Fig. 2.

Sperm size and sperm number in relation to risk of sperm competition. Sperm size is significantly associated with relative testis mass (A) following a nonlinear relationship in passerine birds (presented as ln-transformed sperm total length and residual testes mass from a regression between CTM and BM) and (B) with an accelerating increase in Drosophila (presented as sperm total length and relative testes mass as CTM over BM). Sperm number is significantly associated with relative testis mass (C) following a linear relationship in passerine birds and (D) following a single-peaked curve in Drosophila. The relative position of species belonging to Fringillidae (white circles) and Sylviidae (gray circles) confirm previous findings of opposite patterns in the two families for the relationship between sperm size and risk level of sperm competition. Figures are not controlled for phylogeny.