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1. Fleas Xenopsylla conformis and Xenopsylla ramesis parasitize the rodents Meriones crassus and Gerbillus dasyurus. Previously reported isodar analysis of host selection suggested that at low density, X. conformis parasitizes M. crassus only, but with an increase in flea density, G. dasyurus is also parasitized. Xenopsylla ramesis at low density parasitizes both hosts equally, whereas at high density prefers M. crassus. The ultimate cause of underlying these patterns was suggested to be differential fitness reward of parasitism on a particular host species, while the mechanism can be adaptive host selection by fleas.

2. To justify the suggested fitness-density relations, we studied reproductive success in X. conformis and X. ramesis feeding on either M. crassus or G. dasyurus. We hypothesized that reproductive success would differ between two host species for each flea species.

3. Xenopsylla conformis parasitizing M. crassus produced more eggs than when parasitizing G. dasyurus, whereas time of oviposition and larval hatching did not depend on host species. In contrast, egg production in X. ramesis did not differ between host species. However, fleas fed on M. crassus needed fewer bloodmeals, oviposited earlier and hatching of their larvae took less time than those of fleas fed on G. dasyurus.

4. Patterns of egg production and development time in both fleas were consistent with the hypothesis that their between-host distribution arose from adaptive host selection strategy.