If you ask someone to name an animal that uses camouflage, chances are she may respond with something like a chameleon (we had these in elementary school one year), or maybe a butterfly or a frog. Camouflage and mimicry are widespread in nature. As this article from the NY Times shows, they may be more widespread than most people realize, and they don't just apply to visual camouflage or mimicry.
The article lists several examples of mimicry, such as jungle cats imitating the calls of baby monkeys, baby cockroaches flirting with male cockroaches to obtain food (just when you thought cockroaches couldn't get more disgusting), and octopi that can adopt the coloration of toxic fish over their entire bodies or make an arm look like a sea snake. Interestingly enough, the article concludes with an example of human mimicry; we like other people who unconsciously echo our own gestures better than those who don't.
Shape-shifting aliens are nothing new, but there are other ways to use camouflage and mimicry in science fiction. On Earth, animals use camouflage and mimicry techniques that affect the senses of vision and hearing. Perhaps aliens would rely on different senses and attempt to fool others' senses of smell, touch, or taste. They may even play with senses that we don't have. Would they have other motives for these behaviors besides food and protection? Would they play a part in procreation, for instance, or perhaps alien art? And if they learn to imitate us, will we consider that sincere flattery and accept them? What do you think?