Females tend to be castrated for medical reasons, such as untreatable ovarian cysts or prolapsed uterus. Males are more commonly castrated so they can live with an intact female without reproducing. As Lincoln'sMommy said, spaying a sow is much riskier than the surgery for boys, as it is more along the lines of abdominal surgery. With boys it's usually just a small incision and removal of an 'outer' part of the body.