Sperm Competition - Defensive Adaptations

Defensive Adaptations

Mate-guarding is a defensive behavioral trait that occurs in response to sperm competition; males try to prevent other males from approaching the female (and/or vice versa) thus preventing their mate from engaging in further copulations. Precopulatory and postcopulatory mate-guarding occurs in birds, lizards, insects and primates. In these instances the males guard their female by keeping them in close enough proximity so that if an opponent male shows up in his territory he will be able to fight off the rival male which will prevent the female from engaging in extra-pair copulation with the rival male.

Strategic mate-guarding occurs when the male only guards the female during her fertile periods. This strategy can be more effective because it may allow the male to engage in both extra-pair paternity and within-pair paternity.

Copulatory plugs are frequently observed in insects, reptiles, some mammals, and spiders. Copulatory plugs are inserted immediately after a male copulates with a female, which reduce the possibility of fertilization by subsequent copulations from another male, by physically blocking the transfer of sperm. The Indian Meal Moth (P. interpunctella) are restricted in engaging in further mating activities because the spermatacore serves as a copulatory plug immediately after copulation. Bumblebee mating plugs, in addition to providing a physical barrier to further copulations, contain linoleic acid, which reduces re-mating tendencies of females.

Similarly, Drosophila melanogaster males release toxic seminal fluids, known as ACPs (accessory gland proteins), from their accessory glands to impede the female from participating in future copulations. These substances act as an anti-aphrodisiac causing a dejection of subsequent copulations, and also stimulate ovulation and oogenesis. Seminal proteins can have a strong influence on reproduction, sufficient to manipulate female behavior and physiology.

Another strategy, known as sperm partitioning, occurs when males conserve their limited supply of sperm by reducing the quantity of sperm ejected. In drosophila, ejaculation amount during sequential copulations is reduced; this results in half filled female sperm reserves following a single copulatory event, but allows the male to mate with a larger number of females without exhausting his supply of sperm. To facilitate sperm partitioning, some males have developed complex ways to store and deliver their sperm. In the blue headed wrasse, Thalassoma bifasciatum, the sperm duct is sectioned into several small chambers that are surrounded by a muscle that allows the male to regulate how much sperm is released in one copulatory event.

A strategy common among insects is for males to participate in prolonged copulations. By engaging in prolonged copulations a male has an increased opportunity to place more sperm within the female's reproductive tract and prevent the female from copulating with other males.