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Mammal: the story on HearLore | HearLore
Mammal
The bowhead whale holds the record for the longest lifespan of any mammal, living up to 211 years, a feat that dwarfs the two-year existence of the common shrew. This vast difference in longevity is just one of the many paradoxes that define the class Mammalia, a group of vertebrates that has dominated terrestrial life for the last 66 million years. Mammals are distinguished from reptiles and birds by a suite of unique traits, including the presence of milk-producing mammary glands, a broad neocortex region of the brain, fur or hair, and three middle ear bones. These characteristics set them apart from their ancestors, the synapsids, which diverged from the reptile lineage during the Carboniferous Period over 300 million years ago. While early synapsids like Dimetrodon were once incorrectly labeled as mammal-like reptiles, they were actually the fierce, dominant predators of the early Permian, possessing a single temporal opening in their skulls that allowed for powerful jaw muscles. The lineage leading to modern mammals, known as therapsids, evolved in the Middle Permian about 265 million years ago, eventually giving rise to cynodonts during the Late Triassic to Early Jurassic. These advanced therapsids were the direct ancestors of mammals, and by the time the first true mammals appeared, they had already developed a bony secondary palate and begun the transition toward endothermy, or warm-bloodedness, which allowed them to forage and hunt in climates too cold for ectothermic reptiles. The extinction of non-avian dinosaurs approximately 66 million years ago opened ecological niches that mammals quickly filled, leading to an exponential rise in diversity and body size, a period known as the Cenozoic era. Today, there are around 6,640 extant species of mammals, divided into 27 orders, with the largest groups being rodents, bats, and eulipotyphlans, followed by primates, even-toed ungulates, and carnivores. The study of these animals, known as mammalogy, continues to reveal new insights into their complex evolutionary history, from the earliest fossils like Juramaia sinensis, dated to 160 million years ago, to the molecular phylogenies that suggest placental orders diverged as early as 105 to 120 million years ago. The basic mammalian body type is quadrupedal, with most species using four limbs for terrestrial locomotion, but this has been adapted for life at sea, in the air, in trees, or underground, showcasing the incredible versatility of the mammalian form.
The Evolutionary Bridge
The transition from early synapsids to modern mammals was not a sudden event but a slow, erratic process spanning hundreds of millions of years, characterized by the gradual development of key anatomical features. The earliest clear evidence of a jaw joint formed solely by the squamosal and dentary bones, a hallmark of mammals, comes from the fossil Hadrocodium, dated to approximately 195 million years ago in the early Jurassic. This development freed the articular and quadrate bones, which had previously formed the jaw joint in early therapsids, to become the malleus and incus of the middle ear, allowing for more sensitive hearing. The evolution of hair or fur is equally ancient, with fossils of Castorocauda and Megaconus from 164 million years ago providing the first clear evidence of these integumentary structures. Studies on Permian coprolites suggest that non-mammalian synapsids may have had fur as far back as dicynodonts, indicating that the insulation required for endothermy evolved before the origin of mammals themselves. The development of endothermy was a critical step, as it allowed mammals to maintain a constant body temperature, enabling them to be active in the night and in cold environments where reptiles were sluggish. This metabolic shift required a high-energy diet, leading to the evolution of specialized teeth and digestive systems capable of processing diverse food sources. The dentary gradually became the main bone of the lower jaw, and by the Triassic, the jaw had progressed towards the fully mammalian configuration. The middle ear, constructed from the bones that were previously used to construct the jaws of reptiles, allowed for the detection of higher frequency sounds, including ultrasound, which was inaudible to birds and reptiles and provided a private channel for communication during the Mesozoic. The evolution of erect limbs, which increased stamina by avoiding Carrier's constraint, was another crucial adaptation, though the process was slow and erratic, with modern monotremes still retaining semisprawling limbs. The earliest-known fossil of the Metatheria, or changed beasts, is Sinodelphys, found in 125-million-year-old Early Cretaceous shale in China's northeastern Liaoning Province, which includes tufts of fur and imprints of soft tissues. The oldest-known fossil among the Eutheria, or true beasts, is the small shrewlike Juramaia sinensis, dated to 160 million years ago in the late Jurassic, suggesting that the divergence between marsupials and placentals occurred much earlier than previously thought. The presence of epipubic bones, which extend forwards from the pelvis, was a feature of early mammals that influenced reproduction by preventing the expansion of the torso needed for full pregnancy, a constraint that was later lost in placental mammals to allow for longer gestation periods. The evolution of the placenta was a later development, connecting the developing fetus to the uterine wall to allow nutrient uptake, a feature that enabled the birth of relatively complete and developed young after long gestation periods. The monotremes, which branched early from other mammals, retain some ancestral features such as the cloaca, a single opening for urination, defecation, and reproduction, and they lay eggs that are leathery and uncalcified, similar to those of reptiles and birds. The molecular classification of placentals has revealed three major groups or lineages, Afrotheria, Xenarthra, and Boreoeutheria, which diverged in the Cretaceous, with estimates for the divergence times ranging from 105 to 120 million years ago, depending on the type of DNA used and varying interpretations of paleogeographic data. The relationships between these three lineages remain contentious, with all three possible hypotheses having been proposed with respect to which group is basal, highlighting the ongoing debate and progress in the theoretical underpinnings of mammalian systematics.
