamond 1997; Taylor et al. 1998; Mann 1999; McManus et al. 2002; Vidaver 2002). Intense selection to combat attack by pathogens has resulted in the elaboration of complex and sophisticated immune systems (Du Pasquier 1992; Du Pasquier and Litman 2000; Zuk and Stoehr 2002). While immune responses occur in many types of organisms and there is substantial phylogenetic divergence in the details of immune function (Withers 1992; Knox et al. 1994; Janeway et al. 2005) there is little understanding of exactly how investment in immunocompetence has been shaped by evolutionary forces in most types of D. Llewellyn  M. B. Thompson  G. P. Brown  R. Shine (&) School of Biological Sciences A08, University of Sydney, Sydney, NSW 2006, Australia e-mail: rick.shine@sydney.edu.au B. L. Phillips Centre for Tropical Biodiversity and Climate Change, School of Marine and Tropical Biology, James Cook University, Townsville, QLD 4811, Australia 123 Biol Invasions DOI 10.1007/s10530-011-0135-3