Figure 1

Recorded relationships between head length and flagellum length. Upper left panel interspecific studies: Black squares – mammals ; Red circles – frogs ; Blue up triangles – shorebirds ; Green diamonds – mammals ; Cyan down triangles – frogs. Inset: Violet squares – beetles. Lower left panel intraspecific studies: Pink circles - boar ; Orange up triangles – salmon. Solid lines indicate RMA regression lines, vertical and horizontal lines are non-significant relationships. Right hand panel: Resulting relationships between total sperm length and predicted speed. Colours correspond to the studies in the left had panels. Note the range of possible patterns, dependent on the scaling parameter c : Black – positive (c = 1.0); Red – positive (c = 1.0); Blue – negative (c = 1.0); Green – negative (b = 0.0); Cyan – negative (c = 0.46); Violet – positive (b = 8); Pink – positive (b = 8); and Orange – positive (c = -0.29). Citations for the studies used are given in the Additional file 1.