Just to put my two cents…
Though I never did it (yet), I would love to play animals as PC with GURPS.
Indeed, as ethologists show it more and more, animals are much brighter than we supposed in the past. Of course, they don’t know technology (and maybe don’t care: the body of a feline is a very effective weapon; it doesn’t need to build and use other ones). The problem mainly comes from our conception of intelligence. It got mixed up with logical-mathematical intelligence, which is actually just one part of intelligence: there are several other kinds.
- Naturalist (nature smart)
- Musical (sound smart)
- Existential (life smart)
- Interpersonal (people smart)
- Bodily-kinesthetic (body smart)
- Linguistic (word smart)
- Intra-personal (self smart)
- Spatial (picture smart)
Animals may even be better than humans for some of these other intelligences: bodily-kinesthetic one (felines), spatial one (migratory birds), interpersonal one (dolphins) …
Ditto for languages. Sure, animals don’t speak human languages. But how many human can speak any animal language? Here again, ethologists now know that animal languages are much more complex that what we thought in the past. Dolphin language, for instance, is three-dimensional: they send a sentence (“come, there are fishes here!”), an emotion (yippee!) and a sonar picture (a “photo” of the fishes) at the same time. It explains why scientists needed so much time to understand why they always add echolocation sounds to their “words” …
So, to my mind, playing animals of the same species which can communicate together and communicate with other animals (as well as we, humans, try to do it with cats and dogs) is not at all unrealistic! If you’ve got cats, you perfectly know that they sometimes try to tell us something, looking us right in the eyes, mewing with different tones, and finally abandon, annoyed because we didn’t understand what they wanted …
Of course, while playing animals in a roleplaying game, players will do it in a very anthropocentric manner. But is it really a problem? Don’t we play kobolds, goblins, elves, dwarves, robots, extraterrestrials in a very anthropocentric manner?
Finally, some said above that other roleplaying games are better to play animals than GURPS because they have several mental characteristics instead of just one … I find to the contrary that GURPS is much better here.
Just give your animal an IQ score of 6 or less and TL 0. It will mechanically forbid every human mental skill. Default skills, if any, will fall under 3 (no success roll allowed) while in D&D-like games, a 1 (5% chance) is always successful. GURPS allows playing crafty animals (foxes, craws …) without giving them the ability to use human mental skills.
Furthermore, GURPS offers a lot of advantages and disadvantages which allow perception edges or psychological blockages that no other game offers.
So, playing animals as PC in GURPS is to my mind a very interesting challenge, and something quite easy to do.