摘要We investigated the performance trade-offs of fleas(Siphonaptera)while adapt-ing to a novel host using two host generalists(Xenopsylla conformis and Xenopsylla rame-sis)and one host specialist(Parapulex chephrenis)maintained on their principal hosts(Meriones crassus for Xenopsylla and Acomys cahirinus for P chephrenis).We asked whether,over generations,(i)a host generalist may become a specialist by evolving the ability to exploit a novel host and losing the ability to exploit an original host and(ii)a host specialist can become a generalist by evolving the ability to exploit a novel host without losing the ability to exploit an original host.We established an experimental line of each species on a novel host(Acomys russatus for Xenopsylla and M.crassus for P chephrenis)and maintained this line on this host during 23 generations.We compared reproductive performance of progenitors of each line and their descendants when they exploited ei-ther original or novel host in terms of egg number and size,hatching success,offspring production,and offspring size.We found changes in performance over generations in fe-male offspring size only.Xenopsylla conformis demonstrated a tendency to become a host specialist(increased performance on the novel host with a concomitant decreased perfor-mance on the original host),whereas P chephrenis demonstrated a tendency to become a host generalist(increased performance on the novel host without a concomitant decreased performance on the original host).We conclude that the probability of generalist to spe-cialist transition,and vice versa,is context-dependent and varies between species.