Mammal

• 1. The Class That Nurses

  ›

  A bumblebee bat measuring 30 to 40 millimeters and a blue whale stretching 30 meters belong to the same class. Both produce milk from mammary glands to feed their young. Both grow fur or hair at some point, carry three tiny bones in the middle ear, and possess a broad neocortex in the brain. These four traits define Mammalia, a class of roughly 6,640 described living species sorted into 27 orders. The blue whale may be the largest animal ever to have lived. The shrew may survive only two years while the bowhead whale can reach 211. This documentary follows the story of how such different creatures came to share one lineage. How did a class that now includes humans, bats, and whales emerge from creatures once wrongly called mammal-like reptiles? Why do almost all mammals bear live young, yet five species still lay eggs? And how did a group that spent its early life as small nocturnal insect-eaters end up dominating the land for the last 66 million years?

• 2. From Pelycosaurs To Cynodonts

  ›

  Synapsida split from the reptile lineage during the Pennsylvanian subperiod, between about 323 and 300 million years ago. Mammals are the only living members of this clade, which together with Sauropsida forms the larger Amniota group. Early synapsids were once called pelycosaurs, and Dimetrodon ranked among the largest and fiercest animals of the early Permian. A single temporal fenestra low on each side of the skull marks the synapsid line.

  Therapsids evolved in the Middle Permian, around 265 million years ago, and became the dominant land vertebrates. They differed from basal eupelycosaurs in features of the skull and jaws, including larger skulls and incisors of equal size. The therapsid lineage leading to mammals passed through a series of stages. It ended with probainognathian cynodonts, some of which could easily be mistaken for mammals.

  The Permian-Triassic extinction event, about 252 million years ago, ended the dominance of carnivorous therapsids. Archosaurs took over most medium to large carnivore niches over an extended period of 35 million years, eventually including the crocodylomorphs, the pterosaurs, and the dinosaurs. Yet large cynodonts like Trucidocynodon still occupied sizable carnivorous niches. Mammals would originate from cynodonts during the Late Triassic to Early Jurassic.

• 3. Born In The Dark

  ›

  The first mammals in Kemp's sense appeared in the Late Triassic, about 225 million years ago, 40 million years after the first therapsids. They lived as nocturnal insect-eaters and expanded out of that niche from the mid-Jurassic onwards. Most are thought to have stayed nocturnal, a pattern called the nocturnal bottleneck, which accounts for many typical mammalian traits.

  Castorocauda, a Jurassic close relative of true mammals, had adaptations for swimming, digging, and catching fish. The majority of mammal species in the Mesozoic were multituberculates, eutriconodonts, and spalacotheriids. The earliest-known fossil of the Metatheria is Sinodelphys, found in 125-million-year-old Early Cretaceous shale in China's northeastern Liaoning Province. That fossil is nearly complete and preserves tufts of fur and imprints of soft tissues.

  The oldest-known fossil among the Eutheria is Juramaia sinensis, the small shrewlike Jurassic mother from China, dated to 160 million years ago. A later relative, Eomaia, from 125 million years ago, shared features with marsupials but not with placentals. Among these were epipubic bones extending forward from the pelvis. Teinolophos, one of the earliest-known monotremes, lived about 120 million years ago in Australia.

• 4. The Jaw And The Ear

  ›

  Hadrocodium, whose fossils date from approximately 195 million years ago, gives the first clear evidence of a jaw joint formed solely by the squamosal and dentary bones. There is no space left for the articular, a bone present in the jaws of all early synapsids. In crown-group mammals, the dentary and squamosal meet to form the joint. In most gnathostomes, including early therapsids, the articular and quadrate form it instead.

  The middle ear tells a related story of borrowed bones. Sound passes from the eardrum through a chain of three bones, the malleus, the incus, and the stapes. The malleus and incus are ancestrally derived from the articular and quadrate that once made up the jaw joint of early therapsids. By the Triassic, the lower jaw had progressed toward consisting only of the dentary.

  The earliest clear evidence of hair or fur appears in fossils of Castorocauda and Megaconus, from 164 million years ago. In the 1950s, foramina in the upper jaw bones of cynodonts were suggested as channels for nerves and blood vessels to whiskers. That argument weakened when the modern lizard Tupinambis was found to have nearly identical foramina. Studies on Permian coprolites suggest non-mammalian synapsids already had fur, pushing the evolution of hair possibly back as far as dicynodonts.

• 5. Eggs, Pouches, And Placentas

  ›

  Kayentatherium produced undeveloped perinates, but at far higher litter sizes than any modern mammal, with 38 specimens in one find. The ancestral condition for mammal reproduction was the birthing of relatively undeveloped young, either through direct vivipary or a short period as soft-shelled eggs. This was likely because epipubic bones prevented the torso from expanding.

