The Tavern

How does a species' experience apt actually affect the experience gained and leveling?

The same way as skill aptitude.

Meaning it uses the same modifier formula, 2^(N/4), where N is the aptitude.

Which is to say, a difference of 4 (between any two aptitudes) means the higher apt gains XP twice as fast as the lower.

Or, if you think of a species with 0 XP apt as receiving 100% experience: -2 apt receives 71%, -1 apt receives 85%, and +1 apt receives 119%.

So wait, does the extra 19% XP count towards skills too? So Hu is better and Mu is even worse than I thought they were?

I believe it is 'experience neede to gain levels'

(Really it should be called leveling aptitude)

no - basically character experience and skill experience are separate and the aptitudes for one doesn't affect the other. Pretend a race had +4 EXP apt and -4 to a skill, and killed a 200 exp monster - they get 400 character exp, and their selected skill gets 100 exp. You don't apply both to skill exp gain.