Why? The simple answer is because the female is usually a free-martin (~95% of the time).
What the heck is a free Martin?
A free-martin is an infertile female calf with masculinized behaviour and non-functioning ovaries. This means the female (heifer) calf that was born with a male (bull) twin will not be able to get pregnant and reproduce. The bull calf, however, will develop normal sexually.
Why does this happen?
Cattle fetuses have different type of placenta and vascular connections to their moms uterus (epitheliochorial) than in humans (hemochorial). In cattle, the twins fetal blood vessels become interconnected because they share a placenta while in the uterus. This means that the twins will have a shared blood circulation. If the fetuses are of the same sex this poses no problems. BUT, if they are different, the male (XY) cells will cross over to the female (XX) causing the female twin to result in XY/XX microcimerism. The male hormones that are circulating, will also cause masculinization of the female. These together, result in a infertile heifer or a free-martin.
What does the female look like?
From a distance, the heifer calf appears just like any other heifer calf. BUT, she will usually have some genital tract defects including:
-Ovaries with testicle like cells
-Very small (hypoplastic) uterus
-Absent connection between the vagina and uterus
-Small vulva with male like tuff of hair
Why are they called free-martins?
It is said to have originated in england as it referred to a heifer (female) that was not pregnant after the summer breeding season. She was therefore “free” for fattening and slaughter at “martinmas”— a fall festival in honour of St. martin.”. The more you know!