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Parasite species can adapt to ecological, behavioral, physiological and biochemical traits of a particular host species. The flea Parapulex chephrenis occurs on the spiny mouse Acomys cahirinus, but does not occur on a co-existing gerbil, Gerbillus dasyurus. To test the hypothesis that the host species affects feeding parameters of a host-specific flea, we studied the feeding rate, rate of blood digestion and resistance to starvation of P. chephrenis when feeding on A. cahirinus and G. dasyurus. We predicted that P. chephrenis would: (1) fill its gut with blood faster, (2) digest blood for a shorter time, and (3) survive longer when starved while feeding on its specific host, A. cahirinus, than on a non-specific host, G. dasyurus. These three responses were observed when P. chephrenis fed on the different hosts and, consequently, our predictions were supported. Twenty percent of fleas filled their midgut after feeding for 10 min on A. cahirinus but this occurred only after 25 min on G. dasyurus. The middle stage of blood digestion was significantly shorter in all fleas feeding on A. cahirinus than in fleas feeding on G. dasyurus. Flea survival was shorter when feeding on G. dasyurus than when feeding on A. cahirinus at 25degreesC, but no difference in survival time was found at 15 or 20degreesC. Both A. cahirinus, the specific host, and G. dasyurus, the non-specific host, co-exist in rocky habitats, yet P. chephrenis occurs on one rodent and not the other. The absence of P. chephrenis on G. dasyurus in nature and the decreased foraging efficiency when feeding on this species in the laboratory suggests that some physiological and biochemical differences between hosts can lead to sharp ecological differences in host-parasite relationships.