Correction on the question: Do you know why *the* first (sentence) is (in the) *singular* and the *second* is (in the) plural? The first sentence doesn't have to be in the singular. It could also be written as "Male lions have manes, large hairs around their face." It's written in the singular because it is implied that the author is talking about a typical male lion's characteristics, and is describing those characteristics. The second is in the plural because it is a description that is applicable to *all* female lions.

First subjecy ( a male lion) is singular. Second subject (females) is plural