Generally, olfaction, hearing, tactile perception, and vision are all important sensory modalities in mammals. Olfaction plays a key role in many aspects of mammalian ecology, including foraging, mating and social communication. Many mammals use pheromones and other olfactory cues to communicate information about their reproductive status, territory, or individual or group identity. Scent-marking is commonly used to communicate among mammals. They are often transmitted through urine, feces, or the secretions of specific glands. Some mammals even use odors as defense against mammalian predators (e.g. skunks), which are especially sensitive to foul-smelling chemical defenses.
Typically, mammalian hearing is well-developed. In some species, it is the primary form of perception. Echolocation, the ability to perceive objects in the external environment by listening to echoes from sounds generated by an animal, has evolved in several groups. Echolocation is the main perception channel used in foraging and navigation in microchiropteran bats (Chiroptera) and many toothed whales and dolphins (Odontoceti), and has also evolved to a lesser degree in other species (e.g., some shrews).
Many mammals are vocal, and communicate with one another or with heterospecifics using sound. Vocalizations are used in communication between mother and offspring, between potential mates, and in a variety of other social contexts. Vocalizations can communicate individual or group identity, alarm at the presence of a predator, aggression in dominance interactions, territorial defense, and reproductive state. Communication using vocalizations is quite complex in some groups, most notably in humans.
Mammals also perceive their environment through tactile input to the hair and skin. Specialized hairs (whiskers or "vibrissae") have a sensory function, letting an animal know when it is in contact with an object in its external environment. Vibrissae are often richly innervated and well-supplied with muscles that control their position. The skin is also an important sensory organ. Often, certain portions of the skin are especially sensitive to tactile stimuli, aiding in specific functions like foraging (e.g., the fingers of primates and the nasal tentacles of star-nosed moles). Touch also serves many communication functions, and is often associated with social behavior (e.g., social grooming).
Vision is well-developed in a large number of mammals, although it is less important in many species that live underground or use echolocation. Many nocturnal animals have relatively large, well-developed eyes. Vision can be important in foraging, navigation, entraining biological rhythms to day length or season, communication, and nearly all aspects of mammalian behavior and ecology.
Communication Channels: visual ; tactile ; acoustic ; chemical
Other Communication Modes: mimicry ; duets ; choruses ; pheromones ; scent marks ; vibrations
Perception Channels: visual ; tactile ; acoustic ; ultrasound ; echolocation ; vibrations ; chemical
Overexploitation, habitat destruction and fragmentation, the introduction of exotic species, and other anthropogenic pressures threaten mammals worldwide. In the past five centuries at least 82 mammal species have gone extinct. Currently, the International Union for Conservation of Nature and Natural Resources (IUCN) has listed about 1000 species (roughly 25% of all known mammals), as being at some risk of extinction. Several factors contribute to a species' vulnerability to human-induced extinction. Species that are naturally rare or require large home ranges are often at risk due to habitat loss and fragmentation. Species that are seen to threaten humans, livestock, or crops may be directly targeted for extirpation. Those species that are exploited by humans as a resource (e.g., for their meat or fur) but are not domesticated are often depleted to critically low levels. Finally, global climate change is adversely affecting many mammals. The geographic ranges of many mammals are shifting, and these shifts often correlate with changes in local temperatures and climate. As temperatures rise, which is especially pronounced in polar regions, some mammals are unable to adjust and are consequently at risk of losing their environment.
There are three major groups of mammals, each is united by a major feature of embryonic development. Monotremes (Prototheria) lay eggs, which is the most primitive reproductive condition in mammals. Marsupials (Metatheria) give birth to highly altricial young after a very short gestation period (8 to 43 days). The young are born at a relatively early stage of morphological development. They attach to the mother's nipple and spend a proportionally greater amount of time nursing as they develop. Gestation lasts much longer in placental mammals (Eutheria). During gestation, eutherian young interact with their mother through a placenta, a complex organ that connects the embryo with the uterus. Once born, all mammals are dependent upon their mothers for milk. Aside from these few generalities, mammals exhibit a diversity of developmental and life history patterns that vary among species and larger taxonomic groups.
