In the case of sexual reproduction, the optimal allocation of resources to reproduction actually begins prior to fertilization itself. The theory of mate selection by females is usually described in the context of sexual selection so that the evolution of female mating preferences centers on the genetic consequences of gamete union. Hypotheses to explain the patterns range from females seeking to improve the genetic quality of their offspring by choosing genetically superior male mates to the existence of genetic correlations between female preference and male traits. Indeed, the direct benefits of female choice can be expressed in terms of increased fertility during a breeding season, or the evolution toward selecting particular mate characteristics to increase fecundity. Mate choice is therefore seen as an important component in the evolution of particular life-history strategies.
Models constructed to investigate mate choice can be complicated by the existence of multiple mating. In many species (e.g., the majority of insects), females permit or actively seek multiple mating with different males, even though several female fitness costs of excess mating are known (e.g., additional time and energy devoted to mating, increased predation risk, higher chance of injury, higher probability of disease or parasite transmission, male-originating chemicals reducing female longevity or female fecundity). Nonetheless, the ubiquity and magnitude of this trait suggest large positive effects on female fitness. For example, a fresh supply of sperm can maintain egg fertility, accessory substances can increase egg production rate, and the mating act itself can stimulate egg production.
However, the most studied component of multiple mating concerns sperm competition. This widespread phenomenon occurs when sperm from two or more males compete for a female's ova. Relative sperm numbers are important for sperm competitive success, so that species experiencing higher rates of sperm competition have males with larger testes that produce more sperm. Sperm competition is therefore a selective force shaping optimal ejaculate structure, although spermato-genesis is not unlimited. While males usually possess a greater reproductive potential than females, males have evolved mechanisms for the optimal allocation of finite sperm among females to maximize lifetime reproductive success.
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