Mammals have relatively larger brains than other vertebrates. From monotremes to marsupials to eutherians, the mammal brain increases in size and complexity, primarily by the expansion of the neopallium. The neopallium (or neocortex) is a mantle of gray matter that first appeared as a small region between the olfactory bulb and the larger archipallium.
The neopallium in mammals has expanded over the primitive parts of the vertebrate brain, dominating it as the cerebral cortex. The cerebral cortex is a thin laminar structure consisting of six sheets of neurons. In order to increase the number of neurons in the neocortex it must be folded to fit within the skull of a mammal. For example, the surface area of the human neocortex is about 1.5 ft2 (0.14 m2). With this area, it could not be simply laid over the deeper parts of the brain; folding produces gyri and sulci (folds and grooves, respectively), which gives the eutherian brain a convoluted appearance.
Small mammals do not usually have convolutions, but they are almost always found once a species has reached a particular body size. Some researchers believe that the convolutions simply serve to increase the number of neurons in the neocortex, while others propose that the primary purpose of the convolutions is to increase surface area for heat dissipation. The brain produces a large amount of metabolic heat and must be cooled. Increasing the surface area provides more area for heat transfer (radiation) to occur; i.e., the convolutions produce a “radiator” for the brain. The neocortex may be more developed or less developed depending on the mammalian species. In echolocating bats, for example, it comprises less than 50% of the brain surface because most of the bat brain is devoted to the auditory centers.
Specific regions of the neocortex are specialized for particular functions. For example, the occipital region is a visual center, the temporal region is involved with hearing, and the parietal lobe interprets touch. A structure found only in the eutherian brain is the corpus callosum, a concentration of nerve fibers that connect the two cerebral hemispheres and serve as a communication conduit between them.
Brain structure accounts for mammals’ great ability to learn from their experiences. Their brain structure, combined with other neural characteristics, also accounts for mammals’ acute sensory abilities. For example, mammalian smell is very acute. In some mammals, it is the most developed sense. Mammals have an elongated palate and, consequently, the nasal cavity is elongated as well. A structure in the palate of many mammals, the vomeronasal organ, detects smells from food. The development of turbinal bones covered by sensory mucosa in the nasal cavities has allowed more efficient detection of odors. Even so, some mammals have a poorer sense of smell than others, e.g., insectivorous bats, higher primates, and whales. In fact, dolphins and porpoises completely lack the olfactory apparatus. The receptors for taste are located on the tongue. Taste, interpreted in the brain in conjunction with olfactory stimuli, helps mammals identify whether food is safe to eat or not.
Mammal hearing is highly developed. In mammals, the articular and quadrate bones of the reptilian jaw were modified to become the malleus and incus bones, which, along with the stapes, form the auditory ossicles in the mammal skull. The auditory ossicles conduct vibrations to the inner ear. Another mammal modification is the evolution of a pinna, an external flap that directs sound waves into the ear canal (external acoustic meatus). Many mammal species have mobil pinnae that enable them to pinpoint the location of the sound source. Pinnae are most elaborate in the insectivorous bats, but completely lacking in most marine and subterranean species.
The mammal eye is based on the reptilian eye. Many mammals are able to see very well in low-