The bowhead whale holds the record for the longest lifespan of any mammal, living up to 211 years. This longevity dwarfs the two-year existence of the common shrew, which represents the shortest lifespan among mammals.
When did the class Mammalia first appear in the fossil record?
The earliest clear evidence of a jaw joint formed solely by the squamosal and dentary bones comes from the fossil Hadrocodium, dated to approximately 195 million years ago in the early Jurassic. The oldest-known fossil among the Eutheria is the small shrewlike Juramaia sinensis, dated to 160 million years ago in the late Jurassic.
How many extant species of mammals exist today?
There are around 6,640 extant species of mammals, divided into 27 orders. The largest groups are rodents, bats, and eulipotyphlans, followed by primates, even-toed ungulates, and carnivores.
Which mammals lay eggs instead of giving birth to live young?
The monotremes are the only mammals that lay eggs, with five species existing today. These animals retain ancestral features such as the cloaca and produce leathery, uncalcified eggs similar to those of reptiles and birds.
What anatomical features distinguish mammals from other vertebrates?
Mammals are distinguished by the presence of milk-producing mammary glands, a broad neocortex region of the brain, fur or hair, and three middle ear bones. Their red blood cells lack a nucleus to make space for more haemoglobin, and their lungs are spongy and honeycombed with breathing achieved mainly by the diaphragm.
When was the scientific name Mammalia first coined?
The word mammal is modern, from the scientific name Mammalia coined by Carl Linnaeus in 1758. This name is derived from the Latin mamma, meaning teat or pap, and Linnaeus initially defined the class during that year.
Mammals are defined by a unique set of anatomical features that distinguish them from all other vertebrates, including the presence of sweat glands, specialized to produce milk to nourish their young, and a neocortex region of the brain that is unique to the class. The mammalian heart has four chambers, two upper atria and two lower ventricles, with four valves that ensure blood flows in the correct direction, preventing backflow. The red blood cells of mammals lack a nucleus to make space for more haemoglobin, which significantly increases the cell's oxygen-carrying capacity, allowing for the high metabolic rates required for endothermy. The lungs of mammals are spongy and honeycombed, and breathing is mainly achieved with the diaphragm, which divides the thorax from the abdominal cavity, forming a dome convex to the thorax. The contraction of the diaphragm flattens the dome, increasing the volume of the lung cavity, while relaxing it decreases the volume, causing air to be pushed out of the lungs. The integumentary system, or skin, is made up of three layers: the outermost epidermis, the dermis, and the hypodermis, with the dermis being 15 to 40 times thicker than the epidermis. The hypodermis is made up of adipose tissue, which stores lipids and provides cushioning and insulation, with marine mammals requiring a thick hypodermis, or blubber, for insulation. Hair is a definitive characteristic of mammals, serving multiple functions including thermoregulation, protection, sensory purposes, waterproofing, and camouflage. Different types of fur serve different purposes, from definitive hairs that may be shed after reaching a certain length to vibrissae, or sensory hairs, commonly known as whiskers, and pelage, which includes guard hairs, under-fur, and awn hair. The coloration of mammalian coats is determined by the type and amount of melanin, with eumelanins producing brown and black colors and pheomelanin producing a range of yellowish to reddish colors. Some mammals have more vibrant colors, such as mandrills and vervet monkeys, which have blue skin due to light diffraction in collagen fibers, while others, like sloths, appear green because their fur hosts green algae, a symbiotic relation that affords camouflage. Camouflage is a powerful influence in a large number of mammals, helping to conceal individuals from predators or prey, and seasonal color change between brown in summer and white in winter is driven largely by camouflage in arctic and subarctic mammals. The white, pigmentless fur of arctic mammals, such as the polar bear, may reflect more solar radiation directly onto the skin, while darker coats can absorb more heat from solar radiation, helping smaller mammals stay warmer. The digestive systems of mammals are highly specialized, with herbivores developing diverse physical structures to facilitate the consumption of plant material, including low-crowned teeth for grinding foliage and high-crowned teeth for grinding tough plant tissues. Carnivorous mammals have carnassial teeth, long canines, and