REPRODUCTIVE EFFORT INFLUENCES THE PREVALENCE OF HAEMATOZOAN PARASITES IN GREAT TITS

1. The influence of reproductive effort on host susceptibility to parasitism was examined in great tits, Parus major, by comparing the prevalence of haematozoan parasites with respect to clutch size in male and female parents. 2. Observational and experimental studies were conducted. Observational studies documented the relationship between clutch size and parasite prevalence in males and females in unmanipulated nests. Reproductive effort was manipulated by exchanging complete clutches between pairs of nests during incubation. Parents experienced a maximum manipulation of +/-5 eggs. 3. Observational studies showed that the prevalence of parasites was higher in females than males. The prevalence of parasites in males increased with both increasing clutch size and increasing age. There was no evidence of similar effects in females. 4. Experimental manipulation of clutch sizes showed that males were more likely to be infected if they had naturally large clutches or when their clutch size was artificially increased. There was no evidence of such effects on female infection probability. Reproductive effort thus increases susceptibility of males to parasites throughout the breeding period. 5. The mechanism which relates parasitism to reproductive effort in males is discussed. Reproductive effort might result in greater exposure, decreased ability to control chronic infections, or both. Nevertheless, reproductive effort increases susceptibility to haematozoan infection; whether this represents a cost of reproduction depends on the virulence of these parasites. It may also provide an explanation of previously reported patterns in haematozoan prevalence across bird species.