Rats are various medium-sized, long-tailed rodents. Species of rats are found throughout the order Rodentia, but stereotypical rats are found in the genus Rattus. Other rat genera include Neotoma and Dipodomys. Rats are distinguished from mice by their size; when someone discovers a large muroid rodent, its common name includes the term rat, while if it is smaller, its name includes the term mouse. The common terms rat and mouse are not taxonomically specific. In other words, rat is not a scientific term; the best-known rat species are the brown rat. This group known as the Old World rats or true rats, originated in Asia. Rats are bigger than most Old World mice, which are their relatives, but weigh over 500 grams in the wild; the term rat is used in the names of other small mammals that are not true rats. Examples include the North American pack rats, a number of species loosely called kangaroo rats, others. Rats such as the bandicoot rat are murine rodents related to true rats but are not members of the genus Rattus.
Male rats are called bucks. A group of rats is referred to as a mischief; the common species are opportunistic survivors and live with and near humans. They may cause substantial food losses in developing countries. However, the distributed and problematic commensal species of rats are a minority in this diverse genus. Many species of rats are island endemics, some of which have become endangered due to habitat loss or competition with the brown, black, or Polynesian rat. Wild rodents, including rats, can carry many different zoonotic pathogens, such as Leptospira, Toxoplasma gondii, Campylobacter; the Black Death is traditionally believed to have been caused by the microorganism Yersinia pestis, carried by the tropical rat flea, which preyed on black rats living in European cities during the epidemic outbreaks of the Middle Ages. Another zoonotic disease linked to the rat is foot-and-mouth disease. Rats become sexually reach social maturity at about 5 to 6 months of age; the average lifespan of rats varies by species.
The black and brown rats diverged from other Old World rats in the forests of Asia during the beginning of the Pleistocene. The characteristic long tail of most rodents is a feature, extensively studied in various rat species models, which suggest three primary functions of this structure: thermoregulation, minor proprioception, a nocifensive-mediated degloving response. Rodent tails—particularly in rat models—have been implicated with a thermoregulation function that follows from its anatomical construction; this particular tail morphology is evident across the family Muridae, in contrast to the bushier tails of Sciuridae, the squirrel family. The tail is hairless and thin skinned but vascularized, thus allowing for efficient countercurrent heat exchange with the environment; the high muscular and connective tissue densities of the tail, along with ample muscle attachment sites along its plentiful caudal vertebrae, facilitate specific proprioceptive senses to help orient the rodent in a three-dimensional environment.
Lastly, murids have evolved a unique defense mechanism termed degloving that allows for escape from predation through the loss of the outermost integumentary layer on the tail. However, this mechanism is associated with multiple pathologies that have been the subject of investigation. Multiple studies have explored the thermoregulatory capacity of rodent tails by subjecting test organisms to varying levels of physical activity and quantifying heat conduction via the animals' tails. One study demonstrated a significant disparity in heat dissipation from a rat's tail relative to its abdomen; this observation was attributed to the higher proportion of vascularity in the tail, as well as its higher surface-area-to-volume ratio, which directly relates to heat's ability to dissipate via the skin. These findings were confirmed in a separate study analyzing the relationships of heat storage and mechanical efficiency in rodents that exercise in warm environments. In this study, the tail was a focal point in measuring heat modulation.
On the other hand, the tail's ability to function as a proprioceptive sensor and modulator has been investigated. As aforementioned, the tail demonstrates a high degree of muscularization and subsequent innervation that ostensibly collaborate in orienting the organism; this is accomplished by coordinated flexion and extension of tail muscles to produce slight shifts in the organism's center of mass, etc. which assists it with achieving a state of proprioceptive balance in its environment. Further mechanobiological investigations of the constituent tendons in the tail of the rat have identified multiple factors that influence how the organism navigates its environment with this structure. A particular example is that of a study in which the morphology of these tendons is explicated in detail. Namely, cell viability tests of tendons of the rat's tail demonstrate a higher proportion of living fibroblasts that produce the collagen for these fibers; as in humans, these tendons contain a high density of golgi tendon organs that help the animal assess stretching of muscle in situ and adjust accordingly by relaying the information to higher cortical areas associated with balance and movement.
