Enhanced immune function does not depress reproductive output

Costs of reproduction might be mediated by a physiological (resource alloca tion) trade-off between immune function and reproductive effort, and severa l recent studies have shown that an experimental increase in reproductive e ffort is associated with decreased immune function. Here we test the comple mentary prediction of this hypothesis: that increased immune function (spec ific antibody production) depresses reproductive output. Female European st arlings (Sturnus vulgaris) were injected with a nonpathogenic antigen (shee p red blood cells) following completion of laying of their first clutch, to stimulate an in vivo humoral immune response (primary antibody production) . We induced laying of a second clutch by removing the first clutch, and as sessed changes in reproductive performance in individual females pre- and p ost-treatment. Injection of sheep red blood cells produced a significant an tibody response in 96% (n=29) of treated females, with titres comparable to previous studies (range 1-7). However, increased antibody production did n ot decrease primary or secondary female reproductive effort (re-laying inte rval, egg size, clutch size, chick growth or fledging success), compared wi th control, saline-injected birds (n = 22). These data do not support a sim ple resource allocation model for the cost of reproduction, based on a reci procal, negative relationship between resources allocated to immune functio n and reproduction.