Induced ovulation (animals)

Induced ovulation is when a female animal

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.^[1] 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,^[2] 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

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.[6] This does not explain the evolution of ovulation in all species however, for example some species appear to show estrus synchronisation.

Mechanism

In spontaneous ovulators, estrogen and progesterone secreted by the follicles as they grow and mature affects the release of GnRH, and therefore causes an LH surge. The LH surge then causes the release of the egg.

Ovulation is triggered in induced ovulators by an LH surge from the anterior pituitary that is induced during mating. Animals this has been recorded in include rabbits, voles, ferrets and camels.^[3] In some species such as the ferret, the duration of intromission has no effect on the LH surge, whereas in other species such as the cat these are related and higher levels of LH were produced by mating multiple times. In many species, for a LH surge to occur, little intromission is required.

The pathways in which information reaches the brain and causes GnRH release are not understood well; however, midbrain and brainstem noradrenergic neurones appear to be activated in response to intromission during mating. These neurones then go on to stimulate the mediobasal hypothalamic to release GnRH from the median eminence.^[3] Most experiments on GnRH and LH release have been focused on spontaneous ovulators, though there have been studies completed on some induced ovulators (e.g., rabbits, ferrets). From this, it appears that norepinephrine facilitates GnRH release in the rabbit and ferret and the locus coereuleus which is the part of the brain involved in conveying genital-somatosensory information to the GnRH neurones.^[7] Other substances that have similar effects include neuropeptide Y.

Species

Many species have been found to be induced ovulators and the reasons for this are not always clear. However, one possible reason is that induced ovulation could provide a better reproductive potential for those species that typically have shorter life spans and less encounters resulting in lower mating opportunities throughout their lifetime.^[8] Other species may be 'facultatively-induced ovulators' meaning that while they can spontaneously ovulate, the cycle may speed up or slow down depending on the presence of males, females or mating.^[9]

Some rodents such as squirrels

The koala species are a lesser-known induced ovulator. The koalas require mating in which the presence of ejaculated semen is needed to stimulate the female to produce a LH surge (which would cause ovulation of a follicle). Unlike many other animals, simply being in the presence of a male koala is not enough to induce ovulation itself, nor is vaginal stimulation on its own sufficient to cause induced ovulation to occur.^[10]

Cats are another widely-known induced ovulator. After mating, the LH levels in female cats surge, and the time to ovulation can be predicted to occur between 1–2 days later.^[11]

Wolverines are other known induced ovulators which require physical mating to cause ovulation.^[12]

Induced ovulation occurs in various

Induced ovulation is able to occur in some fish species. In China freshwater fish including a variety of carp types, bream and loach are able to be induced to ovulate by using agonists of dopamine. This induction of ovulation from drugs is able to cause a predictable ovulation period and is very beneficial to farming of these species.^[18]

In cattle

The natural cycle of spontaneous ovulation occurs in species such as cows.^[19] There is a great demand for ovulation to be induced in cattle, as it allows farmers to synchronize their cattle to ovulate at the same time, helping improve the efficiency of dairy farming.^[20] Induced ovulation can be utilized during the warmer seasons to increase plasma progesterone and improve the fertility of the cattle.^[21] However, ovulation can only be induced in cows with mature follicles and merely initiates lutenization, it does not reduce the time for ovulation.

There are a number of methods that are used to induce ovulation in cattle such as: artificial insemination, introducing a number of hormones such as chorionic gonadotrophin, hCG and LH. As well as injecting progesterone by intravaginal devices called PRIDs (progesterone-releasing intravaginal devices)^[22]

In cats

Domestic

It has been known since 1905[24] that domestic rabbits are physically induced ovulators, although they may also ovulate spontaneously. Early reports stated that simply having an oestrous doe in close proximity to a buck can induce ovulation, although there were no data presented in these early reports.^[25]

In camelids

Dromedary camels (Camelus dromedarius), bactrian camels (Camelus bactrianus), llamas (Lama glama) and alpacas (Lama pacos) are all induced ovulators.^[26][27]

Bactrian camel

Bactrian camels ovulate after insemination into the vagina; it is the seminal plasma, but not the spermatozoa, which induces ovulation. Ovulation occurs in 87% of females after insemination: 66% ovulate within 36 hours and the rest by 48 hours (the same as natural mating). The least amount of semen required to elicit ovulation is about 1.0 ml.^[26]

Alpaca

In alpaca, follicles ovulate approximately 26 hours after coital stimulation. Mounting accompanied by intromission is necessary to provide adequate stimulation for LH release and subsequent ovulation.^[28] Deposition of semen, which contains ovulation-inducing factor (OIF),^[4] has been shown to increase the chance of pregnancy. Prolonged copulation, causing abrasion and inflammation of the uterus, may enhance absorption of OIF.

References