According to Buddhism there are, I think, 32 realms of existence. Humans are one realm of existence. Animals, insects, birds and all, that's another realm. One main differentiation between humans and other creatures and animals can be seen physically. Humans are the only creature that regularly move vertically. All animals, even kangaroos that can stand vertical, when they bounce alone, they are horizontal. Monkeys can swing vertical, but when they go on the ground they go on all fours, they move horizontal. That's a physical thing that everyone can see, that all creatures except for humans regularly travel with their bodies horizontal, humans are vertical. It's also obvious that humans have a dexterity in their fingers that are unmatched. As there are differences, Buddhism says humans are a realm of their own, animals are a realm. There are 4 or 5 hell realms, 26 or so heaven realms. Not really heaven in a sense of an eternal realm, yet similar, very pleasant, lots of happiness. In the hell realms, lots of pain.
The human realm, they say, is the best realm for enlightenment, to rise above our Dukkha, to be totally free. It is taught that human realm is the best place because we have a balance between pleasure and pain. Beings in the hell realm experience too much pain, they can't possibly think of Mental Development. In the heaven realm there is too much happiness, almost no physical pain, so they get caught up in too much bliss and then they don't want to develop either, as they get swept away by pleasantness and the thought of permanence.
Thus it is taught that the human realm is the best for getting enlightened. By definition animals can't become enlightened. But they can still do good Kamma. There are many animals that you can easily appreciate, which make good Kamma like a seeing-eye dog. That's a classic example. Even regular dogs that do nice things for their owners, especially little old women who have nice dogs that bark when the noise comes and things like this.
Animals between themselves often make good Kamma. If you don't know in Australia there is a bird named the Kookaburra. They are the ones with a very funny, laughing sound. Kookaburras adopt baby Kookaburras that are lost. Very interesting. This happened to Rosemary's parents once when they were feeding Kookaburras and a few other birds. A neighbor found a baby Kookaburra on the side of the street. Obviously it had fallen off a tree or something, so they picked it up and brought it to Rosemary's parents. They said, "Here, take care of it," yet at that time Rosemary's parents didn't know what the heck was going on. The neighbor said, "No worries, you feed Kookaburras, you just watch, see what happens."
So they put the baby in a box and kept it outside where they fed the other birds. Next time the Kookaburras came, one of them hopped over to the box, looked inside, flew off. It came back 5 minutes later with a big worm. They started shoveling it down the baby Kookaburra's mouth. It adopted it for sure and after a couple of weeks went by, they flew off together. So some birds can make good Kamma. Some animals can make good Kamma, and get a better rebirth for next time.
In general animals can only get reborn as humans when they are in their best Kammic condition but again there is even a story in the scriptures of an animal getting reborn in a heaven realm, jumping over the human realm, and it was due to conditions again. There was a frog that was listening to the Buddha teach and really felt wonderful hearing the sound of the Buddha's voice. An old man walked up with a cane and accidentally speared the frog, he killed it without knowing it. The frog died listening to the Buddha's voice and got reborn in a heaven realm. This ties in to chanting. Some of you know about Buddhist chanting, some of you don't. Chanting or listening to chanting can create a very favorable condition in the mind. And with that favorable aspect in the mind beneficial results can come. But hopefully if you are ever listening to chanting nobody is going to come up and spear you.