Question

Which mammal sleeps on its back?

Answer 1

I think all the great apes sleep in postures homologous to those of human beings. Their bodies are sufficiently similar to human bodies they safely recline in the same postures. The great apes that I am talking about are chimpanzees, bonobos, orangoutangs and gorillas. I am not less whether the gibbons recline like this, as their bodies.

Chimpanzees often weave hammocks in trees on which they sleep. They will sleep on their backs (bellies, sides, etc.) while in the hammocks.

Answer 2

The only animals that can naturally sleep on their backs are humans. No other animal actually does--