Gestation lasts for approximately 335–340 days and usually results in one foal. Twins are rare. Horses are a precocial species, and foals are capable of standing and running within a short time following birth.
Horses, particularly colts, sometimes are physically capable of reproduction at about 18 months, but domesticated horses are rarely allowed to breed before the age of three, especially females. Horses four years old are considered mature, although the skeleton normally continues to develop until the age of six; maturation also depends on the horse’s size, breed, sex, and quality of care. Also, if the horse is larger, its bones are larger; therefore, not only do the bones take longer to actually form bone tissue, but the epuphyseal plates are also larger and take longer to convert from cartilage to bone. These plates convert after the other parts of the bones, and are crucial to development.
Depending on maturity, breed, and work expected, horses are usually put under saddle and to be ridden between the ages of two and four.Although Thoroughbred race horses are put on the track at as young as two years old in some countries, horses specifically bred for sports such as dressage are generally not put under saddle until they are three or four years old, because their bones and muscles are not solidly developed.For endurance riding competition, horses are not deemed mature enough to compete until they are a full 60 calendar months (5 years) old.
Mare
The mare has two ovaries, usually 7–8 cm in length and 3–4 cm thick, that generally tend to decrease in size as the mare ages. The ovaries connect to the fallopian tubes (oviducts), which serve to move the ovum from the ovary to the uterus. To do so, the oviducts are lined with a layer of cilia, which produce a current that flows toward the uterus. Each oviduct attaches to one of the two horns of the uterus, which are approximately 20–25 cm in length. These horns attach to the body of the uterus (18–20 cm long). Caudal to the uterus is the cervix, about 5–7 cm long, which enters the vagina. Usually 3.5–4 cm in diameter, it can expand to allow the passage of the foal. The vagina of the mare is 15–20 cm long, and is quite elastic, allowing it to expand. The vulva is the external opening of the vagina, and consists of the clitoris and two labia. It lies ventral to the rectum. The mare has two mammary glands, which are smaller in virgin mares. They have two ducts each, which open externally.
Stallion
- the testes, which are suspended horizontally within the scrotum. The testes of an average stallion are ovoids 8 to 12 cm long;
- the penis, within the prepuce, also known as the “sheath.” When not erect, the penis is housed within the prepuce, 50 cm long and 2.5 to 6 cm in diameter with the distal end 15 to 20 cm. When erect, the penis doubles in length and thickness and the glans increases by 3 to 4 times. The urethra opens within the urethral fossa, a small pouch at the distal end of the glans.
The internal genitalia comprise the accessory sex glands:
- vesicular glands;
- prostate gland; and
- bulbourethral glands.
These contribute fluid to the semen at ejaculation, but are not strictly necessary for fertility.
(Adapted from Wikipedia)