Fig. 3.

Sperm size–number relationship with increasing risk of sperm competition. In both (A) passerine birds and (B) Drosophila, the overall investment in sperm size (m*) and number (s*) increases with increasing risk of sperm competition (compare with theoretical prediction in Fig. 1A). As predicted (Box 1) (C; compare with Fig. 1C), in passerine birds sperm number increases at the expense of sperm size, whereas (D; compare with Fig. 1D) in Drosophila sperm size increases at the expense of sperm number. Figures are not controlled for phylogeny.