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Post-Coital Sperm Competence in Polygamous Animals: The Role of Sperm Traits in Species–Specific Strategies

Julian Santiago-Moreno, Milagros C Esteso, Cristina Castaño, Adolfo Toledano-Díaz and Antonio López-Sebastián

In some species, the females may mate with more than one male. In these polygamous animals, both pre-coital and post-coital strategies are required for reproductive success. This mini review aims at providing an update of the role of sperm traits in species–specific strategies for post-coital sperm competence. According to the spermcompetition theory, in such species the reproductive success of the males then depends upon both the magnitude of inter-male rivalry and covert sperm competition. The number of inseminated sperm, the percentage of sperm without morphological abnormalities in the ejaculates, and the sperm velocity variables play a significant role in sperm competition. Therefore, males that transfer more sperm per ejaculate or who copulate more often, are more likely to sire offspring. The transfer of large numbers of sperm is also guaranteed by ejaculates with high sperm concentrations. Usually, males of polygamous species have large testes and produce ejaculates with high sperm concentrations. Sperm competition acts as a selective force driving evolutionary change in sperm morphology and function. The degree of sperm pleiomorphy shown by a species varies according to the degree of sperm competition its behaviour entails. For example, the ejaculates of very polygamous species have much more homogenous sperm with lower sperm abnormalities. Finally, recent studies in birds have shown that the straight-line velocity (VSL) value, the most accurate estimate of sperm cell velocity, is of great importance in competition scenarios. Thus, sperm with a higher VSL affords a male fertilization advantages over his rivals.