|MadSci Network: General Biology|
You must be very observant (or else you've been scratched a lot). After receiving your question, I went home and picked up my cats several times. They do indeed stretch out their hind claws, but I had never noticed before. I have noticed that when I pick up small rodents (voles, guinea pigs), they stick their hind legs straight out. Rodents don't have retractable claws, so they can't just extend those. I suspect that the cats and other animals that do this are preparing to grab onto anything within reach to save themselves from the large, scary predator that just grabbed them. (I don't mean to imply that your cats don't like you, or don't want to be picked up, but that they are performing a reflexive action.) This could be something that evolved over millions of years and saved many a wild cat's life in the past. They would have used those extended claws to rake the predator's throat or belly or to get better leverage on a nearby tree-trunk to aid their escape. Desmond Morris has a book on cat watching (I think the title may even be Cat Watching). You might look in it for more information on cats and why they do the things they do (those things that often drive us crazy!).
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