The Class Mammalia includes about 5000 species placed in 26 orders. Systematists do not yet agree on the exact number or on how some orders and families are related to others. The Animal Diversity Web generally follows the arrangement used by Wilson and Reeder (2005). Exciting new information, however, coming from phylogenies based on molecular evidence and from new fossils, is changing our understanding of many groups. For example, skunks have been placed in the new family Mephitidae, separate from their traditional place within the Mustelidae (Dragoo and Honeycutt 1997, Flynn et al., 2005). The Animal Diversity Web follows this revised classification. Whales almost certainly arose from within the Artiodactyla (Matthee et al. 2001; Gingerich et al. 2001). The traditional subdivision of the Chiroptera into megabats and microbats may not accurately reflect evolutionary history (Teeling et al. 2002). Even more fundamentally, molecular evidence suggests that monotremes (Prototheria, egg-laying mammals) and marsupials (Metatheria) may be more closely related to each other than to placental mammals (Eutheria) (Janke et al. 1997), and placental mammals may be organized into larger groups (Afrotheria, Laurasiatheria, Boreoeutheria, etc.) that are quite different from traditional ones (Murphy et al. 2001).
All mammals share at least three characteristics not found in other animals: 3 middle ear bones, hair, and the production of milk by modified sweat glands called mammary glands. The three middle ear bones, the malleus, incus, and stapes (more commonly referred to as the hammer, anvil, and stirrup) function in the transmission of vibrations from the tympanic membrane (eardrum) to the inner ear. The malleus and incus are derived from bones present in the lower jaw of mammalian ancestors. Mammalian hair is present in all mammals at some point in their development. Hair has several functions, including insulation, color patterning, and aiding in the sense of touch. All female mammals produce milk from their mammary glands in order to nourish newborn offspring. Thus, female mammals invest a great deal of energy caring for each of their offspring, a situation which has important ramifications in many aspects of mammalian evolution, ecology, and behavior.
Although mammals share several features in common (see Physical Description and Systematics and Taxonomic History), Mammalia contains a vast diversity of forms. The smallest mammals are found among the shrews and bats, and can weigh as little as 3 grams. The largest mammal, and indeed the largest animal to ever inhabit the planet, is the blue whale, which can weigh 160 metric tons (160,000 kg). Thus, there is a 53 million-fold difference in mass between the largest and smallest mammals! Mammals have evolved to exploit a large variety of ecological niches and life history strategies and, in concert, have evolved numerous adaptations to take advantage of different lifestyles. For example, mammals that fly, glide, swim, run, burrow, or jump have evolved morphologies that allow them to locomote efficiently; mammals have evolved a wide variety of forms to perform a wide variety of functions.
Some mammal species are considered to have a detrimental impact on human interests. Many mammals that eat fruit, seeds, and other types of vegetation are crop pests. Carnivores are often considered to be a threat to livestock or even to human lives. Mammals that are common in urban or suburban areas can become a problem if they cause damage to automobiles when they are struck on the road, or can become household pests. A few species coexist exceptionally well with people, including some feral domesticated mammals (e.g., rats, house mice, pigs, cats, and dogs). As a result of either intentional or unintentional introductions near human habitation, these animals have had considerable negative impacts on the local biota of many regions of the world, especially the endemic biota of oceanic islands.
Many mammals can transmit diseases to humans or livestock. The bubonic plague is perhaps the most well-known example. Plague is spread via fleas that are carried by rodents. Rabies, which can be transmitted among mammalian species, is also a significant threat to livestock and can kill humans as well.
Negative Impacts: injures humans (bites or stings, causes disease in humans , carries human disease); crop pest; causes or carries domestic animal disease ; household pest
Mammals are a vital economic resource for humans. Many mammals have been domesticated to provide products such as meat and milk (e.g., cows and goats) or fiber (sheep and alpacas). Many mammals are kept as service animals or pets (e.g., dogs, cats, ferrets). Mammals are important for the ecotourism industry as well. Consider the many people who travel to zoos or to all corners of the world to see animals like elephants, lions, or whales. Mammals (e.g., bats) often help control populations of crop pests. Some species like Norway rats and domestic mice are vitally important in medical and other scientific research; because humans are mammals, other mammals can serve as models in human medicine and research.
