Almost all female mammals give birth to live young. At birth, the young of some mammals are well developed and able to move immediately. For example, newborn horses and camels can walk within hours of birth. In other mammals, including rabbits and most rodents, the young are mostly developed, but born blind, hairless and defenseless. Many mammals, in the absence of sufficient food requirements in an environment, suppress their metabolism and conserve energy in a process known as hibernation. [196] In the pre-hibernation period, large mammals such as bears become polyphagous to increase fat reserves, while small mammals prefer to collect and store food. [197] The slowing of metabolism is accompanied by a decrease in heart and respiratory rate, as well as a decrease in internal temperatures, which in some cases can be around room temperature. For example, indoor temperatures of overwintering Arctic ground squirrels can drop to -2.9°C (26.8°F), but the head and neck always remain above 0°C (32°F). [198] Some mammals in warm environments summer during periods of drought or extreme heat, such as the dwarf fat-tailed lemur (Cheirogaleus medius). [199] The excretory system of mammals has many components. Like most other land animals, mammals are ureotelic and convert ammonia into urea, which is done by the liver as part of the urea cycle. [94] Bilirubin, a waste product of blood cells, is transmitted through bile and urine using enzymes secreted by the liver. [95] The transmission of bilirubin by bile through the intestinal tract gives mammalian faeces a pronounced brown color.
[96] Mammalian kidney features include the presence of the renal pelvis and renal pyramids, as well as a clearly distinctive cortex and medullas, due to the presence of elongated Henle loops. Only the mammalian kidney is shaped like a bean, although there are some exceptions, such as the multilobed kidneys of pinnipeds, whales and bears. [97] [98] Most adult placental mammals no longer have traces of the cloaca. In the embryo, the embryonic cloaca divides into a posterior region, which becomes a part of the anus, and an anterior region, which has different destinies depending on the sex of the individual: in women, it develops in the vestibule, which receives the urethra and vagina, while in men it forms the entire penile urethra. [98] However, tenrec, golden mole rat and some shrews retain a cesspool as adults. [99] In marsupials, the genital tract is separated from the anus, but externally a trace of the original cloaca remains. [98] The monotremes, which means “single hole” from the Greek, have a veritable cesspool. [100] Mammals (from the Latin mamma `breast`)[1] are a group of vertebrates that form the class of mammals (/məˈmeɪli.ə/), characterized by the presence of mammary glands, which in females produce milk to feed (nurs) their young, a neocortex (a region of the brain), fur or hair, and three bones of the middle ear. These characteristics distinguish them from reptiles (including birds), from which they separated in the Carboniferous more than 300 million years ago. About 6,400 recent species of mammals have been described, divided into 29 orders. The most important orders in terms of number of species are rodents, bats and eulipotyphla (hedgehogs, moles, shrews and others).
The next three are primates (including humans, monkeys, monkeys and others), Artiodactyla (even-toed whales and ungulates) and Carnivora (cats, dogs, seals and others). Some mammals, including ground squirrels and hedgehogs, hibernate in winter. Hibernation is a form of very deep sleep in which an animal`s body temperature drops. In terms of cladistics, which reflects evolutionary history, mammals are the only living members of synapsids (synapsids); this clade forms with Sauropsida (reptiles and birds) the larger clade Amniota. The first synapsids were sphenacodonts, a group that included the famous Dimetrodon. Synapsids split into several different groups of non-mammalian synapsids – traditionally and incorrectly called mammal-like reptiles or by the term pelocaussaurus, and now known as mammalian stem or protomammal – before giving rise to therapsids in the early Middle Permian.
