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1. The fleas Xenopsylla conformis and Xenopsylla ramesis exploit the same rodent host, Meriones crassus, and replace each other between two different habitats situated at the opposite sides of a steep precipitation gradient. It was hypothesised that the reason for this paratopic distribution is competition between larvae of the two species for food resources.

2. This hypothesis was tested by studying the performance of larvae of the two species in terms of their developmental success in mixed-species and single-species treatments under different air temperatures, relative humidities, substrate textures, and food abundance.

3. The number of individuals of X. conformis that survived until emergence depended significantly on the presence of competing species, being, in general, lower in mixed-species compared with single-species treatments. The decrease in developmental success of X. conformis in mixed-species treatments was found mainly during food shortage. In contrast, presence of the competitor did not affect the number of X. ramesis that survived until emergence. No effect of the presence of the competitor on duration of development or sex ratio was found in either species.

4. The results of this study, together with the results of our previous studies, provide an explanation for the paratopic distribution of X. conformis and X. ramesis that exploit the same host species.