Does the mahram have to be a man?

A woman's mahram must be a man, based on the Prophet's (peace be upon him) saying: "It is not permissible for a woman who believes in Allah and the Last Day to travel a distance of one night unless she is accompanied by a mahram." Many scholars permit her to travel for the obligatory Hajj and Umrah with a safe company of righteous women or righteous men, even if there is no mahram among them.