Estrous and Menstrual Cycles
Estrous and Menstrual Cycles
Understanding Female Cycling
When do females cycle?
Females cycle when they:
Reach puberty.
Reach recovery in the post-partum period.
Are in the breeding season.
Reasons for Failure to Cycle
Why do animals fail to cycle?
Possible reasons include:
Too immature.
Too old.
Out of season.
Pregnant.
Postpartum lactation.
Malnourished, diseased, or infertile.
The Postpartum Period
Characteristics of early lactation in mammals:
Most mammals do not cycle or express estrus during early lactation.
Time needed for:
Milk production.
Rebuilding body resources.
Repairing the reproductive tract.
Examples:
Sow: 30 days
Cow: 60 days
Weaning effects:
Weaning removes negative input from nursing on hypothalamus (GnRH).
Aging and the End of Reproductive Cycles
Animal Fertility:
Increases from puberty, reaching a peak at certain parities and declines with age.
End of cycles in females:
Due to:
Depletion of follicles.
Hormonal changes.
Loss of ova.
Senescence in animals:
Most livestock are culled before depletion of ova.
Menopause in women:
Indicated by the absence of estrogen and progesterone, which can lead to bone health issues and uterine health issues.
Amenorrhea = absence of menses.
Testing for menopause:
High FSH levels in women indicate positive menopause test (due to low inhibin).
Classification of Estrous Cycles
Types of estrous cycles:
Diagnosed by observations or hormonal measurements.
Polyestrous: Pigs, cows, mice, and rats.
Seasonally polyestrous: Horses, sheep, and cats (may be long or short day breeders).
Monoestrous: Domestic dogs—non-seasonal with 1-3 cycles per year.
Hormonal and Behavioral Phases of Reproductive Cycle
The reproductive cycle is defined by hormones and behaviors.
Phases are clearer with longer cycles (days).
Key phases of estrous cycle:
Follicle phase
Estrus (sexual receptivity)
Luteal phase
Hormones involved: Estrogen during follicular phase and progesterone during luteal phase.
Major Phases of the Estrous Cycle
Each phase can be applied to most mammals.
Cycle details:
Days of cycle phases:
Proestrus
Estrus
Metestrus
Diestrus
Specific time frames:
Ovulation occurs, followed by phases dominated by estrogen and progesterone.
Proestrus and Estrus Distinctions
Proestrus:
Characterized by rapid follicle growth.
Estrus:
Defined as sexually receptive period, nearing ovulation.
Marked by presence of large follicles and rising estrogen levels.
Changes Associated with Estrus
Sexual receptivity:
Female stands to be mounted, exhibiting various symptoms.
Symptoms can be subtle, especially in sheep and cattle.
Physical changes:
Vulva and cervix become dilated, swollen, and changes in color.
Increased mucus production.
Metestrus Phase
Metestrus (Corpus Hemorrhagicum - CH present):
Occurs following ovulation; characterized by the formation of a transient structure.
Blood vessels break, leading to bleeding and clotting.
Diestrus Phase (Luteal Phase)
Diestrus:
Following ovulation, cells luteinize quickly.
Within days, progesterone levels increase, dominating this phase of the cycle.
Features of Diestrus:
Elevated progesterone (P4) is maintained for the longest duration in the reproductive cycle.
Notable that some species (rodents, dogs, cats) may bypass a traditional luteal phase.
Cycle Length Comparisons (Days)
Cycle length per species:
Cow, pig, horse, goat: 21 days.
Women/primates: 28 days.
Sheep: 17 days.
Cat: 10 days.
Rodent: 5 days.
Consistency of cycles within a species:
Variability exists among individual animals.
Cow Estrous Cycle
Characteristics:
Polyestrous with a 21-day cycle, estrus lasts approximately 18 hours.
Estrus behaviors:
Includes mounting, mucus discharge, vocalizations, and increased activity.
Timing:
Ovulation occurs about 30 hours after the onset of estrus.
