Density dependence of feeding success in haematophagous ectoparasites B. R. KRASNOV 1,2 *, A. HOVHANYAN 1,2,3 , I. S. KHOKHLOVA 3 and A. A. DEGEN 3 1 Mitrani Department of Desert Ecology, Jacob Blaustein Institutes for Desert Research, Ben-Gurion University of the Negev, Sede Boqer Campus, 84490 Midreshet Ben-Gurion, Israel 2 Ramon Science Center, P.O. Box 194, 80600 Mizpe Ramon, Israel 3 Desert Animal Adaptations and Husbandry, Wyler Department of Dryland Agriculture, Jacob Blaustein Institutes for Desert Research, Ben-Gurion University of the Negev, 84105 Beer Sheva, Israel (Received 8 January 2007; revised 25 February 2007; accepted 1 March 2007) SUMMARY We studied the effect of density on feeding success of 2 fleas, Xenopsylla conformis and Xenopsylla ramesis, when exploiting rodents Meriones crassus and Gerbillus dasyurus. We tested 2 alternative hypotheses : (i) that intraspecific interference competition occurs and, thus, feeding success of a flea decreases with an increase in density and (ii) that facilitation via suppression of a host defence system occurs and thus, feeding success of a flea increases with an increase in density. The mean size of a bloodmeal and the proportion of highly engorged individuals in X. conformis feeding on both hosts were affected by density. When on G. dasyurus, both the size of a bloodmeal and the proportion of highly engorged individuals were lower at low (5–15 fleas per host) than at high (25–50 fleas per host) densities. The opposite was true when this flea fed on M. crassus. The mean bloodmeal size and proportions of highly engorged X. ramesis parasitizing either host were not affected by flea density. This study showed that the density dependence of feeding success of a flea (a) varied both between fleas and within-fleas between hosts and (b) indicated either intraspecific competition or facilitation via the host in a particular flea-host association. Key words: density dependence, engorgement, fleas, haematophagy, rodents. INTRODUCTION The distribution of a parasite population across a host population is characterized by its aggregation (Anderson and May, 1978 ; Shaw and Dobson, 1995). Most parasite individuals occur on a few host individuals, while most host individuals have only a few or no parasites. In general, this suggests that some host individuals represent better habitat patches for parasites than other individuals. The aggregation of parasites among their hosts is com- monly thought to arise due to heterogeneities in host populations and/or infection pressure (Shaw and Dobson, 1995). For example, these heterogeneities may include among-host variation in such par- ameters as exposure risk and defensibility. Limited dispersal abilities of parasites can also result in aggregated distribution, although this may be true for some parasite taxa (e.g. lice) but not others (e.g. mosquitoes). A concept of ideal free distribution (IFD ; Fretwell and Lucas, 1970) has also been applied to the distribution of parasites across host individuals (Kelly and Thompson, 2000). This concept predicts that animals that compete for resources distribute themselves among habitat patches in proportion to the amount of resources available to them, so that resource use per individual will be equal across all patches. In other words, animals are (a) ideal in assessing patch quality and (b) free to enter and use the resources on a regular basis. However, given the strict assumptions needed for IFD, it is doubtful that this concept may be applicable for endoparasites and permanent ectoparasites (which spend the entire life on the surface of a host, e.g. lice). However, the IFD approach can be useful in the examination of the distribution of temporary and periodic ecto- parasites. The former are largely free living and visit the host for long enough to take a bloodmeal (e.g. mosquitoes, tabanids), whereas the latter spend a considerably longer time on the hosts than is re- quired merely to obtain a bloodmeal, but nevertheless spend a significant amount of time off-host (e.g. most fleas, mesostigmatid mites) (Lehane, 2005). Kelly and Thompson (2000) developed an IFD model of host choice by blood-sucking insects based on the premise that an individual haematophagous parasite evolved to choose the ‘best’ host to max- imize feeding and, consequently, reproductive suc- cess. They suggested that an individual insect can * Corresponding author : Mitrani Department of Desert Ecology, Jacob Blaustein Institutes for Desert Research, Ben-Gurion University of the Negev, Sede Boqer Campus, 84490 Midreshet Ben-Gurion, Israel. Tel: +972 8 6496841. Fax : +972 8 6586369. E-mail : krasnov@bgu. ac.il 1 Parasitology, Page 1 of 8. f 2007 Cambridge University Press doi:10.1017/S0031182007002739 Printed in the United Kingdom