Why is plural of “mons pubis” not “montes pubum”

Question

Latin newbie here. Was talking with a friend about Martian landforms like Olympus Mons. Then we talked about other uses of mons, like mons pubis. But then I realized I didn’t understand something.

The plural of “mons pubis”, as attested in the Oxford English Dictionary, Merriam-Webster, Wiktionary, and whatever, is “montes pubis”. But the plural genitive of nounal pūbes is not pūbis; it is pūbum. So why is the plural phrase not “montes pubum”? (I’m assuming that it wouldn’t be “montes puberium” just like how the singular isn’t “mons puberis”.)

Answer 1

Pubes, genitive pubis means (as the dictionary tells us) "the signs of manhood, i.e. the hair which appears on the body at the age of puberty". It does not mean a single pubic hair, but - like the English word "hair" - it is a collective noun for the whole hairy covering of the body. That is why it stays in the singular even after a plural noun like montes.

Answer 2

The genitive, in this case, functions like the prepositional phrase "of ..." in English, so the object of the preposition is independent with respect to number. Although the genitive describes or qualifies similar to the way that adjectives do, there is no requirement for numerical agreement as there is with adjectives.

For example, in English, we might speak of a house of cards, with house being singular and cards being plural. The number of cards is not in any way determined by the number of houses.

In the same way, pubis in the singular, is not affected by mons being singular or plural.

Here's some more information on the genitive:

GENITIVE WITH NOUNS

342. A noun used to limit or define another, and not meaning the same person or thing, is put in the Genitive.

This relation is most frequently expressed in English by the preposition of, sometimes by the English genitive (or possessive) case:—

librī Cicerōnis, the books of Cicero, or Cicero's books.

inimīcī Caesaris, Cæsar's enemies, or the enemies of Cæsar.

talentum aurī, a talent of gold.

vir summae virtūtis, a man of the greatest courage.