Reduced investment in immune function in invasion-front populations of the cane toad (Rhinella marina) in Australia

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.