between sperm from different males. The functions of sperm modifications are just beginning to be understood, however, and speculations are far ahead of the data. Discovering sperm function by direct observation is difficult, because both the morphology and the chemical mileu of the female reproductive tract can affect sperm behavior.
Perhaps the most general pattern is that sperm morphology tends to be simple and uniform in externally fertilizing animals and more complex and diverse in those with internal fertilization (Baccetti and Afzelius, 1976). This pattern argues for the importance of sexual selection and the influence of female “playing fields.” Correlations have been found between the length and the form of sperm cells and the length of female storage organs or ducts in birds and insects (Kotrba, 1995, 2006; Miller and Pitnick, 2002; Pitnick et al., 2002; Swallow and Wilkinson, 2002; Minder et al., 2005); the functional effects of longer or shorter female structures are not clear, however, nor are the selective pressures that result in changes in these traits.
Swimming speed is one frequently mentioned sperm competition mechanism that may exercise selection on sperm morphology. Greater flagellum length may correlate in some cases with greater swimming speed or greater force as the sperm nears the egg, but the functional significance of sperm length is often unclear. Sperm length is positively correlated with the probability of encountering sperm competition in some groups of animals, but not in others (Miller and Pitnick, 2002; Oppliger et al., 2003; Minder et al., 2005). Sperm length does not appear to be correlated with the thickness of the zona pellucida in mammals (Gomendio et al., 1997). The sperm of internally fertilizing species of fish and echinoids, which would seem to need less swimming ability, are nevertheless longer than those of external fertilizers in the same groups. In several vertebrate and invertebrate taxa, sperm have traits such as hook-shaped heads that allow individual sperm cells to link up with each other at least temporarily; in several of these species the resulting collaborative groups may swim straighter or more rapidly (Pizzari and Foster, 2008). Groupings of this sort seem to correlate with competition between sperm from different males, rather than between sperm cells in the same ejaculate (Pizzari and Foster, 2008).
Males of some groups routinely produce both fertile sperm, and sperm that are designed to be infertile (parasperm); some even lack nuclei. Parasperm are widespread in Lepidoptera, have evolved repeatedly in other groups (Swallow and Wilkinson, 2002), and constitute more than half of the ejaculate in some species. Hypotheses for parasperm functions (Swallow and Wilkinson, 2002; Holman and Snook, 2006) include provisioning the female with nutrients, displacing or killing rival sperm, blocking access for rival sperm, promoting movement of fertile sperm within the female,