Complex sentence: Mammalian predators often discover tree holes serving as nests and then enlarge the opening in order to obtain and eat the occupant, resulting in strong selective pressure affecting the choice of nest hole opening diameter and deepness.
Split into simpler sentences: Mammalian predators often discover tree holes serving as nests, and enlarge the opening so they can catch and eat the occupant. This puts strong selective pressure on the choice of nest hole opening diameter and deepness.