Females frequently mate with several males, whose sperm then compete to fertilize available ova. Sperm competition represents a potent selective force that is expected to shape male expenditure on the ejaculate. Here, we review empirical data that illustrate the evolutionary consequences of sperm competition. Sperm competition favors the evolution of increased testes size and sperm production. In some species, males appear capable of adjusting the number of sperm ejaculated, depending on the perceived levels of sperm competition. Selection is also expected to act on sperm form and function, although the evidence for this remains equivocal. Comparative studies suggest that sperm length and swimming speed may increase in response to selection from sperm competition. However, the mechanisms driving this pattern remain unclear. Evidence that sperm length influences sperm swimming speed is mixed and fertilization trials performed across a broad range of species demonstrate inconsistent relationships between sperm form and function. This ambiguity may in part reflect the important role that seminal fluid proteins (sfps) play in affecting sperm function. There is good evidence that sfps are subject to selection from sperm competition, and recent work is pointing to an ability of males to adjust their seminal fluid chemistry in response to sperm competition from rival males. We argue that future research must consider sperm and seminal fluid components of the ejaculate as a functional unity. Research at the genomic level will identify the genes that ultimately control male fertility.

Sperm competition, the contest among ejaculates from rival males to fertilize ova of a female, is a common and powerful evolutionary force influencing ejaculate traits. During competitive interactions between ejaculates, longer and faster spermatozoa are expected to have an edge; however, to date, there has been mixed support for this key prediction from sperm competition theory. Here, we use the spectacular radiation of cichlid fishes from Lake Tanganyika to examine sperm characteristics in 29 closely related species. We provide phylogenetically robust evidence that species experiencing greater levels of sperm competition have faster-swimming sperm. We also show that sperm competition selects for increases in the number, size, and longevity of spermatozoa in the ejaculate of a male, and, contrary to expectations from theory, we find no evidence of trade-offs among sperm traits in an interspecific analysis. Also, sperm swimming speed is positively correlated with sperm length among, but not within, species. These different responses to sperm competition at intra-and interspecific levels provide a simple, powerful explanation for equivocal results from previous studies. Using phylogenetic analyses, we also reconstructed the probable evolutionary route of trait evolution in this taxon, and show that, in response to increases in the magnitude of sperm competition, the evolution of sperm traits in this clade began with the evolution of faster (thus, more competitive) sperm.sperm competition ͉ sperm size ͉ sperm swimming speed ͉ sexual selection ͉ correlated evolution P ostcopulatory sexual selection in the form of sperm competition occurs whenever access to ova of a female is contested by ejaculates from Ͼ1 male (1). Because fertilization success is thought to be influenced by characteristics of competing ejaculates, sperm competition theory predicts that the strength of sperm competition will be reflected in sperm traits. Specifically, theory predicts a positive relation between the size of spermatozoa and the strength of sperm competition (2), based on the assumptions that (i) longer spermatozoa swim faster, and (ii) faster-swimming sperm achieve higher fertilization success during competitive matings between multiple males (2, 3). Most comparative studies have shown that species experiencing greater levels of sperm competition have longer sperm (4, 5), although one widely-cited study demonstrated a negative relation between sperm competition and sperm length in bony fishes (6). Surprisingly, there is scant empirical evidence demonstrating that sperm competition promotes the evolution of fasterswimming sperm across species, and little evidence of a link between sperm morphology and swimming speed (4,7,8). For example, evidence of a positive relation between sperm length and swimming speed has been reported only from mammals, using data obtained by various methods (5, 9). Also, numerous intraspecific studies on diverse taxa provide almost no evidence for sperm size influencing sperm swimming speed (7). The only cle...

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Theory predicts a trade-off between investments in precopulatory (ornaments and armaments) and postcopulatory (testes and ejaculates) sexual traits due to the costs associated with their growth and maintenance within the finite energy resources available. Empirical studies, however, have revealed considerable inconsistency in the strength and direction of relationships among these sexual traits. Ambiguity may result from variance in the marginal benefits gained by increasing investments in either pre-or postcopulatory sexual traits. Here, in a broad comparative study, we test the prediction that the relationship between pre-and postcopulatory sexual traits differs among taxa relative to the importance of male-male contest competition within them. We find that covariance between pre-and postcopulatory sexual traits gradually shifts from strongly positive to strongly negative with increasing male-male contest competition. Thus, our findings reveal a potentially unifying explanation for the oftentimes inconsistent relationships in the strength and direction of covariance among sexual traits.