Games : How do monsters level in your farm?

(This question adresses DWM2.)

Hi folks. While carrying around up to three monsters and gaining their experience through fights, your monsters on the monster also level.

Is it known how they level? As I never payed attention to exactly when the farm monster get experience, I can think of shared experience (when your carried monsters get experience, your farm monster also get some of that) or some mechanic like for every few steps, there is some experience.

How do the monsters on the monster farm get their experience?