Ejaculate dynamics in butterflies: a strategy for maximizing fertilization success?

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Abstract
The butterfly spermatophore is a complex structure containing both sperm and accessory substances. Most of the sperm are anucleated `apyrene' sperm, which do not fertilize the eggs. The fertilizing `eupyrene' sperm comprise only 10-15% of the total sperm number transferred to the female at mating. In addition to sperm, some butterfly species also provide the female with nutrients that are used for egg production and/or somatic maintenance. Many butterflies eclose with most resources available for reproduction already present. Consequently, resources have to be allocated to the different spermatophore components over consecutive matings. This study is the first to quantify eupyrene and apyrene sperm numbers in relation to spermatophore mass, mating history and larval diet. Spermatophore mass was significantly reduced on the male's second mating. However, both eupyrene and apyrene sperm numbers were dramatically increased, as was the proportion of eupyrene sperm. Theory predicts that sperm numbers should increase under sperm competition. On males' second matings, the probability of encountering virgin females is reduced. Males may therefore maximise fertilization success by providing these females with large numbers of sperm. Simultaneously, it may be advantageous for males to reduce their nutrient donation because of their lower paternity assurance. Larval diet affected mass of the first spermatophore but had no effect on sperm number. This suggests that males maintain sperm numbers when resources are limited, prioritising fertilization success over investment in offspring.