About this photo:

This was the first creature that we saw on our Chan Chich night walk, a young snake that our guide wasn't able to name right away. He promised to check the book, but we left before talking to him again. The yellow mark by the head should make it fairly easy to identify for someone with access to a good snake book. This is not a poisonous snake. The snake is on a palm tree, which gives you some idea of the size.