Oestrus vs Menstrual Cycles
There are two types of reproductive cyclicity in animals: oestrus cycles, which occur in non-primates, and menstrual cycles, which occur in primates. During an oestrus cycle, the body reabsorbs the endometrium if conception fails and a bloody discharge comes from blood vessels. There is also a significant behaviour change in these animals during oestrus and animals are only receptive to mating during 'heat'. An oestrus cycle lasts approximately 20 days and may continue until death.
Menstrual cycles includes bleeding which results from the endometrium being shed if conception fails. These animals are theoretically receptive to mating throughout the cycle which lasts approximately 28 days. Menstruation ceases at menopause.
Phases of the Oestrus Cycle
This cycle can be divided into two major stages which both include two stages:
- Follicular Phase: this includes:
- Proestrus: this involves the formation of ovulatory follicles and oestradiol secretion. It begins after the corpus lutea are destroyed and follicle stimulating hormone (FSH) and lutenising hormone (LH) are prominent. This lasts for about 2-5 days.
- Oestrus: the animal becomes sexually receptive and oestrus behaviour is demonstrated. Peak oestradiol secretion occurs. Ovulation occurs once oestrogen levels reach a threshold level.
- Luteal Phase, which includes:
- Metoestrus: after ovulation, one or more corpus lutea (CL) develop in the ovaries and progesterone secretion begins. This also lasts for about 2-5 days
- Dioestrus: A mature CL secretes large amounts of progesterone which prompts the uterus to prepare for embryo development. This stage terminates when the CL is destroyed. It is the longest stage and lasts for 10-14 days in polyoestrus animals.
Variations in Cyclicity
Anoestrus refers to when a female does not exhibit regular oestrus cycles, the ovaries and inactive and both ovulatory follicles and CL are absent. Anoestrus is caused by insufficient gonadotropin releasing hormone (GnRH) secretion and there are three forms:
- True anoestrus: this is caused by insufficient nutrition, lactation or stress.
- Apparent anoestrus: this is a result of the failure to detect oestrus or pregnancy.
- Seasonal anoestrus: this is normal in animals who are seasonal breeders.
Oestrus cycles are categorised according to how frequently they occur throughout the year:
- Monoestrus: these are animals with only one oestrus cycle per year. Dogs are an example of an animal with this type of cycle, although they may display 1-2 cycles per year.
- Continually Polyoestrus: this is when oestrus cycles occur regularly throughout the entire year and pregnancy can occur regardless of season. Cattle, cats and pigs demonstrate this type of cycle.
- Seasonally Polyoestrus (short-day breeders): these animals display clusters of oestrus cycles in a particular season and they begin to cycle as the day length decreases (autumn/winter). Sheep and goats demonstrate this.
- Seasonally Polyoestrus (long-day breeders): These animals display clusters of oestrus cycles in a season and they begin to cycle as day length increases (spring). Horses display this type of cycle.
A domestic female displays 1-2 oestrus cycles per year. This consists of a proestrus and oestrus lasting 9 days each, dioestrus lasting 2 months and anoestrus lasting 5 months. Ovulation occurs during oestrus three days after the LH surge. The female is receptive to the male while estradiol is decreasing and progesterone in the body is increasing to coincide with ovulation.
The feline oestrus cycle consists of proestrus, oestrus, postoestrus, dioestrus and anoestrus. They are induced ovulators which means they require copulation in order to ovulate. Postoestrus occurs between oestrus periods in cats that have not ovulated. The female enters oestrus, which lasts for 9 days, every 17 days.
Melatonin secretion from the pineal gland occurs during hours of darkness and translates the changes in photoperiod into neural impulses. Melatonin is progonadotropic in the ewe as it causes GnRH to be secreted which results in the secretion of LH and FSH. In sheep, the first ovulation following anoestrus is 'silent' and no behavioural changes occur. In order for behavioural oestrus to happen progesterone needs to be present for a period of time before oestrogen is secreted. Progesterone acts as a primer to increase the brain's sensitivity to oestrogen.
In horses, melatonin is antigonadotropic as it inhibits the secretion of GnRH and hence the secretion of LH and FSH. In these animals, anoestrus is caused by melatonin which is secreted in response to increasing darkness (winter). During spring time, melatonin secretion is decreased and this allows GnRH to be released which leads to LH and FSH secretion. The transition between anoestrus and oestrus periods is called the vernal transition and results in overt sexual or oestrus behaviour.
Teasers and Artificial Lighting
Pheromones provide information on the sexual or social status of the emitter and are released in bodily fluids or produced and kept on the emitter's body. Female reproductive cycles in animals can be stimulated and synchronised by introducing a male teaser. A teaser is a surgically prepared male used to sexually tease but not impregnate females. They can be vasectomised males or castrates that have been injected with testosterone.
Day length can also be adjusted in order to manage fertility, particularly in seasonally polyoestrus animals. With horses, artificial lighting can be used to shift the vernal period earlier in the year. This increases the number of breedings possible in a season.
That's it for this post, see you next time :)