Fig. 1.

Summary of sperm production parameters under variable cues of sperm competition risk and likely mating rate. Mean (± s.e.m.) sperm production parameters: a testis size, b epididymal sperm numbers and c daily sperm production rate. We simultaneously manipulated social cues of sperm competition risk (SC) and potential mating rate (MR), consistently finding that only the former impacts on sperm production parameters (all P < 0.05, see main text for test statistics). Included for comparison in each right-hand panel are previously published data from Ramm and Stockley (R&S 09, [17]), who reported a complementary experiment in which mating rate cues were held constant at a low level and only sperm competition cues were varied. Note that overall these experiments point to a primary influence of sperm competition cues on plasticity in sperm production parameters, with the mean parameter estimates for the three treatment groups classified as high sperm competition (blue shaded bars) differing significantly from those for the three treatment groups classified as low sperm competition (red shaded bars) for two out of three sperm production parameters (testis mass: t ₄ = 2.63, P = 0.058; epididymal sperm numbers: t ₄ = 5.94, P = 0.004; daily sperm production: t ₄ = 3.70, P = 0.02). HSC high sperm competition, LSC low sperm competition, HMR high mating rate, LMR low mating rate