A group of scientists are studying a group of gorillas in the wild. They are trying to determine if young gorillas display signs of hyperactivity after eating certain types of foods. The study takes place over several months. They observe the gorillas each day during the same time period. They note what types of food the gorillas eat each day and document how much time the gorillas spend playing and sleeping. Why is studying animals in the wild considered to be somewhat experimentally "out of control"?