Positive Impacts: pet trade ; food ; body parts are source of valuable material; ecotourism ; research and education; produces fertilizer; controls pest population
The ecological roles, or niches, filled by the nearly 5000 mammal species are quite diverse. There are predators and prey, carnivores, omnivores, and herbivores, species that create or greatly modify their habitat and thus the habitat and structure of their communities [e.g., beavers damming streams, large populations of ungulates (Artiodactyla and Perissodactyla) grazing in grasslands, moles digging in the earth]. In part because of their high metabolic rates, mammals often play an ecological role that is disproportionately large compared to their numerical abundance. Thus, many mammals may be keystone predators in their communities or play important roles in seed dispersal or pollination. The ecosystem roles that mammals play are so diverse that it is difficult to generalize across the group. Despite their low species diversity, compared to other animal groups, mammals have a substantial impact on global biodiversity.
Ecosystem Impact: disperses seeds; pollinates; creates habitat; biodegradation ; soil aeration ; keystone species
As a group, mammals eat an enormous variety of organisms. Many mammals can be carnivores (e.g., most species within Carnivora), herbivores (e.g., Perissodactyla, Artiodactyla), or omnivores (e.g., many primates). Mammals eat both invertebrates and vertebrates (including other mammals), plants (including fruit, nectar, foliage, wood, roots, seeds, etc.) and fungi. Being endotherms, mammals require much more food than ectotherms of similar proportions. Thus, relatively few mammals can have a large impact on the populations of their food items.
Foraging Behavior: stores or caches food ; filter-feeding
Primary Diet: carnivore (Eats terrestrial vertebrates, Piscivore , Eats eggs, Sanguivore , Eats body fluids, Insectivore , Eats non-insect arthropods, Molluscivore , Scavenger ); herbivore (Folivore , Frugivore , Granivore , Lignivore, Nectarivore ); omnivore ; planktivore ; mycophage ; coprophage
Mammals can be found on all continents, in all oceans, and on many oceanic islands of the world.
Biogeographic Regions: nearctic (Introduced , Native ); palearctic (Introduced , Native ); oriental (Introduced , Native ); ethiopian (Introduced , Native ); neotropical (Introduced , Native ); australian (Introduced , Native ); antarctica (Native ); oceanic islands (Introduced , Native ); arctic ocean (Native ); indian ocean (Native ); atlantic ocean (Native ); pacific ocean (Native ); mediterranean sea (Native )
Other Geographic Terms: cosmopolitan
Different species of mammals have evolved to live in nearly all terrestrial and aquatic habitats on the planet. Mammals inhabit every terrestrial biome, from deserts to tropical rainforests to polar icecaps. Many species are arboreal, spending most or all of their time in the forest canopy. One group (bats) have even evolved powered flight, which represents only the third time that this ability has evolved in vertebrates (the other two groups being birds and extinct Pterosaurs).
Many mammals are partially aquatic, living near lakes, streams, or the coastlines of oceans (e.g., seals, sea lions, walruses, otters, muskrats, and many others). Whales and dolphins (Cetacea) are fully aquatic, and can be found in all oceans of the world, and some rivers. Whales can be found in polar, temperate, and tropical waters, both near shore and in the open ocean, and from the water's surface to depths of over 1 kilometer.
Habitat Regions: temperate ; tropical ; polar ; terrestrial ; saltwater or marine ; freshwater
Terrestrial Biomes: tundra ; taiga ; desert or dune ; savanna or grassland ; chaparral ; forest ; rainforest ; scrub forest ; mountains ; icecap
Aquatic Biomes: pelagic ; reef ; lakes and ponds; rivers and streams; coastal ; brackish water
Wetlands: marsh ; swamp ; bog
Other Habitat Features: urban ; suburban ; agricultural ; riparian ; estuarine ; intertidal or littoral
Just as mammals vary greatly in size, they also vary greatly in lifespan. Generally, smaller mammals live short lives and larger mammals live longer lives. Bats (Chiroptera) are an exception to this pattern, they are relatively small mammals that can live for one or more decades in natural conditions, considerably longer than natural lifespans of significantly larger mammals. Mammalian lifespans range from one year or less to 70 or more years in the wild. Bowhead whales may live more than 200 years.