simple digestive tracts adapted to digest primarily meat, while omnivores display varying degrees of carnivory and herbivory. The stomach of even-toed ungulates is divided into four sections, the rumen, the reticulum, the omasum, and the abomasum, where cellulolytic microbes produce cellulase to break down the cellulose in plants. The excretory system of mammals involves the conversion of ammonia into urea, which is done by the liver as part of the urea cycle, and the passing of bilirubin via bile through the intestinal tract gives mammalian feces a distinctive brown coloration. The mammalian kidney has a bean shape, with the presence of the renal pelvis and renal pyramids, and of a clearly distinguishable cortex and medulla, due to the presence of elongated loops of Henle. The reproductive system of mammals is characterized by internal fertilization and the production of live young, with the exception of the five species of monotremes, which lay eggs. The mammary glands of mammals are specialized to produce milk, the primary source of nutrition for newborns, with the milk of marsupials changing greatly in both production rate and in nutrient composition due to the underdeveloped young. The monotremes do not have the teats seen in most mammals, but they do have mammary glands, and the young lick the milk from a mammary patch on the mother's belly. The lactose is the main sugar in placental milk, while monotreme and marsupial milk is dominated by oligosaccharides, highlighting the diversity of reproductive strategies within the class.
The Locomotion of Life
Mammals exhibit a vast range of locomotor adaptations, from the quadrupedal gait of most species to the bipedalism of humans and great apes, and the specialized swimming of cetaceans and sirenians. Most vertebrates are plantigrade, walking on the whole of the underside of the foot, but many mammals, such as cats and dogs, are digitigrade, walking on their toes, which allows for greater stride length and speed. Some animals, such as horses, are unguligrade, walking on the tips of their toes, which further increases their stride length and speed. A few mammals, namely the great apes, are also known to walk on their knuckles, at least for their front legs, while giant anteaters and platypuses are also knuckle-walkers. Bipedal species have a larger field of vision than quadrupeds, conserve more energy, and have the ability to manipulate objects with their hands, which aids in foraging. Instead of walking, some bipeds hop, such as kangaroos and kangaroo rats, while others, like the great apes, employ many other gaits occasionally, including a four-legged crawl in tight spaces. Arboreal animals frequently have elongated limbs that help them cross gaps, reach fruit, or other resources, and test the firmness of support ahead, with many species using prehensile tails to grasp branches. The spider monkey, for example, has a tip of the tail with either a bare patch or adhesive pad, which provides increased friction, while squirrels have evolved highly mobile ankle joints that permit rotating the foot into a reversed posture, allowing the claws to hook into the rough surface of the bark. Aquatic mammals, the cetaceans and sirenians, have lost their legs and have a tail fin to propel themselves through the water, with whales swimming by moving their tail fin and lower body up and down, propelling themselves through vertical movement. Their skeletal anatomy allows them to be fast swimmers, and most species have a dorsal fin to prevent themselves from turning upside-down in the water. The flukes of sirenians are raised up and down in long strokes to move the animal forward, and can be twisted to turn, while the forelimbs are paddle-like flippers which aid in turning and slowing. Semi-aquatic mammals, like pinnipeds, have two pairs of flippers on the front and back, the fore-flippers and hind-flippers, with the elbows and ankles enclosed within the body. Pinnipeds have several adaptations for reducing drag, including smooth networks of muscle bundles in their skin that may increase laminar flow and make it easier for them to slip through water. Fossorial mammals, or animals adapted to digging, have a fusiform body, thickest at the shoulders and tapering off at the tail and nose, with degenerated eyes and small or absent external ear flaps. Truly fossorial mammals have short, stout legs as strength is more important than speed to a burrowing mammal, while semi-fossorial mammals have cursorial legs. The front paws are broad and have strong claws to help in loosening dirt while excavating burrows, and the back paws have webbing, as well as claws, which aids in throwing loosened dirt backwards. Bats are the only mammals that can truly fly, moving their wings up and down to generate lift and thrust, with wings that are much thinner and consist of more bones than those of birds, allowing bats to maneuver more accurately and fly with more lift and less drag. The membranes of bat wings are delicate, ripping easily, but the tissue is able to regrow, such that small tears can heal quickly, and the surface of their wings is equipped with touch-sensitive receptors on small bumps called Merkel cells, allowing the bat to detect and collect information about the air flowing over its wings.
