A baby kangaroo at birth is tiny. Deaf, blind, and bald, it clambers with
great difficulty up mum's fur to her pouch, and there it lives, warm,
protected, and attached to a nipple, for up to eight months. During this
period, all its excrement has to go into the bottom of the (waterproof)
pouch.
I had imagined a filling cesspool in there, but in fact mum regularly licks baby and pouch clean, and leaves both wet with saliva. This helps keep the baby warmed by good contact with her skin, until it starts to grow fur at around five months.