  The five species of monotreme, the platypus and the four species of echidna, lay eggs. Their eggs are leathery and uncalcified, and they retain a true cloaca, a single opening that translates from Greek as single hole. A platypus has sex chromosomes more like those of a chicken than those of a therian mammal. Monotremes lack teats, so the young lick milk from a mammary patch on the mother's belly.

  Marsupials have a short gestation period, typically shorter than the estrous cycle, and give birth to undeveloped newborns. In many species, further development takes place within a pouch-like sac, the marsupium, at the front of the mother's abdomen. Placentals give birth to relatively complete young after long gestation, drawing nutrition through the placenta. In placentals the epipubic bone is either lost or converted into the baculum, freeing the torso to expand. Lactose dominates placental milk, while monotreme and marsupial milk is dominated by oligosaccharides.

• 6. Inheriting The Earth

  ›

  The Cretaceous-Paleogene extinction event, approximately 66 million years ago, emptied ecological space once filled by non-avian dinosaurs. Therians took over the medium to large niches and underwent an exponential rise in body size, producing megafauna. Their diversity also rose exponentially. The earliest-known bat dates from about 50 million years ago, only 16 million years after the dinosaurs vanished.

  No placental fossils have been found from before the end of the Cretaceous. The earliest undisputed placental fossils come from the early Paleocene. An early Paleocene animal named Protungulatum donnae was once identified as one of the first placental mammals, then later reclassified as a non-placental eutherian. Recalibrations suggest a Late Cretaceous origin for placentals and a Paleocene origin for most modern clades.

  The earliest-known ancestor of primates is Archicebus achilles, from around 55 million years ago. This tiny primate weighed 20 to 30 grams and could fit within a human palm. Molecular studies reveal three major placental lineages, Afrotheria, Xenarthra, and Boreoeutheria, which diverged in the Cretaceous. Estimates for their divergence range from 105 to 120 million years ago. From 66 million years ago to the present, mammals have been the dominant terrestrial animal group.

• 7. Bodies Built To Move

  ›

  Most vertebrates are plantigrade, walking on the whole underside of the foot. Cats and dogs are digitigrade, walking on their toes for greater stride and speed. Horses are unguligrade, walking on the tips of their toes, and show four natural gaits, from the walk to the trot, canter, and gallop. The great apes walk on their knuckles for their front legs, as do giant anteaters and platypuses.

  Bats are the only mammals that can truly fly. By folding their wings inward on the upstroke, they use 35 percent less energy during flight than birds. Their wings are thinner and contain more bones than those of birds, allowing finer maneuvering. The wing surface carries touch-sensitive receptors on small bumps called Merkel cells, also found on human fingertips, though in bats each bump has a tiny hair at its center.

  Fully aquatic cetaceans and sirenians have lost their legs and propel themselves with a tail fin. The rear limbs of cetaceans and sea cows are mere internal vestiges. Fossorial mammals adapted for digging have a fusiform body, thickest at the shoulders and tapering at tail and nose. In the fully fossorial marsupial mole the eyes are degenerated and useless, while the Cape golden mole has skin covering the eyes. Many such diggers, including shrews, hedgehogs, and moles, were once placed in the now obsolete order Insectivora.

• 8. Minds, Voices, And Societies

  ›

  Sperm whales have the largest brain mass of any animal on earth, averaging 8000 cubic centimeters and 7.8 kilograms in mature males. All mammalian brains possess a neocortex, a region unique to the class, and placental brains add a corpus callosum absent in monotremes and marsupials. The sea otter uses rocks to smash abalone and break open shells, with some populations spending 21 percent of their time making tools. Chimpanzees use twigs to fish for termites, possibly a true example of animal teaching.

  The mirror test measures self-recognition. Mammals that have passed it include Asian elephants, where some pass and some do not, chimpanzees, bonobos, orangutans, humans from 18 months, common bottlenose dolphins, orcas, and false killer whales. The vervet monkey gives a distinct alarm call for each of at least four predators. A python call sends the monkeys into the trees, while an eagle call sends them to hide on the ground.

  Eusociality, the highest level of social organization, appears in two rodent species, the naked mole-rat and the Damaraland mole-rat. In 1958, Harry Harlow set up an experiment with rhesus monkeys that showed social encounters were necessary for the young to develop mentally and sexually. Some elephant rumbles are infrasonic and can be heard by other elephants up to 6 miles away near sunrise and sunset. The domestication of many mammals drove the Neolithic Revolution, turning nomadic human societies sedentary and leading toward the first civilizations.