Does testis size track expected mating success in yellow dung flies?

1. We quantitatively compare the fitness function linking male mating success in the field to body size and the allometric relationship linking testis length (estimating sperm production) to body size, both empirically derived, for the yellow dung fly Scathophaga stercoraria (Diptera: Scathophagidae). 2. We find that both relationships are similarly non-linear (quadratic) and increasing. The most parsimonious interpretation of this congruence is that the evolution of testis size (and hence sperm numbers) evolutionarily 'tracks' the number of copulations a male of a given size can expect, resulting in the particular non-linear testis size-body size allometry evident. 3. We offer a general, two-tiered statistical approach for this kind of quantitative comparison.