An udder is a single mass of specialized mammary glands containing two pairs of milk-producing quarters, typically found in mature female ruminants. While a fully functional, milk-producing udder is exclusive to the mature female, the foundational anatomical structures are present in young cattle of both sexes from birth. Therefore, a calf possesses the potential for an udder, but not the developed organ itself.
Anatomy of Young Cattle
The physical structures related to the udder begin forming in the fetal stage. At birth, both male and female calves are born with four small teats located in the inguinal area near the rear legs. These external projections are the only visible signs of the underlying mammary system.
Female Calves
In a female calf, known as a heifer calf, the underlying structure includes four separate mammary glands, often referred to as quarters, along with a supporting ligament system. Within these glands are the basic components of the future milk-producing organ, such as the fat pad and a rudimentary duct system. This undeveloped structure is correctly termed the mammary system, as it lacks the extensive secretory tissue necessary for milk production.
Male Calves
Male calves, whether they remain intact as bulls or are castrated as steers, also possess these four teats. However, the underlying glandular tissue in the male is minimal and remains undeveloped throughout life. This is due to the absence of the required female hormones, making the male mammary system non-functional and vestigial.
Maturation of the Mammary System
The rudimentary mammary system in a female calf remains largely dormant, growing at a rate similar to the rest of the body until about three months of age. A period of rapid development, or allometric growth, then begins, driven by hormonal shifts associated with puberty. During this time, the duct system starts to branch out and expand into the mammary fat pad, laying the groundwork for the future milk-secreting tissue.
The transformation into a functional udder is closely tied to the reproductive cycle, specifically pregnancy. The most significant development of the milk-secreting structures, called the alveoli, occurs during gestation. Hormones like estrogen and progesterone, which surge during pregnancy, stimulate the proliferation and differentiation of mammary epithelial cells.
Approximately 80% of the growth in milk-secretory tissue happens during the final two months of pregnancy. The udder is considered fully formed and functional only after the calf is born, which triggers the hormonal release of prolactin necessary for the initiation of milk synthesis. The male’s mammary tissue never receives this cascade of reproductive hormones, ensuring it never develops beyond its initial rudimentary form.
Defining Young Bovines
The general term “baby cow” is imprecise within agricultural terminology, which uses specific names based on age and sex. Any young bovine is referred to as a calf until it is approximately one year old. A young female bovine that has not yet given birth is called a heifer.
The term “cow” is strictly reserved for a female that has already given birth to at least one calf. Conversely, an intact adult male is a bull, while a castrated male is known as a steer.