Nice concept, you can really feel sorry for the little monkey I also like it that you included a banana, and that it likewise feels sorry for him. There's a few things I'd like to point out, of course only to help you advance this (you asked for critique )
-The contrast on the bars could be a bit stronger, to make them look a bit more shiny. -The shadow of the banana is a bit off, I think you should keep the shadow of his head away. -The shadow of the monkey when he jumps up and down is a bit turned around I think. When he lands his shadow goes small and when he jumps up it's big.
I also think you should play a bit more with the frame speeds. Your work looks good most of the time, but the frame speeds are rather quick. With that I mean there's never a rest point in your animations, as well as this one (like a 1 second frame or a 0,50 second frame) If you want I can show you what I mean with this animation. I can adjust it a bit, but of course only with your permission. I can send you a link to ImageShack with the animation so you can study it if you want
I love the way the picture motions itself as the monkey in a cage trying to get a banana, eagerly to strive for it. The vision is easy to see, handle-able, and quite cute. I haven't seen monkeys in cages that want bananas, often, since I'm not even used to seeing things like that. Not even in the zoo! I bet making those animated pictures can be quite tough to get around, but since this is nicely done, it's so noticeable, and easily enjoyable. The impact however, is alright, for the matter of its time. It's just that, the image is fairly small for a gif. image, but I'm not used to seeing small images like that. As far as I can see, it's cute, yet not extremely attention-grabbing all the time.