Mammals started off as these mammals like reptiles, which were great, big er creatures. But as the time wore on, and this was befogt just before the dinosaurs happened, am the ancestors of mammals became smaller  and more nocturnal. Now we'll just go from the ear to the jaw. Mammals opened up an entirely new universe of sensation. This is why dog whistles are so loud - you can't hear them, but the dog can. Most birds can't hear sounds more than eight to ten killa hurts. E, small children can hear up to 20 killer hurts. But people like me, who grow up listening to deep purple, we