Rodents are mammals of the order Rodentia, which are characterized by a single pair of continuously growing incisors in each of the upper and lower jaws. About 40% of all mammal species are rodents, they are the most diversified mammalian order and live in a variety of terrestrial habitats, including human-made environments. Species can be fossorial, or semiaquatic. Well-known rodents include mice, squirrels, prairie dogs, chinchillas, beavers, guinea pigs, hamsters and capybaras. Other animals such as rabbits and pikas, whose incisors grow continually, were once included with them, but are now considered to be in a separate order, the Lagomorpha. Nonetheless and Lagomorpha are sister groups, sharing a most recent common ancestor and forming the clade of Glires. Most rodents are small animals with robust bodies, short limbs, long tails, they use their sharp incisors to gnaw food, excavate burrows, defend themselves. Most eat seeds or other plant material, they tend to be social animals and many species live in societies with complex ways of communicating with each other.
Mating among rodents can vary from monogamy, to polygyny, to promiscuity. Many have litters of altricial young, while others are precocial at birth; the rodent fossil record dates back to the Paleocene on the supercontinent of Laurasia. Rodents diversified in the Eocene, as they spread across continents, sometimes crossing oceans. Rodents reached both South America and Madagascar from Africa and were the only terrestrial placental mammals to reach and colonize Australia. Rodents have been used as food, for clothing, as pets, as laboratory animals in research; some species, in particular, the brown rat, the black rat, the house mouse, are serious pests and spoiling food stored by humans and spreading diseases. Accidentally introduced species of rodents are considered to be invasive and have caused the extinction of numerous species, such as island birds isolated from land-based predators; the distinguishing feature of the rodents is their pairs of continuously growing, razor-sharp, open-rooted incisors.
These incisors little enamel on the back. Because they do not stop growing, the animal must continue to wear them down so that they do not reach and pierce the skull; as the incisors grind against each other, the softer dentine on the rear of the teeth wears away, leaving the sharp enamel edge shaped like the blade of a chisel. Most species have up to 22 teeth with no canines or anterior premolars. A gap, or diastema, occurs between the cheek teeth in most species; this allows rodents to suck in their cheeks or lips to shield their mouth and throat from wood shavings and other inedible material, discarding this waste from the sides of their mouths. Chinchillas and guinea pigs have a high-fiber diet. In many species, the molars are large, intricately structured, cusped or ridged. Rodent molars are well equipped to grind food into small particles; the jaw musculature is strong. The lower jaw is pulled backwards during chewing. Rodent groups differ in the arrangement of the jaw muscles and associated skull structures, both from other mammals and amongst themselves.
The Sciuromorpha, such as the eastern grey squirrel, have a large deep masseter, making them efficient at biting with the incisors. The Myomorpha, such as the brown rat, have enlarged temporalis muscles, making them able to chew powerfully with their molars; the Hystricomorpha, such as the guinea pig, have larger superficial masseter muscles and smaller deep masseter muscles than rats or squirrels making them less efficient at biting with the incisors, but their enlarged internal pterygoid muscles may allow them to move the jaw further sideways when chewing. The cheek pouch is a specific morphological feature used for storing food and is evident in particular subgroups of rodents like kangaroo rats, hamsters and gophers which have two bags that may range from the mouth to the front of the shoulders. True mice and rats do not contain this structure but their cheeks are elastic due to a high degree of musculature and innervation in the region. While the largest species, the capybara, can weigh as much as 66 kg, most rodents weigh less than 100 g.
