Mammals

Mammals are a class of vertebrate animals characterized by the presence of milk-producing mammary glands, a neocortex region of the brain, fur or hair, and three middle ear bones. They are distinguished from reptiles and birds, from which their ancestors diverged over 300 million years ago. Around 6,400 extant species of mammals have been described and divided into 29 orders. The largest orders of mammals, by number of species, are the rodents, bats, and Eulipotyphla. Mammals range in size from the bumblebee bat to the blue whale, and have a maximum lifespan that varies from two years for the shrew to 211 years for the bowhead whale. Most mammals give birth to live young, except the five species of monotremes, which are egg-laying mammals. Mammals are intelligent, with some possessing large brains, self-awareness, and tool use. Domestication of many types of mammals by humans played a major role in the Neolithic Revolution. Mammal classification has been through several revisions since Carl Linnaeus initially defined the class, and at present, no classification system is universally accepted. Most mammals, including the six most species-rich orders, belong to the placental group. The first mammals appeared in the Late Triassic epoch, 40 million years after the first therapsids. The oldest known fossil among the Eutheria is the small shrewlike Juramaia sinensis, or "Jurassic mother from China", dated to 160 million years ago in the late Jurassic.Evolution of Mammals: Key Facts and Features

• Epipubic bones, found in marsupials, monotremes, and other non-placental mammals, are an ancestral feature that stiffens muscles during locomotion and reduces space needed for gestation.

• Monotremes, such as Teinolophos, lay leathery eggs and have a cloaca, a feature inherited from original amniotes.

• Hadrocodium, dating back to 195 million years ago, provides the first clear evidence of a jaw joint formed solely by the squamosal and dentary bones.

• Castorocauda and Megaconus, from 164 million years ago, provide the earliest clear evidence of hair or fur in mammals.

• Endothermy likely evolved in non-mammalian therapsids, and some ancestors of therians may have had body temperatures similar to modern therians.

• The parasagittal limb posture first appeared in the eutherian Eomaia and the metatherian Sinodelphys, both dated to 125 million years ago.

• Lactation may have originally evolved to keep eggs moist, based on evidence from monotremes.

• After the Cretaceous-Paleogene extinction event, therian mammals diversified quickly and underwent an exponential increase in body size (megafauna).

• The earliest undisputed fossils of placentals come from the early Paleocene, after the extinction of non-avian dinosaurs.

• Archicebus achilles, dating back to around 55 million years ago, is the earliest known ancestor of primates.

• All living mammals have sweat glands and produce milk to nourish their young, but fossils must be classified using other features.

• Sexual dimorphism and tooth structures reflect feeding preferences, and many herbivores have specialized stomachs.

• Mammals convert ammonia into urea, and only the mammalian kidney has a bean shape.Overview of Mammals

• Mammals have a larynx and supralaryngeal vocal tract to produce sounds, with the lungs and surrounding musculature providing the air stream and pressure required to phonate.

• Mammals can change the position of the larynx, allowing them to breathe through the nose while swallowing through the mouth, and to form both oral and nasal sounds.

• Different types of fur serve different purposes, including thermoregulation, protection, sensory purposes, waterproofing, and camouflage.

• The denseness of fur can increase an animal's insulation value, and arctic mammals especially have dense fur.

• Mammalian coats are colored for a variety of reasons, the major selective pressures including camouflage, sexual selection, communication, and thermoregulation.

• Mammals are solely gonochoric and have different reproductive systems, with marsupials having a short gestation period and generally giving birth to a number of undeveloped newborns that then undergo further development.

• Nearly all mammals are endothermic ("warm-blooded"), which requires plenty of food energy, so mammals eat more food per unit of body weight than most reptiles.

• Species maximum lifespan varies significantly among mammals, with DNA repair capability being an important determinant.

• Most vertebrates are plantigrade, walking on the whole of the underside of the foot, while many mammals are digitigrade, walking on their toes.

• Animals will use different gaits for different speeds, terrain, and situations.

• Mammals have a wide range of adaptations for different environments, including aquatic mammals that trap air in their fur to conserve heat by keeping the skin dry, and some small mammals that have the ability to produce ultrasound.

• The vocal production system is controlled by the cranial nerve nuclei in the brain, and supplied by the recurrent laryngeal nerve and the superior laryngeal nerve, branches of the vagus nerve.Mammal Locomotion, Behavior and Feeding Habits

Locomotion:

• Mammals have a vast range of gaits, and they can be grouped into categories according to their patterns of support sequence.
• Quadrupeds have three main categories: walking, running, and leaping gaits.
• Arboreal animals have elongated limbs, prehensile tails, and claws to interact with rough substrates.
• Aerial animals like bats have thin and more bones in their wings to maneuver more accurately and fly with more lift and less drag.
• Fossorial animals have a fusiform body, thickest at the shoulders and tapering off at the tail and nose, and short, stout legs.
• Aquatic mammals like cetaceans and sirenians have lost their legs and have a tail fin to propel themselves through the water.
• Semi-aquatic mammals like pinnipeds use their flippers to move in a wing-like manner similar to penguins and sea turtles.

Behavior:

• Mammals communicate by vocalizing for various purposes, including mating rituals, warning calls, indicating food sources, and for social purposes.
• Vocal communication includes the territorial calls of gibbons, frequency in greater spear-nosed bats, and the alarm calls of vervet monkeys and prairie dogs.
• Mammals signal by visual anti-predator signals such as stotting, and by scent-marking.
• Mammals have different feeding habits, including carnivorous, herbivorous, and omnivorous diets.
• The digestive tract of an herbivore is host to bacteria that ferment complex substances, and make them available for digestion.
• Some mammals are coprophagous, consuming feces to absorb the nutrients not digested when the food was first ingested.
• Mammals can hibernate in the absence of sufficient food requirements in an environment, and some mammals in hot environments aestivate in times of drought or extreme heat.

Feeding:

• The size of an animal is a factor in determining diet type, with smaller mammals being mostly insectivorous and larger animals being mostly herbivorous.
• Some mammals are omnivores and display varying degrees of carnivory and herbivory, generally leaning in favor of one more than the other.
• The dentition of hypocarnivores consists of dull, triangular carnassial teeth meant for grinding food, while hypercarnivores have conical teeth and sharp carnassials meant for slashing.
• Some physiological carnivores consume plant matter and some physiological herbivores consume meat.