Cycle stages:
3 days proestrus, 1 day estrus, 2 days metestrus, 15 days diestrus.
Cattle Estrus Detection and Aids
Human observation techniques:
AM/PM checking.
Estrus aids include:
Chalk, KAMAR, V or Gomer bull, and pedometers used for tracking.
Mare Reproductive Cycle
Characteristics:
Seasonally polyestrus as long-day breeders.
Estrus and ovulation primarily occur in the spring.
Cycle details:
21-day cycle with estrus lasting 5-7 days.
Detection:
Heat detection includes teasing with a stallion, with significant behavior changes noted (e.g., raising tail, urination, vocalization).
Timing:
Ovulation occurs within the last 2 days of estrus.
Cycle stages:
2 days proestrus, 5 days estrus, 2 days metestrus, 12 days diestrus.
Pig Estrous Cycle
Characteristics:
Polyestrous with a 21-day cycle.
Estrus lasts 2 days during which the female exhibits backpressure behavior for the boar.
Timing:
Ovulates late in estrus.
Cycle stages:
6 days proestrus, 2 days estrus, 2 days metestrus, 11 days diestrus.
Sheep and Goat Estrous Cycles
Characteristics:
Seasonally polyestrous, short-day breeders.
Cycle specifics for sheep:
Cycle length is about 17 days.
Estrus is subtle and lasts approximately 1 day.
Ovulation occurs after the onset and toward the end of estrus.
Detection aids:
Use of teaser ram, harness markers, and chin balls.
Rodent Reproductive Cycle
Characteristics:
Polyestrous with a cycle length of 5 days.
Estrus lasts less than 1 day, marked by interest in males and specific behaviors such as hopping and lordosis.
Timing:
Ovulation occurs approximately 12 hours after the start of estrus.
Unique feature:
Corpus luteum only forms after day 5 if mated and pregnant.
Hormonal dynamics:
Estrogen (E2) and progesterone (P4) increase from follicles during estrus.
Canine Reproductive Cycle
Characteristics:
Mono-/polyestrous with 1-3 cycles per year, timing varies by breed and season.
Cycle stages:
Proestrus (approximately 9 days), noted for follicle development and uterine bleeding (though not menses).
Estrus lasts about 9 days; females accept males from the first day.
Ovulation occurs on the 2nd or 3rd day of estrus.
Diestrus begins on the first day of refusal, with a CL forming that lasts approximately 60 days, regardless of breeding.
Feline Reproductive Cycle
Characteristics:
Seasonally polyestrous with no distinct season.
Cycle specifics:
Proestrus lasts 10 days, occurring repeatedly for follicle growth and estrogen peak.
Estrus lasts 7 days, marked by vocalization and physical behaviors such as rubbing and tail elevation.
If mated, estrus can be shorter (around 4 days).
Cats are induced ovulators; ovulation and CL formation occur only when needed.
CL lifespan is about 60 days in gestation, possibly shorter if not pregnant.
Menstrual Cycle in Primates
Characteristics:
In primates (including humans), females are sexually receptive throughout the cycle, with no defined estrus.
Cycle lengths by species:
Women: 28 days
Chimpanzee: 35 days
Gorillas: 32 days
Orangutans: 29 days.
Details of the Menstrual Cycle
Cycle specifics:
Day 1 is marked by the first day of bleeding (menstruation) lasting about 5 days.
Day 5:
Start of follicle phase with selection of follicles and increased estrogen, rebuilding the endometrium.
Day 14:
Ovulation occurs.
Days 15-28:
Cholesterol and corpus luteum form, leading to increased progesterone during the luteal phase.
At Day 28:
If not pregnant, the corpus luteum regresses, and progesterone levels decline.
Endometrium and Ovary in Menstrual Cycle
Cycle phases:
Follicle phase
Ovulation
Luteal phase dominated by progesterone.
Tissue undergoes thickening with gland development.
Proliferative stage leads to successful rebuilding of the tissue post-menstruation.