All mammals have hair at some point during their development, and most mammals have hair their entire lives. Adults of some species lose most or all of their hair but, even in mammals like whales and dolphins, hair is present at least during some phase of ontogeny. Mammalian hair, made of a protein called keratin, serves at least four functions. First, it slows the exchange of heat with the environment (insulation). Second, specialized hairs (whiskers or "vibrissae") have a sensory function, letting an animal know when it is in contact with an object in its environment. Vibrissae are often richly innervated and well-supplied with muscles that control their position. Third, hair affects appearance through its color and pattern. It may serve to camouflage predators or prey, to warn predators of a defensive mechanism (for example, the conspicuous color pattern of a skunk is a warning to predators), or to communicate social information (for example, threats, such as the erect hair on the back of a wolf; sex, such as the different colors of male and female capuchin monkeys; or the presence of danger, such as the white underside of the tail of a white-tailed deer). Fourth, hair provides some protection, either simply by providing an additional protective layer (against abrasion or sunburn, for example) or by taking on the form of dangerous spines that deter predators (porcupines, spiny rats, others).
Mammals are typically characterized by their highly differentiated teeth. Teeth are replaced just once during an individual's life (a condition called diphyodonty). Other characteristics found in most mammals include: a lower jaw made up of a single bone, the dentary; four-chambered hearts; a secondary palate separating air and food passages in the mouth; a muscular diaphragm separating thoracic and abdominal cavities; a highly developed brain; endothermy and homeothermy; separate sexes with the sex of an embryo being determined by the presence of a Y or 2 X chromosomes; and internal fertilization.
Often, characteristics of skulls and dentition are used to define and differentiate mammalian groups. To make these easier to comprehend within the accounts of lower mammalian taxa, we provide links to dorsal, ventral, and lateral views of the skull of a dog on which the major bones, foramina, and processes have been labelled. Closeups of the basicranial region, orbital region, and lingual and labial views of a mandible are also available. A partially labeled full skeleton of a raccoon has also been prepared.
Other Physical Features: endothermic ; heterothermic ; homoiothermic; bilateral symmetry ; polymorphic ; venomous
Sexual Dimorphism: sexes alike; female larger; male larger; sexes colored or patterned differently; female more colorful; male more colorful; sexes shaped differently; ornamentation
Predation is a significant source of mortality for many mammals. Except for those few species that are top predators, mammals are preyed upon by many other organisms, including other mammals. Other groups that typically eat mammals are predatory birds and reptiles. Many species cope with predation through avoidance strategies such as cryptic coloration, by restricting foraging to times when predators may not be abundant, or through their sociality. Some mammals also have defensive chemicals (e.g., skunks) or bear some type of protective armor or physical defense (e.g., armadillos, pangolins, New World porcupines and Old World porcupines).
Known Predators:
Anti-predator Adaptations: aposematic ; cryptic
Generally, most mammalian species are either polygynous (one male mates with multiple females) or promiscuous (both males and females have multiple mates in a given reproductive season). Because females incur such high costs during gestation and lactation, it is often the case that male mammals can produce many more offspring in a mating season than can females. As a consequence, the most common mating system in mammals is polygyny, with relatively few males fertilizing multiple females and many males fertilizing none. This scenario sets the stage for intense male-male competition in many species, and also the potential for females to be choosy when it comes to which males will sire her offspring. As a consequence of the choices females make and the effort males put into acquiring matings, many mammals have complex behaviors and morphologies associated with reproduction. Many mammal groups are marked by sexual dimorphism as a result of selection for males that can better compete for access to females.
About 3 percent of mammalian species are monogamous, with males only mating with a single female each season. In these cases, males provide at least some care to their offspring. Often, mating systems may vary within species depending upon local environmental conditions. For example, when resources are low, males may mate with only a single female and provide care for the young. When resources are abundant, the mother may be able to care for young on her own and males will attempt to sire offspring with multiple females.
Other mating systems such as polyandry can also be found among mammals. Some species (e.g. common marmosets and African lions) display cooperative breeding, in which groups of females, and sometimes males, share the care of young from one or more females. Naked mole rats have a unique mating system among mammals. Like social insects (Hymenoptera and Isoptera), naked mole rats are eusocial, with a queen female mating with several males and bearing all of the young in the colony. Other colony members assist in the care of her offspring and do not reproduce themselves.
