Re: neck to femur angle is exactly 125*, why?

According to "Clinically Oriented Anatomy," by Moore and Dalley, the angle between the head of the femor and the shaft is between 115 and 140 degrees, averaging at 126 degrees. You are correct in that the angle is widest at birth and gradually diminishes until the adult angle is reached.

The most obvious advantage of this alignment is that is allows the femor greater mobility at the hip joint. If there was no angle, the femor would be inclined to rub againt the Ischium when moved back and forth.

The main disadvantage is that this angle generates tremendous strain on the neck of the femor. Fractures of the femoral neck can occur easily in older people because of a slight stumble.