Semantics

back
Он нёс на спине тяжёлый рюкзак. He had a heavy backpack on his back.
spine
Кошка зашипела и выгнула спину. The cat hissed and arched its spine.
This entry is concerned with the use of the word спина denoting the back of humans and animals. No specific words or expressions exist for the back of animals. Common sense will dictate which of the expressions in this entry can or cannot be used to refer to animals.