Resource predictability and host specificity in fleas : the effect of host body mass B. R. KRASNOV 1 *, S. MORAND 2 , D. MOUILLOT 3 , G. I. SHENBROT 1 , I. S. KHOKHLOVA 4 and R. POULIN 5 1 Ramon Science Center and Mitrani Department of Desert Ecology, Jacob Blaustein Institutes for Desert Research, Ben-Gurion University of the Negev, P.O. Box 194, Mizpe Ramon 80600, Israel 2 Center for Biology and Management of Populations, Campus International de Baillarguet, CS 30016 34988 Montferrier-sur-Lez cedex, France 3 UMR CNRS-UMII 5119 Ecosystemes Lagunaires, University of Montpellier II, CC093, FR-34095 Montpellier Cedex 5, France 4 Desert Animals Adaptations and Husbandry, Wyler Department of Dryland Agriculture, Jacob Blaustein Institutes for Desert Research, Ben-Gurion University of the Negev, Beer Sheva 84105, Israel 5 Department of Zoology, University of Otago, P.O. Box 56, Dunedin, New Zealand (Received 22 November 2005; revised 28 December 2005; accepted 24 January 2006; first published online 28 March 2006) SUMMARY Ecological specialization is hypothesized to result from the exploitation of predictable resource bases. For parasitic organisms, one prediction is that parasites of large-bodied host species, which tend to be long-lived, should specialize on these hosts, whereas parasites of small host species, which represent more ephemeral and less predictable resources, should become generalists. We tested this prediction by quantifying the association between the level of host specificity of fleas and the mean body mass of their mammalian hosts, using published data from 2 large, distinct geographical regions (South Africa and northern North America). In general, we found supporting evidence that flea host specificity, measured either as the number of host species exploited or their taxonomic distinctness, became more pronounced with increasing host body mass. There were, however, some discrepancies among the results depending on the different measures of host specificity, the geographical region studied, or whether we used the raw values or phylogenetically independent contrasts. These are discussed with respect to other forces acting on the evolution of host specificity in parasites, as well as in the context of the regions’ contrasting evolutionary histories. Overall, though, our findings indicate that the exploitation of large-bodied, and therefore long-lived, host species has promoted specialization in fleas, most likely because these hosts represent predictable resources. Key words: fleas, host body size, host specificity, mammals, North America, South Africa. INTRODUCTION Host specificity is one of the most important characteristics of a parasite species. Although host specificity can vary to some degree among different populations of the same parasite species across its geographical range, it still represents a true character of a parasite species, being less variable within parasite species than among parasite species (Krasnov et al. 2004 a). This suggests that the level of host specificity is subjected to natural selection rather than merely reflecting the local restrictions caused by a variety of ecological, morphological, chemical and/ or genetic factors (Fox and Morrow, 1981). From an evolutionary perspective, selection for higher or lower levels of host specificity (or, in a broader sense, ecological specialization) is affected by a variety of both parasite ( =forager)-related and host ( =resource)-related factors (Fox and Morrow, 1981 ; Futuyama and Moreno, 1988 ; Fry, 1996 ; Desdevises et al. 2002). In particular, the models of Ward (1992) suggested that species tend to specialize on predict- able resources, i.e. resources that are relatively stable in both space and time. This likely minimizes extinction rate for an optimal forager. The main resource for a parasitic species is its host, which provides a parasite with food, habitat and mating grounds. Consequently, specialization in parasites is expected to be associated with the level of predictability of its host resources (Ward, 1992). Persistence of a host individual in time, in turn, is associated with its size. In general, larger host species live longer and, thus, represent a more predictable resource for a parasite (Peters, 1983). In addition, larger hosts offer more niches for parasites. As a result, larger host species are expected to harbour mainly parasite species with higher host specificity, whereas small-bodied hosts should be exploited mainly by generalist parasites. Indeed, it has been shown that larger fish species harbour more species * Corresponding author : Ramon Science Center, P.O. Box 194, Mizpe Ramon 80600, Israel. Tel : +972 8 6588764. Fax: +972 8 6586369. E-mail : krasnov@bgu.ac.il 81 Parasitology (2006), 133, 81–88. f 2006 Cambridge University Press doi:10.1017/S0031182006000059 Printed in the United Kingdom