I'm not sure whether this belongs in Health, because it impacts on the health of possible space colonists, or Tech Talk because it's about the space program, or Skepticism/Science because it's about that, too, so I'll just put it here in General, and lets the mods move it if they like.
Episode 446 of Astronomycast is about small animals that have been sent to space to study the effects of microgravity on animals. (The previous episode was creepy crawlies in space, and the next one will be about larger animals (dogs, cats, chimps) in space.
So lots of animals have serious problems in microgravity, and reproduction is a biggie. Mice show deterioration of the skin (as well as of the bones), fish don't know which way to swim, frogs jump, flinging themselves all over the place, birds often fail to hatch, and they get all bollixed up. Jellyfish (not really fish) get completely disoriented and space-sick. But newts get birth defects, often developing without heads. Headless newts die. Of course. The take-away from all of this is you better have artificial gravity for long-term space flight.
One animal that seems to do pretty well in space are lizards. They happily crawl around on the walls.
Giving some credence to the notion that if there are space aliens visiting Earth, they are probably lizard people. (Joking.) (But the rest of the above is real.)