SPERM SIZE OR NUMBERS - EFFECTS OF NUTRITIONAL STRESS UPON EUPYRENE AND APYRENE SPERM PRODUCTION STRATEGIES IN THE MOTH PLODIA-INTERPUNCTELLA (LEPIDOPTERA, PYRALIDAE)

1. Sperm numbers are important for fertilization and sperm competition success but the importance of sperm size is not known. 2. We forced males of a meal moth (Plodia interpunctella) to develop on a low-protein diet with intense competition for food and compared their sperm sizes and numbers with sperm produced by males developing on a normal diet. 3. We find that males suffering resource restrictions for spermatogenesis reduce sperm numbers but not sperm sizes for both the fertile 'eupyrene' and non-fertile 'apyrene' sperm types. Under a restrictive larval diet, males do not alter the relative proportion of apyrene to eupyrene sperm produced from a 9:1 ratio. 4. We conclude that apyrene sperm play a critical role in the ejaculate and that both sperm sizes are constrained about narrow optima. The results suggest that males would suffer more drastic fitness losses for their ejaculate if they varied sperm sizes rather than numbers, as predicted by recent ESS sperm competition models.