Animals should increase their present reproductive output if their chances for future reproduction are low. However, an animal's ability to make this adjustment may be constrained by the physiological mechanisms mediating the response. To examine this hypothesis, I infected 2- and 5-week-old female crickets, Acheta domesticus, with either a pathogen (the bacterium Serratia marcescens) that induces antimicrobial immune responses, or a parasite (larvae of the parasitoid fly Ormia ochracea) that induces an encapsulation immune response. Females of both age groups infected with bacteria laid more eggs the day after injection than did saline-injected crickets. A similar increase was elicited by injecting components of the bacterial cell wall (lipopolysaccharides). The bacteria-induced increase in egg laying (1) was not the result of physical stress, (2) did not appear to be a nonspecific response to the infection, and (3) was probably not mediated by octopamine. Females did not increase egg laying when infested with O. ochracea, even though this parasitoid invariably kills its host. Injections of sephadex beads, which induced an immune response similar to that created by the parasitoids, also had no effect on egg laying. These results are consistent with the hypotheses that crickets can increase egg laying in response to infection and that increased egg output correlates with the activation of some, but not all, immune responses.
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