What causes twins in mares?

Twins in mares are primarily caused by the ovulation and fertilization of more than one egg. In equine reproduction, a mare can release multiple eggs during her estrous cycle, which can then be fertilized by sperm from a stallion. If both eggs are successfully fertilized, this results in the development of two embryos, leading to a twinning situation.

Understanding the reproductive physiology of mares is crucial in this context. A mare typically ovulates one egg per cycle, but in some instances, multiple eggs can be released, resulting in twin pregnancies. This phenomenon is relatively rare in horses compared to other species and can pose significant risks to both the mare and the embryos, as the mare's reproductive system is not naturally adapted to carrying twins.

The other options, while they may seem plausible at first glance, do not accurately describe the primary cause of twinning in mares. For instance, the division of an embryo refers to identical twins or cloning and is not a common occurrence in horses. Inadequate nutrition and genetic predisposition can contribute to various reproductive issues but do not directly cause twin pregnancies.