If Heaven is only for humans then what happens to the animals spirits when when they die?
Animals do not have a spirit in the sense that humans have spirits. Humans were the only entities in all creation into which the divine breath of God was breathed. This does not mean that animals do not have a spirit of sorts. As for what happens when an animal dies it is not known. The Bible, however, talks about animal like creatures in heaven as well as horses so I cannot say that animals don’t go to heaven when they die.