The smallest rodent is the Baluchistan pygmy jerboa, which averages only 4.4 cm in head and body length, with adult females weighing only 3.75 g. Rodents have wide-ranging morphologies, but have squat bodies and short limbs; the fore limbs have five digits, including an opposable thumb, while the hind limbs have three to five digits. The elbow gives the forearms great flexibility; the majority of species are plantigrade, walking on both the palms and soles of their feet, have claw-like nails. The nails of burrowing species tend to be long and strong, while arboreal rodents have shorter, sharper nails. Rodent species use a wide variety of methods of locomotion including quadrupedal walking, burrowing, bipedal hopping and gliding. Scaly-tailed squirrels and flying squirrels, although not related, can both glide from tree to tree using parachute-like membranes that stretch from the fore to the hind limbs; the agouti is antelope-like, being digitigrade and having hoof-like nails. The majority of rodents have tails, which can be of many shapes and siz
Domestic sheep are quadrupedal, ruminant mammals kept as livestock. Like most ruminants, sheep are members of the even-toed ungulates. Although the name sheep applies to many species in the genus Ovis, in everyday usage it always refers to Ovis aries. Numbering a little over one billion, domestic sheep are the most numerous species of sheep. An adult female sheep is referred to as a ewe, an intact male as a ram or a tup, a castrated male as a wether, a younger sheep as a lamb. Sheep are most descended from the wild mouflon of Europe and Asia. One of the earliest animals to be domesticated for agricultural purposes, sheep are raised for fleeces and milk. A sheep's wool is the most used animal fiber, is harvested by shearing. Ovine meat is called lamb when from younger animals and mutton when from older ones in Commonwealth countries, lamb in the United States. Sheep continue to be important for wool and meat today, are occasionally raised for pelts, as dairy animals, or as model organisms for science.
Sheep husbandry is practised throughout the majority of the inhabited world, has been fundamental to many civilizations. In the modern era, New Zealand, the southern and central South American nations, the British Isles are most associated with sheep production. Sheepraising has a large lexicon of unique terms which vary by region and dialect. Use of the word sheep began in Middle English as a derivation of the Old English word scēap. A group of sheep is called a herd or mob. Many other specific terms for the various life stages of sheep exist related to lambing and age. Being a key animal in the history of farming, sheep have a entrenched place in human culture, find representation in much modern language and symbology; as livestock, sheep are most associated with pastoral, Arcadian imagery. Sheep figure in many mythologies—such as the Golden Fleece—and major religions the Abrahamic traditions. In both ancient and modern religious ritual, sheep are used as sacrificial animals; the exact line of descent between domestic sheep and their wild ancestors is unclear.
The most common hypothesis states. Sheep were among the first animals to be domesticated by humankind. C in Mesopotamia; the rearing of sheep for secondary products, the resulting breed development, began in either southwest Asia or western Europe. Sheep were kept for meat and skins. Archaeological evidence from statuary found at sites in Iran suggests that selection for woolly sheep may have begun around 6000 BC, the earliest woven wool garments have been dated to two to three thousand years later. Sheep husbandry spread in Europe. Excavations show that in about 6000 BC, during the Neolithic period of prehistory, the Castelnovien people, living around Châteauneuf-les-Martigues near present-day Marseille in the south of France, were among the first in Europe to keep domestic sheep. From its inception, ancient Greek civilization relied on sheep as primary livestock, were said to name individual animals. Ancient Romans kept sheep on a wide scale, were an important agent in the spread of sheep raising.
Pliny the Elder, in his Natural History, speaks at length about wool. European colonists spread the practice to the New World from 1493 onwards. Domestic sheep are small ruminants with a crimped hair called wool and with horns forming a lateral spiral. Domestic sheep differ from their wild relatives and ancestors in several respects, having become uniquely neotenic as a result of selective breeding by humans. A few primitive breeds of sheep retain some of the characteristics of their wild cousins, such as short tails. Depending on breed, domestic sheep may have no horns at all, or horns in both sexes, or in males only. Most horned breeds have a single pair. Another trait unique to domestic sheep as compared to wild ovines is their wide variation in color. Wild sheep are variations of brown hues, variation within species is limited. Colors of domestic sheep range from pure white to dark chocolate brown, spotted or piebald. Selection for dyeable white fleeces began early in sheep domestication, as white wool is a dominant trait it spread quickly.