The Minds of Mammals
Intelligence in mammals is a complex trait that varies widely across species, with indications of intelligence including the ability to learn, matched with behavioral flexibility. Rats, for example, are considered to be highly intelligent, as they can learn and perform new tasks, an ability that may be important when they first colonize a fresh habitat. Food gathering appears to be related to intelligence, with a deer feeding on plants having a brain smaller than a cat, which must think to outwit its prey. Tool use by animals may indicate different levels of learning and cognition, with the sea otter using rocks as essential and regular parts of its foraging behavior, with some populations spending 21% of their time making tools. Other tool use, such as chimpanzees using twigs to fish for termites, may be developed by watching others use tools and may even be a true example of animal teaching. Tools may even be used in solving puzzles in which the animal appears to experience a Eureka moment, a phenomenon observed in other mammals that do not use tools, such as dogs. Brain size was previously considered a major indicator of the intelligence of an animal, but allometric analysis indicates that mammalian brain size scales at approximately the 0.75 exponent of the body mass. Comparison of a particular animal's brain size with the expected brain size based on such allometric analysis provides an encephalization quotient that can be used as another indication of animal intelligence. Sperm whales have the largest brain mass of any animal on earth, averaging 8,000 grams in mature males, yet brain size alone does not determine intelligence. Self-awareness appears to be a sign of abstract thinking, and the traditional method for measuring this is the mirror test, which determines if an animal possesses the ability of self-recognition. Mammals that have passed the mirror test include Asian elephants, some of which pass and some do not, chimpanzees, bonobos, orangutans, humans from 18 months, common bottlenose dolphins, orcas, and false killer whales. Social structure in mammals ranges from solitary animals that defend a territory and avoid social interactions with the members of its species, except during breeding season, to eusocial societies, which are the highest level of social organization. Eusociality is demonstrated in two rodent species, the naked mole-rat and the Damaraland mole-rat, and involves an overlap of adult generations, the division of reproductive labor, and cooperative caring of young. Presociality is when animals exhibit more than just sexual interactions with members of the same species, but fall short of qualifying as eusocial, with humans and some species of Callitrichidae, such as marmosets and tamarins, being unique among primates in their degree of cooperative care of young. A fission-fusion society is a society that changes frequently in its size and composition, making up a permanent social group called the parent group, with many mammals exhibiting this, such as primates, elephants, spotted hyenas, lions, and dolphins. In a hierarchy, individuals are either dominant or submissive, with a despotic hierarchy where one individual is dominant while the others are submissive, as in wolves and lemurs. Communication and vocalization serve many purposes, including in mating rituals, as warning calls, to indicate food sources, and for social purposes, with males often calling during mating rituals to ward off other males and to attract females, as in the roaring of lions and red deer. The songs of the humpback whale may be signals to females, and they have different dialects in different regions of the ocean, while the vervet monkey gives a distinct alarm call for each of at least four different predators, and the reactions of other monkeys vary according to the call. Elephants communicate socially with a variety of sounds, including snorting, screaming, trumpeting, roaring, and rumbling, with some of the rumbling calls being infrasonic, below the hearing range of humans, and can be heard by other elephants up to 10 kilometers away at still times near sunrise and sunset. Microbats and toothed whales, including oceanic dolphins, vocalize both socially and in echolocation, with the ability to learn new vocalizations being exemplified in humans, seals, cetaceans, elephants, and possibly bats.
