29.6 Mammals

Mammalian Diversity and Characteristics

Mammals are vertebrates defined by two features no other group shares: mammary glands that produce milk and hair (or fur) covering at least part of the body. Beyond those hallmarks, mammals are endothermic, possess differentiated teeth, and have a neocortex that supports complex behavior. The roughly 6,000 living species are split into three groups based on how they reproduce: monotremes, marsupials, and placentals.

Monotremes vs. Marsupials vs. Placentals

Monotremes are the only mammals that lay eggs.

• Just five extant species: the platypus and four species of echidna.
• They retain a cloaca, a single opening shared by the reproductive, digestive, and urinary tracts.
• Monotremes lack nipples. Instead, mothers secrete milk from mammary patches on the skin, and the young lap it up.

Marsupials give birth to tiny, underdeveloped (altricial) young that crawl into a pouch (marsupium) to finish developing.

• Gestation is much shorter than in placental mammals. A kangaroo joey, for example, is born after only about 33 days and is roughly the size of a jellybean.
• Other familiar marsupials include koalas, wombats, and opossums. The Virginia opossum is the only marsupial native to North America.

Placental mammals (eutherians) nourish the developing fetus internally through a placenta, allowing for a longer gestation and more developed offspring at birth.

• This is by far the largest and most diverse mammalian group, including rodents, bats, whales, primates, and many others.
• All placentals are viviparous (live birth). Offspring range from highly precocial (a wildebeest calf can stand within minutes) to quite altricial (newborn mice are blind and hairless), so "precocial" is a tendency, not a rule, for the whole group.

Evolutionary Path of Mammals

Mammals trace back to the synapsids, a lineage that split from the sauropsids (the line leading to reptiles and birds) during the Carboniferous period, around 310 million years ago.

1. Pelycosaurs were early synapsids recognizable by a single temporal fenestra (opening) behind each eye socket. Think of Dimetrodon with its large sail.
2. Therapsids evolved from pelycosaurs and developed increasingly mammal-like traits: teeth differentiated into incisors, canines, premolars, and molars; stronger, more efficient jaw muscles; and a more upright limb posture. Many therapsids were likely at least partially endothermic.
3. Cynodonts, a subgroup of therapsids, are the direct ancestors of mammals. They evolved a secondary palate (allowing them to breathe while chewing) and a dentary-squamosal jaw joint, which is the jaw joint all living mammals share.
4. Mammal-like synapsids survived the devastating Permian-Triassic extinction (~252 mya), and the first true mammals appeared during the Triassic period. Early mammals were mostly small, nocturnal insectivores that diversified explosively after the non-avian dinosaurs went extinct at the end of the Cretaceous (~66 mya).

Mammalian Physiology and Diversity

Unique Mammalian Characteristics

• Mammary glands: Specialized exocrine glands that produce milk, providing offspring with nutrition, antibodies, and hydration.
• Hair: Made of the protein keratin. Hair serves multiple roles: thermal insulation, sensory input (whiskers are modified hairs), camouflage, and protection.
• Homeothermy: Mammals maintain a relatively constant internal body temperature regardless of the environment. In most species this is around 36–38 °C.
• Neocortex: An expanded outer layer of the cerebral cortex found only in mammals. It's the seat of higher-order functions like reasoning, sensory perception, and language (in humans).

Features Supporting Mammalian Metabolism

Endothermy is energetically expensive. Mammals have a suite of physiological features that keep up with their high metabolic demands:

• Respiratory system: Highly branched bronchi end in millions of tiny alveoli, creating an enormous surface area for gas exchange. A muscular diaphragm drives efficient ventilation.
• Circulatory system: A four-chambered heart completely separates oxygenated and deoxygenated blood, ensuring tissues receive fully oxygenated blood at high pressure.
• Insulation: Hair or fur traps a layer of warm air against the skin. A subcutaneous fat layer adds further insulation (especially important in marine mammals like seals and whales).
• Thermoregulation: Sweat glands allow evaporative cooling. Brown adipose tissue generates heat through non-shivering thermogenesis, which is particularly important in newborns and hibernating species.
• Diet and digestion: High metabolic rates require energy-rich food. Mammals have specialized, differentiated teeth for processing diverse diets and a relatively long gut for efficient nutrient absorption.

Groups of Placental Mammals

Molecular phylogenetics divides placental mammals into four major superorders. You don't need to memorize every member, but understanding the groupings shows how diverse placental mammals really are.

• Xenarthra: Armadillos, sloths, and anteaters. Members tend to have reduced or absent teeth and unique extra articulations between their lumbar vertebrae (the name literally means "strange joints").
• Afrotheria: Elephants, manatees, aardvarks, and hyraxes. This is a surprisingly diverse group that originated in Africa. A hyrax looks like a large rodent but is actually more closely related to an elephant.
• Euarchontoglires: Rodents, rabbits, primates, tree shrews, and colugos (flying lemurs). Rodents alone account for about 40% of all mammal species. Primates are notable for grasping hands, forward-facing eyes, and large brains relative to body size.
• Laurasiatheria: Bats, carnivorans (dogs, cats, bears), pangolins, ungulates (both even-toed and odd-toed), and cetaceans (whales and dolphins). This superorder contains some of the most dramatic adaptive radiations: powered flight in bats, full aquatic life in cetaceans, and hooved locomotion in ungulates.