What happens with indígena as an adjective for masculine nouns?

Question

I think I understand how indígena works when used as a noun.

No soy un indígena. Ella es una indígena.
I am not an indigenous person. She is an indigenous person.

But how does indígena work as an adjective? I'm confused with what to do when the form given in the dictionary ends in "a". Several dictionaries list indígena, but not indígeno. Do you say

Su padre es indígena.

or

Su padre es indígeno.

Also, if indígena uses "a" when used as an adjective for a masculine noun, are there other adjectives that behave this way?

Answer 1

Indígena is a “common gender” noun that can be used as an adjective. This is what the RAE says about common gender nouns:

Sustantivos comunes en cuanto al género. Son los que, designando seres animados, tienen una sola forma, la misma para los dos géneros gramaticales. En cada enunciado concreto, el género del sustantivo, que se corresponde con el sexo del referente, lo señalan los determinantes y adjetivos con variación genérica: el/la pianista; ese/esa psiquiatra; un buen/una buena profesional. Los sustantivos comunes se comportan, en este sentido, de forma análoga a los adjetivos de una sola terminación, como feliz, dócil, confortable, etc., que se aplican, sin cambiar de forma, a sustantivos tanto masculinos como femeninos: un padre/una madre feliz, un perro/una perra dócil, un sillón/una silla confortable.

The ending doesn't change, because the final -a is in fact not the mark of the feminine gender. It comes directly from Latin, where it was already common gender of sorts (it only distinguished between masculine/feminine vs. neuter in the accusative case). The noun doesn't have a fixed gender, and the corresponding adjective doesn't either; we only "see" their gender when they appear together with gendered words, like articles or other nouns or adjectives with which they must agree.