I've been going to live drawing sessions at school, where there's a group of artists gathering around a model. The model stays in one pose for a particular length of time, starting with a few 2 minutes sessions for gesture drawings, then 5 and 10 minute sessions, and finally 3 20-minute sessions. It's wonderful to see how the artists in the group work. They each have their style and what they like to focus on, which can change from session to session depending on the model and how she or he poses.
The models do their best to stay in one position for the time period, and it isn't easy. This cannot be said of zoo animals. They would pose sometimes, but only for a brief second. You'd start drawing them in profile, and before you're done with the first line, you see them completely facing you. Or showing off their butts.