However, colored sheep do appear in many modern breeds, may appear as a recessive trait in white flocks. While white wool is desirable for large commercial markets, there is a niche market for colored fleeces for handspinning; the nature of the fleece varies among the breeds, from dense and crimped, to long and hairlike. There is variation of wool type and quality among members of the same flock, so wool classing is a step in the commercial processing of the fibre. Depending on breed, sheep show a range of weights, their rate of growth and mature weight is a heritable trait, selected for in breeding. Ewes weigh between 45 and 100 kilograms, rams between 45 and 160 kilograms; when all deciduous teeth have erupted, the sheep has 20 teeth. Mature sheep have 32 teeth; as with other ruminants, the front teeth in the lower jaw bite against a hard, toothless pad in the upper jaw. These are used to pick off vegetation the rear
The estrous cycle or oestrus cycle is the recurring physiological changes that are induced by reproductive hormones in most mammalian therian females. Estrous cycles start after sexual maturity in females and are interrupted by anestrous phases or by pregnancies. Estrous cycles continue until death; some animals may display bloody vaginal discharge mistaken for menstruation. Mammals share the same reproductive system, including the regulatory hypothalamic system that produces gonadotropin-releasing hormone in pulses, the pituitary gland that secretes follicle-stimulating hormone and luteinizing hormone, the ovary itself that releases sex hormones including estrogens and progesterone. However, species vary in the detailed functioning. One difference is that animals that have estrous cycles resorb the endometrium if conception does not occur during that cycle. Animals that have menstrual cycles shed the endometrium through menstruation instead. Another difference is sexual activity. In species with estrous cycles, females are only sexually active during the estrus phase of their cycle.
This is referred to as being "in heat". In contrast, females of species with menstrual cycles can be sexually active at any time in their cycle when they are not about to ovulate. Humans have menstrual cycles rather than estrous cycles. They, unlike most other species, have concealed ovulation, a lack of obvious external signs to signal estral receptivity at ovulation. There are, subtle signs to which human males may favorably respond, including changes in a woman's scent and facial appearance; some research suggests that women tend to have more sexual thoughts and are more prone to sexual activity right before ovulation. Animals with estrous cycles have unmistakable outward displays of receptivity, ranging from engorged and colorful genitals to behavioral changes like mating calls. See Menstrual cycle § Cycles and phases for more information. Estrus is derived from Greek οἶστρος oîstros; this refers to the gadfly in Ancient Greek mythology that Hera sent to torment Io, won in her heifer form by Zeus.
Euripides used oestrus to indicate'frenzy', to describe madness. Homer uses the word to describe panic. Plato uses it to refer to an irrational drive and to describe the soul "driven and drawn by the gadfly of desire". Somewhat more aligned to current meaning and usage of estrus, Herodotus uses oîstros to describe the desire of fish to spawn; the earliest use in English was with a meaning of'frenzied passion'. In 1900, it was first used to describe'rut in animals. In British and most Commonwealth English, the spelling is œstrus. In all English spellings, the noun ends in - the adjective in - ous, thus in North American English, a mammal may be described as "in estrus" when it is in that particular part of the estrous cycle. A four-phase terminology is used in reference to animals with estrous cycles. One or several follicles of the ovary start to grow, their number is species specific. This phase can last as little as one day or as long as three weeks, depending on the species. Under the influence of estrogen the lining in the uterus starts to develop.
