Sperm size and number variation in the red flour beetle

Disruptive selection between large, nutritive gametes and numerous, competing gametes may have driven the evolution and maintenance of anisogamy. Sperm competition can explain why there are so many tiny sperm because numerical competition between rival gametes drives males to maximize sperm number and this may be achieved by minimizing sperm size. Since males operate within a finite reproductive budget and ejaculate production is limited, we might predict that, when variation in sperm size exists, males must trade increases in sperm size against a decrease in sperm number. We use Tribolium castaneum as our model to investigate the existence of a sperm size-number trade-off. We sampled 14 different populations that have been isolated for different periods (up to 39 years) and find across this sample of 70 males that there is significant variation in both sperm length and ejaculate sperm number between males. Despite this significant variance, we find no evidence for any relationship between sperm size and number across males. There is some evidence for a trade-off when we analyse across 14 population means, but this relationship is not robust and disappears when a single outlier is omitted. We conclude that sperm size and ejaculate sperm number vary independently, but that differential allocation to gonadal tissue and/or ejaculation frequency would permit this independent variation.