Animals and humans die for the same reason, because their bodies wear out. Solomon is often said to be blessed with wisdom from Jehovah. In Eccl 3:19-21 he wrote "The fate of people and animals is the same, one dies just like the other. The all have the same breath, and people have no advantage over animals; its all just vanity. All go to the same place, all are made from the earth, and all return to the earth again. Who can say whether the spirit of a person ascends, or whether the spirit of an animal descends?
I would agree with Solomon. I don't see a difference in KIND of intelligence between animals and people, just a difference in degree. We write books because we have hands with opposable thumbs, but out mental capabilities are just more complex examples of what other creatures do. Koko the gorilla cannot speak, because her larnyx is not constructed to make the same sounds we do. Still, she carries on conversations using ASL (American Sign Language), constructs sentences, and expresses love toward her keepers and her own pets. She once called another gorilla who had not learned ASL a "stupid monkey". Her conversational abilities have been compared to a human child about five years of age, perhaps due to the smaller capacity of her brain compared with a human.
Conceivably, whales could learn to converse with humans, or humans could learn to converse with whales. Perhaps WE are too stupid to learn the whale language, which appears to consist of complex vocalizations which are learned and repeated to other whales.
Creatures which we think of as "lower" life forms are primarily lower in the complexity of their brains and nervous systems. Yet even in these, they have a sense of self, they recognize themselves as different from others of the same species, and from the creatures they may need to eat, or to escape from.
Anyone with an aquarium can watch how the fish behave when a new fish is added to the tank. Some fish are nervous about the newcomer, and shy away, as it may try to eat them. If this were merely instinct, then they would never get over it, yet when the new fish doesn't demonstrate aggressive behavior, they learn to tolerate its presence, even swimming right in front of or next to it. Others fish,(gouramis, for example), demonstrate curiosity, and will swim over to the newcomer and investigate it, touching it gently all over with their fins.
On a recent PBS broadcast, a herd of hippos found a dead cow on the bank of a river. They gently licked the body, then arranged themselves around it in a protective ring, and stood vigil for hours. Were they having a funeral for the cow (not even one of their own species), or protecting the body from crocodiles? If they were just dumb animals, why would hippos even care if crocodiles ate the body?
Elephants have been filmed caressing the bones of dead elephants which they had known in life. Does the living elephant remember who the bones belonged to?
My own view is that the spirit or life force comes from the same place, regardless of the creature which contains it. When the physical body expires, the spirit returns to the same place.