Why don't animals(animal realm) commit suicide? We all know that suicide has been totally refused in Buddhism as unwholesome act.
suicide can be observed in human realm but why not in animal realm?
I'm not sure that they don't, not sure that it is never observed -- sometimes an old dog walks in front of a car for example, and is killed, and I wonder if they knew what they were doing.
But your question reminds me of a poem, by a non-Buddhist author, it's short and says:

Self-Pity
I never saw a wild thing
sorry for itself.
A small bird will drop frozen dead from a bough
without ever having felt sorry for itself.

It's a famous poem, striking, maybe worth considering from a Buddhist perspective.
I'm not even sure about that poem, though, not sure that it's true either: its author didn't always write kindly even about people (perhaps he was wrong about animals, too) -- it's hard to be sure of others' motives.
Animals sometimes feel sad, I'm pretty sure of that, and apparently mourn their dead; but perhaps self-pity depends on the ability to tell (or fabricate) a story about oneself: which, maybe, animals don't.