Aren’t they cute? They’re Brown Swiss calves and they are 4 month old twins. The cream-colored one is a bull calf (male) and the fawn colored one is a heifer calf (female). The heifer calf is known as a “freemartin”.
A freemartin is an infertile female mammal.
In cattle a freemartin is the normal outcome of mixed sexed twins. And 90% -95% of heifers born from mixed sexed twins will be sterile. Sometimes a single heifer calf will be made sterile from the death of a male twin during the early part of the gestation period.
The female is made infertile in utero due to an interconnection and fusion of chorions and shared blood vessels which permits the blood from each twin to flow around the other twin.
The action of male hormones upon the female fetus usually renders the heifer calf with non-functioning ovaries and often a very short vagina. The heifer often displays a masculine appearance and behavior.
The action of the female hormones on the male fetus usually has no effect on the bull calf except sometimes the testicles may be smaller than normal. This is important because testicle size is associated with cattle fertility.
The freemartin effect has been recorded in goats, sheep and pigs but it is very rare. The 18th century Scottish physician John Hunter was the first to observe that a freemartin heifer always has a male twin.
Freemartinism is usually diagnosed by vaginal examination with a probe at 3 – 6 months of age. Vaginal length in a normal heifer calf is usually greater than 5 1/2 inches. In a freemartin heifer the vaginal length is usually between 2 1/2 to 4 inches long.
Sometimes genetic testing will be used with a valuable heifer to determine breeding ability. But most often a visual examination of the placental membranes shortly after birth will confirm the probability of sterility of the female calf.