Ebbie, it's a fairly complex answer, but here are some of the factors. A) Monkey troops, in fact, do not spend that much time feeding, at least compared to herbivores. The more the animal grazes or browses, as opposed to eating concentrated food sources like fruits or animals, the longer they spend getting food. Humans and apes usually gather from more concentrated sources. B) Humans have tools, which greatly speed the gathering of many types of food. C) The 10-15 hour figure is time spent gathering food. Preparing and eating the food may take longer.