Mammals (Class Mammalia) are a group of vertebrates that includes about 5,000 species that range in size from the minute Bumblebee Bat which measures a mere 4cm to the collasal Blue Whale which can reach lengths of 29m and weights of 176 tons. The Class Mammalia is further broken down into the various orders of mammals which include:
- Monotremata (egg-laying marsupials)
- Marsupialia (marsupials)
- Insectivora (insectivores)
- Chiroptera (bats)
- Dermoptera (flying lemurs)
- Macroscelidea (elephant shrews)
- Scandentia (tree shrews)
- Primates (primates)
- Xenarthra (anteaters and relatives)
- Pholidota (pangolins)
- Lagomorpha (rabbits, hares, and pikas)
- Rodentia (rodents)
- Cetacea (cetaceans)
- Carnivora (carnivores)
- Pinnipedea (seals and sea lions)
- Proboscidea (elephants)
- Tubulidentata (aardvark)
- Hyracoidea (hydraxes)
- Sirenia (dugongs and manatees)
- Perissodactyla (odd-toed hoofed mammals)
- Artiodactyla (even-toed hoofed mammals)
Mammals have successfully adapted to and colonized a wide variety of habitats including land, air, and water. Their widespread success is largely due to their ability to regulate their own body temperature. But mammals have also evolved many other unique adaptations to help them succeed in numerous environments, as is illustrated by their many forms of locomotion.
Mammals are endothermic, that is, they maintain a constant body temperature independent of their environment. This characteristic has enabled mammals to occupy a wide range of habitats since they can remain active in extreme temperatures. The hypothalamus, an area of the mammalian brain, maintains and monitors body temperature. By increasing metabolic rate, body temperature can be increased. By decreasing metabolic rate, body temperature can be decreased. Additionally, the ability to widen or constrict blood vessels provides another means by which heat can be carried to or restricted from parts of a mammal's body. Behavior also can help to regulate body temperature. Groups of mammals can huddle together for warmth or descend into cool burrows to avoid extreme heat.
Mammals are the only animals that have hair covering their body, a rod of cells that are strengthened by the protein keratin. The different types of hair mammals have include fur, whiskers, spines, and horns. Fur provides protection for the skin and insulates the animal. Whiskers provide sensory information for animals. Spines (such as the quills of a porcupine) provide protection.
The ways in which mammals move from place to place is greatly varied. Some mammals use four limbs to walk (such animals are referred to as quadrapedal) while others use only two (such animals are referred to as bipedal). The greatest differences in locomotion adaptation can be seen when comparing mammals from very different habitats. Dolphins, adapted for an aquatic lifestyle, have flippers to help them glide through the water. The jaguar, a terrestrial carnivore with explosive running power, is adapted for speed on land. The limbs of bats have evolved into wings enabling them to inhabit the skies.
A characteristic unique to mammals is that they all feed their young with milk produced by the female's mammary glands. Most mammals give birth to live young and almost all species have hair on their body. Some types of mammals form complex social structures. A quick summary of mammalian characteristics include:
- mammary glands
- hair
- diaphragm
- three middle-ear ossicles
- heterodont dentation
- sweat, sebaceous, and scent glands
- four-chambered heart
- large cerebral cortex
Mammals are believed to have evolved from an ancient group of reptiles called the therapsids. Therapsids lived about 225 million years ago during the Triassic Period. As they evolved, therapsids developed a lighter, more flexible skeleton and the alignment of their limbs changed over time to be directly beneath their body (unlike reptiles whose limbs are at the sides of their body).
The first mammals are thought to have been relatively small creatures (less than 10 cm long) with light, delicate skeletons. The fossil record suggests that these creatures had teeth and skulls characteristic of herbivores or predators that fed on arthropods or other vertebrates. Well-developed senses of hearing and smell suggest they may have been nocturnal creatures. another important adaptation mammals displayed was endothermy or the ability to regulate their own body temperatures.
When climate change took place in the Mesozoic Era, temperatures fluctuated and dropped. The reptiles that once dominated the land suffered in the low temperatures, while mammals were able to better compete since their body temperatures were less effected by temperature fluctuation.
The Tertiary period, which began about 70 million years ago, brought with it the dawning of the "age of mammals", a time following the extinction of many reptile species. This is when mammals diversified significantly in number and kind.
