Sheep do not experience a menstrual period or bleed externally as a normal part of their reproductive cycle. This distinction is rooted in the type of reproductive cycle that governs the female sheep, or ewe, which is shared by most mammals. Understanding this process requires examining the biological mechanisms that prepare the uterus for potential pregnancy.
Understanding Reproductive Cycles
Mammals generally follow one of two main reproductive strategies: the menstrual cycle or the estrous cycle. The menstrual cycle, characteristic of primates including humans, prepares the inner lining of the uterus, the endometrium, for a fertilized egg. If pregnancy does not occur, the body sheds this thickened lining. This shedding involves the rupture of small blood vessels, resulting in external bleeding.
The estrous cycle, followed by sheep and most other non-primate mammals, handles the uterine lining differently. If fertilization does not happen, the endometrium is not shed but is instead reabsorbed by the body. This reabsorption prevents the external flow of blood and tissue. The estrous cycle also defines a specific, limited time of sexual receptivity.
The Sheep Estrous Cycle
The reproductive activity of the ewe is regulated by the estrous cycle, which has an average length of approximately 17 days, typically ranging from 13 to 19 days. Sheep are considered seasonally polyestrous, meaning they cycle multiple times within a specific breeding season. Most breeds are short-day breeders, starting their cycles when the daylight hours decrease, usually in the autumn.
The cycle is divided into phases, including Proestrus, Estrus, Metestrus, and Diestrus, all controlled by fluctuating hormone levels. Estrus, or “heat,” is the brief window when the ewe is sexually receptive to the ram, typically lasting between 24 and 36 hours. Rising estrogen levels cause the ewe to become receptive to mating.
If the ewe is not successfully bred, a hormone called prostaglandin causes the regression of the corpus luteum, a temporary structure that produces progesterone. This hormonal shift signals the body to begin a new cycle. The uterine lining is quietly reabsorbed, avoiding the need for external discharge. This internal recycling of the endometrium ensures that the reproductive system is quickly prepared for the next cycle.
Signs of Estrus in Sheep
Since ewes do not bleed, their reproductive readiness, or estrus, is indicated by subtle behavioral and physical changes. Ewes in heat often become more restless and may actively seek out the ram. They may also exhibit a characteristic rapid tail wagging, especially when the ram is nearby.
Physical signs can include a slightly swollen vulva, which may appear pinker or more congested than usual due to increased blood flow. Occasionally, a clear or thin mucous discharge may be present, but this is a secretion, not blood from a shedding uterine lining. These signs are often hard to notice without a ram’s presence, as ewes tend to be less demonstrative than other livestock.
Any visible external bleeding in a ewe is not a normal reproductive event. If blood is observed, it suggests a problem such as an injury, a complication from lambing, or a health issue, and should be investigated. The reabsorption mechanism of the estrous cycle means that a healthy ewe will not show the blood-shedding characteristic of a menstrual period.