The Human Connection
The domestication of many types of mammals by humans played a major role in the Neolithic Revolution, resulting in farming replacing hunting and gathering as the primary source of food for humans. This led to a major restructuring of human societies from nomadic to sedentary, with more cooperation among larger and larger groups, and ultimately the development of the first civilizations. Domesticated mammals provided, and continue to provide, power for transport and agriculture, as well as food, including meat and dairy products, fur, and leather. Mammals are also hunted and raced for sport, kept as pets and working animals of various types, and are used as model organisms in science. Mammals have been depicted in art since Paleolithic times, and appear in literature, film, mythology, and religion, reflecting their deep integration into human culture and history. The decline in numbers and extinction of many mammals is primarily driven by human poaching and habitat destruction, primarily deforestation, which threatens the survival of many species. The International Union for Conservation of Nature (IUCN) completed a five-year Global Mammal Assessment for its IUCN Red List, which counted 5,488 species in 2008, highlighting the urgent need for conservation efforts. The study of mammalogy continues to reveal new insights into the complex evolutionary history of mammals, from the earliest fossils to the molecular phylogenies that suggest placental orders diverged as early as 105 to 120 million years ago. The classification of mammals has been through several revisions since Carl Linnaeus initially defined the class in 1758, and at present, no classification system is universally accepted. McKenna & Bell (1997) and Wilson & Reeder (2005) provide useful recent compendiums, while Simpson (1945) provides systematics of mammal origins and relationships that had been taught universally until the end of the 20th century. However, since 1945, a large amount of new and more detailed information has gradually been found, with the paleontological record being recalibrated and the intervening years seeing much debate and progress concerning the theoretical underpinnings of systematization itself, partly through the new concept of cladistics. Though fieldwork and lab work progressively outdated Simpson's classification, it remains the closest thing to an official classification of mammals, despite its known issues. The word mammal is modern, from the scientific name Mammalia coined by Carl Linnaeus in 1758, derived from the Latin mamma, meaning teat or pap. In an influential 1988 paper, Timothy Rowe defined Mammalia phylogenetically as the crown group of mammals, the clade consisting of the most recent common ancestor of living monotremes and therians and all descendants of that ancestor. Since this ancestor lived in the Jurassic Period, Rowe's definition excludes all animals from the earlier Triassic, despite the fact that Triassic fossils in the Haramiyida have been referred to the Mammalia since the mid-19th century. T. S. Kemp has provided a more traditional definition, describing synapsids that possess a dentary-squamosal jaw articulation and occlusion between upper and lower molars with a transverse component to the movement, or, equivalently in Kemp's view, the clade originating with the last common ancestor of Sinoconodon and living mammals. The earliest-known synapsid satisfying Kemp's definitions is Tikitherium, dated to the Late Triassic, so the appearance of mammals in this broader sense can be given this date, though some researchers suggest that this animal may have actually evolved during the Neogene. The molecular classification of placentals has revealed three major groups or lineages, Afrotheria, Xenarthra, and Boreoeutheria, which diverged in the Cretaceous, with estimates for the divergence times ranging from 105 to 120 million years ago, depending on the type of DNA used and varying interpretations of paleogeographic data. The relationships between these three lineages remain contentious, with all three possible hypotheses having been proposed with respect to which group is basal, highlighting the ongoing debate and progress in the theoretical underpinnings of mammalian systematics. The classification of mammals continues to evolve, with new discoveries and molecular studies challenging traditional views and providing a more nuanced understanding of the evolutionary history of this diverse and fascinating class of animals.