פירוש על קידושין 70:2

The Torah refers to the sons of Aaron when transmitting the prohibition of defiling oneself to take care of the dead. Therefore, this mitzvah applies only to males, not to females.

The Talmud now turns to the prohibitions of marring and rounding—both of which refer to parts of the head. This source teaches that since they are not subject to the prohibition of marring, they are not subject to the prohibition of rounding. Below, we will address how we know they are not subject to the prohibition of marring.