I believe there is nothing much to explain about the suffering which going through by animals as we can observe them daily.  Animal realm is visible on the earth, water, trees & sky.  Various kind of animals are living among us and it's identify as a separate world which is categorized under the lower realms by subjecting to the suffering they are going through & the level of wisdom.

"If one is reborn in the animal realm as a result of doing evil deeds, one will be endlessly gripped by constant fear. 

There will not be the slightest peace of mind at any time.  How then can one direct one's attention to merit?"

{From - Lowada Sangarawa (towards a better world); a Sinhalese poem of the 15th century by Ven. Vĩdãgama Maitreya Tero & translated by Bhikku K. Nãnananda}