The ruby-throated hummingbird beats its wings at the incredibly rapid speed of 50 to 70 times a second. If a 170-pound man expended energy at the rate of the hummingbird, he would have to eat 285 pounds of hamburger or twice his weight in potatoes each day in order to maintain his weight. He would have to evaporate 100 pounds of perspiration per hour to keep his skin temperature below the ----------?