* We don't have data on all species: Scientists haven't observed every animal species to document their sleeping positions.
* Sleeping positions vary: Many animals may sleep on their backs sometimes, but not consistently.
* Different definitions of "sleeping on their back": Do we mean fully on their back, or just slightly tilted?
However, we know that several animals sleep on their backs, including:
* Domestic dogs: Some dogs sleep on their backs with all four legs in the air.
* Cats: Cats can sleep on their backs, often with their legs stretched out and their paws curled.
* Horses: Horses can sleep on their backs while lying down, but it's usually a very brief sleep.
* Bats: Some bats sleep hanging upside down with their backs facing the ground.
* Sea otters: Sea otters often sleep on their backs while floating in the water.
* Certain primates: Some monkeys and apes have been observed sleeping on their backs.
Important note: This is not an exhaustive list. There may be many other animal species that sleep on their backs, but more research is needed.