Induced ovulation (animals)

Induced ovulation (animals)
Classification and external resources
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Ovulation occurs at the ovary surface and is described as the process in which an oocyte (female germ cell) is released from the follicle. Ovulation is a non-deleterious 'inflammatory response' which is initiated by a luteinizing hormone (LH) surge. The mechanism of ovulation varies between species. In humans the ovulation process occurs around day 14 of the menstrual cycle, this can also be referred to as 'cyclical spontaneous ovulation'. However the monthly menstruation process is typically linked to humans and primates, all other animal species ovulate by various other mechanisms.

Spontaneous ovulation is the ovulatory process in which the maturing ovarian follicles secrete ovarian steroids to generate pulsatile GnRH (the neuropeptide which controls all vertebrate reproductive function) release into the median eminence (the area which connects the hypothalamus to the anterior pituitary gland) to ultimately cause a pre-ovulatory LH surge. Spontaneously ovulating species go through menstrual cycles and are fertile at certain times based on what part of the cycle they are in. Species in which the females are spontaneous ovulators include rats, mice, guinea pigs, sheep, monkeys, and humans.

Induced ovulation is the process in which the pre-ovulatory LH surge and therefore ovulation is induced by some component of coitus e.g. receipt of genital stimulation. Usually spontaneous steroid-induced LH surges are not observed in induced ovulator species throughout their reproductive cycles, which indicates that GnRH release is absent or reduced due to lack of positive feedback action from steroid hormones. However, by contradiction some spontaneously ovulating species can occasionally undergo mating-induced preovulatory LH surges. Species in which the females are induced ovulators include cats, rabbits, ferrets, and camels. In 1985, Chen et al., used Bactrian camels to investigate the factor(s) that induce ovulation during breeding season. They monitored the camel ovaries for ovulation by rectal palpation following insemination of semen samples. Chen et al., concluded that in this particular camel species ovulation was induced by the seminal plasma, and not by the spermatozoa.

Although the evolution of these two types of ovulation is not well understood, the type of ovulation that would be advantageous in some species can be understood by looking at social groups. Animals that have large, complex social groups benefit from spontaneous ovulation as only the best males get to breed with females. If there are few males suitable for breeding it makes sense to spread out the times at which females are fertile, therefore increasing the proportion in which conception occurs. This does not explain the evolution of ovulation in all species however, for example some species appear to show estrus synchronisation.

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