Strategic sperm allocation in the Small White butterfly Pieris rapae (Lepidoptera: Pieridae)

1. In species where females mate multiply, it is important for males to recuperate quickly in order to maximize their fertilization success. Butterflies produce a spermatophore at mating containing accessory secretions and sperm of two types: a large number of non-fertile.'apyrene' sperm and fewer fertile 'eupyrene' sperm. Many butterfly species eclose with most nutrients for reproduction already present. Males must therefore decide how to allocate resources to the various spermatophore components at any given mating. 2. Recovery rates of apyrene and eupyrene sperm number and spermatophore size was studied in the polyandrous Small White butterfly Pieris rapae. The mass of the first spermatophore increases with time since eclosion, as does the number of both types of sperm. Similarly, on a male's second mating, both the mass of the spermatophore and the number of sperm increases with time since the first mating. 3. However, the rate of increase in eupyrene sperm numbers is higher after the first mating. The difference in rate of increase may be the result of different probabilities of virgin and non-virgin males obtaining future matings. 4. Males have a sperm storage organ, the duplex, in which they retain sperm after their first mating. This ensures that high sperm numbers are available for their second mating, even when remating only 1 h later. Thus, males do not ejaculate all available sperm on any given mating, and seem to have different strategies on their first and second matings. 5. It can be argued that Small White butterfly males allocate sperm strategically according to the probability of obtaining subsequent matings, and the level of sperm competition.