Some animals may experience vaginal secretions. The female is not yet sexually receptive. Variant terms for proestrus include pro-oestrus and pro-oestrum. Estrus or oestrus refers to the phase. Under regulation by gonadotropic hormones, ovarian follicles mature and estrogen secretions exert their biggest influence; the female exhibits sexually receptive behavior, a situation that may be signaled by visible physiologic changes. Estrus is seen in the mammalian species, including primates, it is thought that this increased sexual receptivity serves the function of helping the female obtain mates with superior genetic quality. This phase is sometimes called oestrum. In some species, the labia are reddened. Ovulation may occur spontaneously in some species. Among quadrupeds, a signal trait of estrus is the lordosis reflex, in which the animal spontaneously elevates her hindquarters; this phase is characterized by the activity of the corpus luteum. The signs of estrogen stimulation subside and the corpus luteum starts to form.
The uterine lining begins to appear. In the absence of pregnancy the diestrus phase terminates with the regression of the corpus luteum; the lining in the uterus is reorganized for the next cycle. Other spellings include metoestrus, metoestrum, dioestrus and dioestrum. Anestrus refers to the phase; this is a seasonal event and controlled by light exposure through the pineal gland that releases melatonin. Melatonin may repress stimulation of reproduction in long-day breeders and stimulate reproduction in short-day breeders. Melatonin is thought to act by regulating the hypothalamic pulse activity of the gonadotropin-releasing hormone. Anestrus is induced by time of year, lactation, significant illness, chro
Fertilisation or fertilization known as generative fertilisation, pollination, fecundation and impregnation, is the fusion of gametes to initiate the development of a new individual organism or offspring. This cycle of fertilisation and development of new individuals is called sexual reproduction. During double fertilisation in angiosperms the haploid male gamete combines with two haploid polar nuclei to form a triploid primary endosperm nucleus by the process of vegetative fertilisation. In Antiquity, Aristotle conceived the formation of new individuals through fusion of male and female fluids, with form and function emerging in a mode called by him as epigenetic. In 1784, Spallanzani established the need of interaction between the female's ovum and male's sperm to form a zygote in frogs. In 1827, von Baer observed a therian mammalian egg for the first time. Oscar Hertwig, in Germany, described the fusion of ova from sea urchin; the evolution of fertilisation is related to the origin of meiosis, as both are part of sexual reproduction, originated in eukaryotes.
There are two conflicting theories on how the couple meiosis–fertilisation arose. One is; the other is. The gametes that participate in fertilisation of plants are the pollen, the egg cell. Various families of plants have differing methods. In Bryophyte land plants, fertilisation takes place within the archegonium. In flowering plants a second fertilisation event involves another sperm cell and the central cell, a second female gamete. In flowering plants there are two sperm from each pollen grain. In seed plants, after pollination, a pollen grain germinates, a pollen tube grows and penetrates the ovule through a tiny pore called a micropyle; the sperm are transferred from the pollen through the pollen tube to the ovule. Pollen tube growth Unlike animal sperm, motile, plant sperm is immotile and relies on the pollen tube to carry it to the ovule where the sperm is released; the pollen tube penetrates the stigma and elongates through the extracellular matrix of the style before reaching the ovary.
Near the receptacle, it breaks through the ovule through the micropyle and the pollen tube "bursts" into the embryo sac, releasing sperm. The growth of the pollen tube has been believed to depend on chemical cues from the pistil, however these mechanisms were poorly understood until 1995. Work done on tobacco plants revealed a family of glycoproteins called TTS proteins that enhanced growth of pollen tubes. Pollen tubes in a sugar free pollen germination medium and a medium with purified TTS proteins both grew. However, in the TTS medium, the tubes grew at a rate 3x that of the sugar-free medium. TTS proteins were placed on various locations of semi in vevo pollinated pistils, pollen tubes were observed to extend toward the proteins. Transgenic plants lacking the ability to produce TTS proteins exhibited slower pollen tube growth and reduced fertility. Rupture of pollen tube The rupture of the pollen tube to release sperm in Arabidopsis has been shown to depend on a signal from the female gametophyte.
