Is the soul immortal? In that it surpasses human mortality and steadily grows, learning and increasing in mass and wisdom with each incarnation - it is immortal. But what is mortality for a soul? Souls do not die during nuclear explosions, nor do they lose themselves in Black Holes. Souls are of material that can affect its surroundings while remaining untouched.
Souls, once formed, do not die. They may grow rapidly or slowly, may stagnate or plateau for a long time or forever, but they do not die, nor do they shrink.
an existing soul may chose to incarnate in an animal that does not have the potential of sparking a soul, for education reasons. Thus, the dog or snake or dolphin does not spark a soul, but can be incarnated on occasion.
To spark, the biological entity must have the capacity to envision itself as separate, and where many animals seem to have this trait, they do not. A dog will understand that it is being yelled at by its master, or snarled at by the leader of the pack, and by groveling and crawling away is indicating they seem to understand another has interacted with them, as a separate being.