Putting a comma after dress up here is wrong, because it implies the trainers simply dressed up, but doesn't say how they dressed up. The comma separates the clause where it explains "like one of them." I knew this while writing it, but I put a comma there because of the two commas after– one before eating bamboo and one before climbing trees. I was paranoid that someone who doesn't understand English well would see that, and presume me to have made a mistake because there was no comma between dress up and act like one of them, yet there were two commas later in the sentence. I decided to put the comma there to cater to those people. Oh, and this factoid is not true, it's a first draft of a joke I'm working on.