A penis may be contained in a prepuce when not erect, and some placentals also have a penis bone (baculum). This milk provides young with much-needed proteins, sugars, fats, vitamins, and salts. [307], Class of animals with milk-producing glands, This article is about the animal class. Some give birth to a single offspring or twins, while others bring forth a litter of young ones. Mammals generate heat mainly by keeping their metabolic rate high. Mammals can communicate and vocalize in several ways, including the production of ultrasound, scent-marking, alarm signals, singing, and echolocation. Gaits can be grouped into categories according to their patterns of support sequence. These sensitive areas are different in bats, as each bump has a tiny hair in the center, making it even more sensitive and allowing the bat to detect and collect information about the air flowing over its wings, and to fly more efficiently by changing the shape of its wings in response. [132], Among mammals, species maximum lifespan varies significantly (for example the shrew has a lifespan of two years, whereas the oldest bowhead whale is recorded to be 211 years). Most of the land mammals cannot survive in water. Like their amphibious tetrapod predecessors, they had lungs and limbs. [283], Various species are predicted to become extinct in the near future,[284] among them the rhinoceros,[285] primates,[286] pangolins,[287] and giraffes. [214] Harry Harlow set up an experiment with rhesus monkeys, presocial primates, in 1958; the results from this study showed that social encounters are necessary in order for the young monkeys to develop both mentally and sexually. This generates a lift force vector pointing forwards and upwards, and a drag force vector pointing rearwards and upwards. Mammals (from Latin mamma "breast") are a group of vertebrate animals constituting the class Mammalia (/məˈmeɪliə/), and characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur or hair, and three middle ear bones. Around 6,400 extant species of mammals have been described. McKenna & Bell (1997) and Wilson & Reader (2005) provide useful recent compendiums. [174] 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. Some hybrids have been recognized as species, such as the red wolf (though this is controversial). One idea is that sleep in mammals has to do with body size and diet, according to a 2005 study in the journal Nature. Other mammals, called herbivores, eat plants, which contain complex carbohydrates such as cellulose. Today, mammals are the only existing synapsids. [97] In marsupials, the genital tract is separate from the anus, but a trace of the original cloaca does remain externally. [37] A later eutherian relative, Eomaia, dated to 125 million years ago in the early Cretaceous, possessed some features in common with the marsupials but not with the placentals, evidence that these features were present in the last common ancestor of the two groups but were later lost in the placental lineage. [76], On average, male mammals are larger than females, with males being at least 10% larger than females in over 45% of investigated species. Animals may also have unusual gaits that are used occasionally, such as for moving sideways or backwards. Endothermy requires plenty of food energy, so mammals eat more food per unit of body weight than most reptiles. [120], The mammary glands of mammals are specialized to produce milk, the primary source of nutrition for newborns. Although these patterns hold across mammals as a whole, there is considerable variation across orders. [65] Recalibrations of genetic and morphological diversity rates have suggested a Late Cretaceous origin for placentals, and a Paleocene origin for most modern clades. In 1997, the mammals were comprehensively revised by Malcolm C. McKenna and Susan K. Bell, which has resulted in the McKenna/Bell classification. [53], The evolution of erect limbs in mammals is incomplete—living and fossil monotremes have sprawling limbs. [92] Bilirubin, a waste product derived from blood cells, is passed through bile and urine with the help of enzymes excreted by the liver.