Paternal Instinct?

I understand why mothers are constantly tuned into the needs of their babies, why they are born multi-taskers, why they have eyes in the backs of their heads, and why they can survive on very little sleep for extended periods of time. It is all built into women, with few exceptions.

But can anyone tell me what it is, exactly, that keeps a sleeping father from rolling over and smothering his 4-month old baby night after night? Can anyone tell me why the whole night long he seems not to move from the very edge of the bed?