Specific proteins called FER protein kinases present in the ovule control the production of reactive derivatives of oxygen called reactive oxygen species. ROS levels have been shown via GFP to be at their highest during floral stages when the ovule is the most receptive to pollen tubes, lowest during times of development and following fertilization. High amounts of ROS activate Calcium ion channels in the pollen tube, causing these channels to take up Calcium ions in large amounts; this increased uptake of calcium causes the pollen tube to rupture, release its sperm into the ovule. Pistil feeding assays in which plants were fed diphenyl iodonium chloride suppressed ROS concentrations in Arabidopsis, which in turn prevented pollen tube rupture. Bryophyte is a traditional name used to refer to all embryophytes that do not have true vascular tissue and are therefore called "non-vascular plants"; some bryophytes do have specialised tissues for the transport of water. A fern is a member of a group of 12,000 species of vascular plants that reproduce via spores and have neither seeds nor flowers.
They differ from mosses by being vascular. They leaves, like other vascular plants. Most ferns have what are called fiddleheads that expand into fronds, which are each delicately divided; the gymnosperms are a group of seed producing plants that includes conifers, Cycads and Gnetales. The term "gymnosperm" comes from the Greek composite word γυμνόσπερμος, meaning "naked seeds", after the unenclosed condition of their seeds, their naked condition stands in contrast to the seeds and ovules of flowering plants, which are enclosed within an ovary. Gymnosperm seeds develop either on the surface of scales or leaves modified to form cones, or at the end of short stalks as in Ginkgo. After being fertilised, the ovary starts to develop into the fruit. With multi-seeded fruits, multiple grains of pollen are necessary for syngamy with each ovule; the growth of the pollen tube is controlled by the vegetative cytoplasm. Hydrolytic enzymes are secreted by the pollen tube that digest the female tissue as the tube grows down the stigma and style
The Poeciliidae are a family of freshwater fishes of the order Cyprinodontiformes, the tooth-carps, include well-known live-bearing aquarium fish, such as the guppy, molly and swordtail. The original distribution of the family was the southeastern United States to north of Río de la Plata and Africa, including Madagascar. However, due to release of aquarium specimens and the widespread use of species of the genera Poecilia and Gambusia for mosquito control, poeciliids can today be found in all tropical and subtropical areas of the world. In addition and Gambusia specimens have been identified in hot springs pools as far north as Banff, Alberta. Although the whole family Poeciliidae is known as "live bearers", some species are egg-scattering with external fertilization. All African species are egg-layers, all American species are live-bearers. Among the three subfamilies, Aplocheilichthyinae is restricted to Africa, Poeciliinae is from the Americas, Procatopodinae is from Africa; this distribution suggests that the Poeciliidae predate the split between Africa and South America 100 million years ago, that live-bearing subsequently evolved in South America.
Poeciliids colonized North America through the Antilles while they were connected 44 million years ago. Poeciliids moved to Central America by the Aves land bridge on the Caribbean Plate; when South America connected to Central America three million years ago, some further dispersal southward occurred, but South American species did not move into Central America. Among the live-bearing species, differences in the mode and degree of support the female gives the developing larvae occur. Many members of the family Poeciliidae are considered to be lecithotrophic, but others are matrotrophic. Members of the genus Poeciliopsis, for example, show variable reproductive life history adaptations. Poeciliopsis monacha, P. lucida, P. prolifica form part of the same clade within that genus. However, their modes of maternal provisioning vary greatly. P. monacha can be considered to be lecithotrophic because it does not provide any resources for its offspring after fertilization - the pregnant female is a swimming egg sac.
P. lucida shows an intermediate level of matrotrophy, meaning that to a certain extent the offspring's metabolism can affect the mother's metabolism, allowing for increased nutrient exchange. P. prolifica is considered to be matrotrophic, all of the nutrients and materials needed for fetal development are supplied to the oocyte after it has been fertilized. This level of matrotrophy allows Poeciliopsis to carry several broods at different stages of development, a phenomenon known as superfetation; because the space for developing embryos is limited, viviparity reduces brood size. Superfetation can compensate for this loss by keeping embryos at various stages and sizes during development. P. Elongata, P. turneri and P. presidionis form another clade which could be considered an outgroup to the P. monacha, P.lucida, P. prolifica clade. These three species are highly matrotrophic - so much so that in 1947, C. L. Turner described the follicular cells of P. turneri as "pseudo-placenta, pseudo-chorion, pseudo-allantois".
