Nervous System

A mammal's brain is about 15 times heavier than the brain of a similarly sized fish, amphibian, or reptile. Of all animals, humans have one of the highest ratios of brain size to body size. Whales, dolphins, and some primates also have high ratios. These differences are due mostly to the size of the cerebrum. The cerebrum is the outer region of the brain and the largest part of the brain in mammals. The cerebrum's surface is usually folded and fissured, which greatly increases its surface area without increasing its volume. The cerebrum evaluates input from the sense organs, controls movement, initiates and regulates behavior, and functions in memory and learning.

As with other terrestrial vertebrates, a mammal's survival depends on five major senses: vision, hearing, smell, touch, and taste. The importance of each sense depends on the mammal's environment. For example, most bats, which are active at night, rely largely on sound rather than vision for navigating and finding food. Using a process called echolocation (EK-oh-loh-KAY-shuhn), these bats emit high-frequency sound waves, which bounce off objects, including potential prey. The bat then analyzes the returning echoes to determine the size, distance, direction, and speed of the objects.

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