Maybe it's not so surprising that an Earth-evolved land mammal wouldn't thrive in microgravity. From the article: "33 percent of [Shuttle/ISS] astronauts had expansion of the cerebral spinal fluid space surrounding the optic nerve, 22 percent had flattening of the rear of the eyeball, 15 percent had bulging of the optic nerve, and 11 percent had changes in the pituitary gland and its connection to the brain."