Size and sex matter: infection dynamics of an invading parasite (the pentastome Raillietiella frenatus) in an invading host (the cane toad Rhinella marina) CRYSTAL KELEHEAR*, GREGORY P. BROWN and RICHARD SHINE School of Biological Sciences, A08, University of Sydney, NSW 2006, Australia (Received 11 March 2012; revised 18 April 2012; accepted 20 April 2012; first published online 20 July 2012) SUMMARY Correlations between host phenotype and vulnerability to parasites can clarify the processes that enhance rates of parasitism, and the effects of parasites on their hosts. We studied an invasive parasite (the pentastome Raillietiella frenatus, subclass Pentastomida, order Cephalobaenida) infecting a new host (the invasive cane toad Rhinella marina), in tropical Australia. We dissected toads over a 27-month period to investigate seasonal changes in pentastome population dynamics and establish which aspects of host phenotype are related to infection. Pentastome prevalence and intensity varied seasonally; male toads were 4 times more likely to be infected than were females; and prevalence was highest in hosts of intermediate body size. The strong sex effect may reflect habitat or dietary divergence between the sexes, resulting in males encountering parasites more often. The relationship between pentastome prevalence and host size likely reflects a role for acquired immunity in preventing re-infection. Infection did not influence host body condition (fatbody size), suggesting that R. frenatus does not impose high energy costs in cane toads. Infected toads had heavier spleens (likely an immune response to infection) and larger testes (perhaps since reproductively active hosts have altered microhabitat use and/or immunocompetence) than did uninfected conspecifics. Although experimental studies are required to identify the causal bases of such patterns, our data confirm that infection status within a population can be strongly linked to host phenotypic traits. Key words: anuran, Bufo marinus, gonad, lung, organ mass, pentastomid, season, spleen, testes, toad. INTRODUCTION Our understanding of host-parasite dynamics has lagged behind that of many other ecological inter- actions such as competition, predation, and intrasex- ual and intersexual conflict. Only in the last 20 years has robust evidence begun to emerge revealing that parasites can substantially affect the viability of their hosts (e.g., Hudson et al. 1998; Moore and Wilson, 2002). Specific features of host phenotype (e.g., age, sex, size and body condition), ecology and behaviour (e.g., diet, habitat use, self-cleaning) can influence the probability that an individual will become infected, as well as its ability (based on immunocompetence and/or energy reserves) to combat that infection (Wilson et al. 2002). Variation in such traits among individuals within a population is reflected in field surveys that show strong among-individual variance in infection intensities (Shaw and Dobson, 1995; Wilson et al. 2002). Commonly, specific age groups, body sizes or sexes within the host popu- lation may exhibit disproportionately high infection rates (Freeland, 1983; Wilson et al. 2002). Such correlations between host phenotype and infection parameters can clarify the processes that render an organism vulnerable to infection; and also, can reveal consistent costs to the host inflicted by infection. Biological invasions provide ideal model systems with which to explore this issue. They can alter local host-parasite dynamics through numerous pathways, and any impacts of interactions between host and parasite may be extreme since they have not been blunted by long periods of co-adaption (Lee and Klasing, 2004). Introduced species may bring new parasites that can host-switch to infect local suscep- tible species, and/or they may act as hosts for previously introduced or endemic parasites in the new range (Daszak et al. 2000). The toxic cane toad (Rhinella marina, previously Bufo marinus) was introduced to northeastern Australia in 1935 in a failed attempt at controlling beetle pests of sugar cane (Lever, 2001). It has since spread widely across the tropics of Queensland, the Northern Territory and most recently, Western Australia (Kearney et al. 2008). Our study sites were at the forefront of the expanding range of the invasive cane toad in the tropics of the Northern Territory. Toads arrived at the study sites in 2005 and have since acquired a parasite (the pentastomid Raillietiella frenatus) from sympatric invasive Asian house geckos (Hemidactylus frenatus). Although the identity of this parasite has been clarified with molecular and morphological data (Kelehear et al. 2011b), ongoing * Corresponding author: Tel: + 61 2 9351 3772. Fax: + 61 2 9351 5609. E-mail: crystal.kelehear@hotmail.com 1596 Parasitology (2012), 139, 1596–1604. © Cambridge University Press 2012 doi:10.1017/S0031182012000832