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The energetic demands of the immune system and reproduction are often high and can lead to trade-offs between these 2 life-history traits. In decorated crickets, Gryllodes sigillatus, much of a male's reproductive effort is devoted to calling, and to the synthesis of a spermatophylax, a large, gelatinous, non–sperm-containing mass forming part of the spermatophore and consumed by the female after mating. We employed a reciprocal design in which we experimentally induced an immune response in some males by injecting them with lipopolysaccharides and experimentally induced increased spermatophore production in others to determine if male G. sigillatus trade off immunity and reproduction. Immune-challenged males produced significantly smaller spermatophores, although they did not call less than controls. We also found that males that synthesized more spermatophores exhibited lower immunity. These results reveal a fundamental trade-off between immunity and reproductive effort in male G. sigillatus, specifically with respect to a male's ability to synthesize a costly nuptial food gift. However, we found no correlation between the mass of a male's gift and his ability to survive an acute bacterial infection; this result, coupled with the trade-off identified above, raises the possibility that the spermatophylax serves as a Zahavian handicap. [ABSTRACT FROM PUBLISHER]