Sperm competition: linking form to function

Abstract

Background: Using information from physics, biomechanics and evolutionary biology, we explore the implications of physical constraints on sperm performance, and review empirical evidence for links between sperm length and sperm competition (where two or more males compete to fertilize a female's eggs). A common theme in the literature on sperm competition is that selection for increased sperm performance in polyandrous species will favour the evolution of longer, and therefore faster swimming, sperm. This argument is based on the common assumption that sperm swimming velocity is directly related to sperm length, due to the increased thrust produced by longer flagella.

Results: We critically evaluate the evidence for links between sperm morphology and swimming speed, and draw on cross-disciplinary studies to show that the assumption that velocity is directly related to sperm length will rarely be satisfied in the microscopic world in which sperm operate.

Conclusion: We show that increased sperm length is unlikely to be driven by selection for increased swimming speed, and that the relative lengths of a sperm's constituent parts, rather than their absolute lengths, are likely to be the target of selection. All else being equal, we suggest that a simple measure of the ratio of head to tail length should be used to assess the possible link between morphology and speed. However, this is most likely to be the case for external fertilizers in which females have relatively limited opportunity to influence a sperm's motility.

Figure 2

Percentage difference in drag between a sphere and a prolate spheroid of identical volume at Re << 1. As the ratio of length to diameter of the spheroid increases (i.e. the shape elongates) there is an initial decrease in drag, but this difference only results in drag for the spheroid dropping to a minimum of 95.66% of that of the sphere.

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.

Figure 3

Computed sperm velocity as a function of channel width. Swimming speed is predicted to increase dramatically as the channel walls become closer. Non-dimensionalised terms are velocity/wave speed of the flagellum and channel width/amplitude of flagellar beat. Redrawn from [81].