Mating System: monogamous ; polyandrous ; polygynous ; polygynandrous (promiscuous) ; cooperative breeder ; eusocial
Many mammals are seasonal breeders, with environmental stimuli such as day length, resource intake and temperature dictating when mating occurs. Females of some species store sperm until conditions are favorable, after which their eggs are fertilized. In other mammals, eggs may be fertilized shortly after copulation, but implantation of the embryo into the uterine lining may be delayed (“delayed implantation”). A third form of delayed gestation is "delayed development", in which development of the embryo may be arrested for some time. Seasonal breeding and delays in fertilzation, implantation, or development are all reproductive strategies that help mammals coordinate the birth of offspring with favorable environmental conditions to increase the chances of offspring survival.
Some mammals give birth to many altricial young in each bout of reproduction. Despite being born in a relatively underdeveloped state, young of this type tend to reach maturity relatively quickly, soon producing many altricial young of their own. Mortality in these species tends to be high and average lifespans are generally short. Many species that exemplify this type of life history strategy can be found among the rodents and insectivores. At the other end of the life history spectrum, many mammals give birth to one or a few precocial young in each bout of reproduction. These species tend to live in stable environments where competition for resources is a key to survival and reproductive success. The strategy for these species is to invest energy and resources in a few, highly developed offspring that will grow to be good competitors. Cetaceans, primates and artiodactyls are examples of orders that follow this general pattern.
Among mammals, many reproductive strategies can be observed, and the patterns listed above are the extremes of a continuum encompassing this variation. Environmental factors, as well as physiological and historical constraints all contribute to the pattern of reproduction found in any population or species. Differences in these factors among species have led to the diversity of life history traits among mammals.
Key Reproductive Features: semelparous ; iteroparous ; seasonal breeding ; year-round breeding ; gonochoric/gonochoristic/dioecious (sexes separate); sexual ; induced ovulation ; fertilization (Internal ); viviparous ; oviparous ; sperm-storing ; delayed fertilization ; delayed implantation ; embryonic diapause ; post-partum estrous
A fundamental component of mammalian evolution, behavior, and life history is the extended care females must give to their offspring. Investment begins even before a female's eggs become fertilized. All female mammals undergo some form of estrus cycle in which eggs develop and become ready for potential fertilization. Hormones regulate changes in various aspects of female physiology throughout the cycle (e.g., the thickening of the uterine lining) and prepare the female for possible fertilization and gestation. Once fertilization occurs, females nurture their embryos in one of three ways--either by attending eggs that are laid externally (Prototheria), nursing highly altricial young (often within a pouch, or "marsupium"; Metatheria), or by nourishing the developing embryos with a placenta that is attached directly to the uterine wall for a long gestation period (Eutheria). Gestation in eutherians is metabolically expensive. The costs incurred during gestation depend upon the number of offspring in a litter and the degree of development each embryo undergoes.
Once the young are born (or hatch, in the case of monotremes) females feed their newborn young with milk, a substance rich in fats and protein. Because females must produce this high-energy substance, lactation is far more energetically expensive than gestation. Once mammals are born they must maintain their own body temperatures, no longer being able to depend on their mother for thermoregulation, as was the case during pregnancy. Lactating females must provide enough milk for their offspring to maintain their body temperatures as well as to grow and develop. In addition to feeding their young, females must protect them from predators. In some species, young remain with their mothers even beyond lactation for an extended period of behavioral development and learning.
Depending upon the species and environmental conditions, male mammals may either provide no care, or may invest some or a great deal of care to their offspring. Care by males often involves defending a territory, resources, or the offspring themselves. Males may also provision females and young with food.
Mammalian young are often born in an altricial state, needing extensive care and protection for a period after birth. Most mammals make use of a den or nest for the protection of their young. Some mammals, however, are born well-developed and are able to locomote on their own soon after birth. Most notable in this regard are artiodactyls such as wildebeest or giraffes. Cetacean young must also swim on their own shortly after birth.
Parental Investment: pre-fertilization (Provisioning, Protecting: Female); pre-hatching/birth (Provisioning: Female, Protecting: Female); pre-weaning/fledging (Provisioning: Male, Female, Protecting: Male, Female); pre-independence (Provisioning: Male, Female, Protecting: Male, Female); post-independence association with parents; extended period of juvenile learning; inherits maternal/paternal territory; maternal position in the dominance hierarchy affects status of young
