Mammals display a remarkable array of adaptations that enable them to inhabit a wide range of habitats. Some of their characteristics are shared by no other groups of animals: hair, mammary glands, three specialized middle-ear bones. Note: Characteristics listed below that are marked with an asterisk * are unique, defining characteristics of mammals.
Hair *
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Hair is a defining characteristic of mammals: no other organisms possess true hair and all mammals have hair covering at least part of their body at some time during their life. Hair grows from skin cells known as follicles. The hair shaft is made of a protein called keratin. Hair serves numerous functions.
Mammary Glands *
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Mammary glands, like hair, are a uniquely mammalian trait. Though present in both males and females, mammary glands only fully develop in females. Mammary glands consist of ducts and glandular tissues that secrete milk through nipples. Young mammals obtain milk from their mother by feeding from her nipples. The milk provides the young with much needed protein, sugars, fat, vitamins, and salts.
Three Middle Ear Bones *
The three middle ear bones in mammals are the malleus, incus, and stapes. These bones—commonly referred to as the hammer, anvil, and stirrup—are unique to mammals, no other animal group has them. The middle ear bones transmit sound vibrations from the tympanic membrane or eardrum to the inner ear and transforms them into neural impulses. The malleus and incus are derived bones that were once part of the lower jaw in mammal ancestors.Diaphram
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The diaphram is a layer of muscle located at the base of the ribcage that separates the thoracic cavity from the adominal cavity in mammals. Mammals are not the only vertebrates to posess a diaphram, amphibians and reptiles also have diaphragms or diaphragm-like structures. It should be noted that the anatomy of the diaphram and its position vary among the different classes of vertebrates.
Four-Chambered Heart
Large Cerebral Cortex
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Lower Jaw Made of a Single Bone
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In mammals, the lower jaw consists of a single bone.
Diphyodonty
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Diphyodonty is a fancy term describing a pattern of tooth replacement in animals in which the teeth are replaced only once throughout the lifetime. Young mammals have a set of teeth that are smaller and weaker than their adult teeth. These teeth are later replaced by larger permanent teeth.
