The Allen and Greenough is still under construction;
so some links may not work quite the way you would expect.
Gender in the Fourth Declension.
90. Most nouns of the Fourth Declension in -us are Masculine.
Exceptions: The following are Feminine: acus, anus, colus, domus, ídús
(plural), manus, nurus, porticus,
quínquátrús (plural), socrus, tribus, with a few names of plants and
trees. Also, rarely, penus, specus.