Peculiar mechanisms Induced ovulation

Induced ovulation occurs when release of eggs in females is triggered by a stimulus, most often physical such as copulation, but also behavioral or pheromonal such as the vicinity of males. In contrast to spontaneous ovulators, or species where the release of eggs depends on the seasonal photope-

African lioness (Panthera leo) with her cubs. (Photo by Joe McDonald. Bruce Coleman, Inc. Reproduced by permission.)

riod signal, species with induced ovulation develop ova that are ready for release but require a stimulus for release.

Induced ovulation occurs in many species, but is best understood in mammalian carnivores. Examples of species with induced ovulation include cats (Felidae), bears (Ursidae), and numerous Mustelidae such as wolverine (Gulo gulo), striped skunk (Mephitis mephitis), and North American river otter (Lontra canadensis). For species with induced ovulation, it appears that a certain level of stimulation is required for eggs to be released, and thus it has been hypothesized that females may use the ability of a male to induce her ovulation as an indicator of male vigor, hence male quality. In these species, females may not be able to compare males simultaneously and because of the severity of the environment, may not be sure of her ability to find mates. In this case, the best strategy for the females would be to mate with all males encountered, and bear offspring from the male that induces the greatest stimulus. Evidence in black bears (Ursus americanus) of multiple paternity within single litters suggests that induced ovulation may be used by females as a mate choice strategy within the reproductive tract. For males, inducing ovulation may be a method of ascertaining paternity when pair bonds must be short to allow for encounters with other females. It is also possible that induced ovulation evolved as a strategy against sexual coercion in carnivores. In this case, females forced into copulation by males of lesser quality could abort eggs if they subsequently bred with a better quality male that provided a greater stimulus. Although the complexity of induced ovulation is still being investigated, it appears that benefits may exist for both males and females of species living in highly seasonal environments.

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