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Background: Parasite transmission rate often increases with population density, and selection is expected to favour individuals that differentially allocate immune resources according to future population density (density-dependent prophylaxis). Laboratory studies uphold these predictions, but. field studies sometimes contradict them.

Question: Do wild populations of Mormon crickets show density-dependent prophylaxis?

Organisms: Cryptically coloured low-density, and darkly melanized high-density, populations of Mormon crickets (Anabrus simplex).

Methods: Over 2 years, we assessed two measures of immunity, encapsulation ability and lysozyme activity, in populations with known genetic relationships.

Results: Individuals from consistently high-density populations showed greater immune responses. Increases in each immune parameter were positively associated with cuticular melanization. Despite the broad correspondence between high population density and increased immunity, immune variation in wild Mormon crickets appears to be predominated by population-level effects, as opposed to the short-term flexibility in immunity expected under a conventional interpretation of density-dependent prophylaxis.