Reference for Bekhorot 58:17

אם איתא דבכור הוה לא הוה טרח ביה סבר

What is the reason? - It is easily recognized.<span class="x" onmousemove="('comment',' The difference between the skin of a male and a female. A female animal is not subject to the law of the firstling.');">⁷</span>

Jastrow