Froese and Daniel Pauly, eds.. "Poeciliidae" in FishBase. October 2004 version. "Poeciliidae". Integrated Taxonomic Information System. Retrieved 4 June 2004
A primate is a eutherian mammal constituting the taxonomic order Primates. Primates arose 85–55 million years ago from small terrestrial mammals, which adapted to living in the trees of tropical forests: many primate characteristics represent adaptations to life in this challenging environment, including large brains, visual acuity, color vision, altered shoulder girdle, dexterous hands. Primates range in size from Madame Berthe's mouse lemur, which weighs 30 g, to the eastern gorilla, weighing over 200 kg. There are 190 -- 448 species of living primates, depending on. New primate species continue to be discovered: over 25 species were described in the first decade of the 2000s, eleven since 2010. Primates are divided into two distinct suborders; the first is the strepsirrhines - lemurs and lorisids. The second is haplorhines - the "dry-nosed" primates - tarsier and ape clades, the last of these including humans. Simians are monkeys and apes, cladistically including: the catarrhines consisting of the Old World monkeys and apes.
Forty million years ago, simians from Africa migrated to South America by drifting on debris, gave rise to the New World monkeys. Twenty five million years ago the remaining Old World simians split into Old World monkeys. Common names for the simians are the baboons, macaques and great apes. Primates have large brains compared to other mammals, as well as an increased reliance on visual acuity at the expense of the sense of smell, the dominant sensory system in most mammals; these features are more developed in monkeys and apes, noticeably less so in lorises and lemurs. Some primates are trichromats, with three independent channels for conveying color information. Except for apes, primates have tails. Most primates have opposable thumbs. Many species are sexually dimorphic. Primates have slower rates of development than other sized mammals, reach maturity and have longer lifespans. Depending on the species, adults may live in solitude, in mated pairs, or in groups of up to hundreds of members; some primates, including gorillas and baboons, are terrestrial rather than arboreal, but all species have adaptations for climbing trees.
Arboreal locomotion techniques used include leaping from tree to tree and swinging between branches of trees. Primates are among the most social of animals, forming pairs or family groups, uni-male harems, multi-male/multi-female groups. Non-human primates have at four types of social systems, many defined by the amount of movement by adolescent females between groups. Most primate species remain at least arboreal: the exceptions are some great apes and humans, who left the trees for the ground and now inhabit every continent. Close interactions between humans and non-human primates can create opportunities for the transmission of zoonotic diseases virus diseases, including herpes, ebola and hepatitis. Thousands of non-human primates are used in research around the world because of their psychological and physiological similarity to humans. About 60% of primate species are threatened with extinction. Common threats include deforestation, forest fragmentation, monkey drives, primate hunting for use in medicines, as pets, for food.
Large-scale tropical forest clearing for agriculture most threatens primates. The English name "primates" is derived from Old French or French primat, from a noun use of Latin primat-, from primus; the name was given by Carl Linnaeus. The relationships among the different groups of primates were not understood until recently, so the used terms are somewhat confused. For example, "ape" has been used either as an alternative for "monkey" or for any tailless human-like primate. Sir Wilfrid Le Gros Clark was one of the primatologists who developed the idea of trends in primate evolution and the methodology of arranging the living members of an order into an "ascending series" leading to humans. Used names for groups of primates such as "prosimians", "monkeys", "lesser apes", "great apes" reflect this methodology. According to our current understanding of the evolutionary history of the primates, several of these groups are paraphyletic: a paraphyletic group is one which does not include all the descendants of the group's common ancestor.
In contrast with Clark's methodology, modern classifications identify only those groupings that are monophyletic. The cladogram below shows one possible classification sequence of the living primates: groups that use common names are shown on the right. All groups with scientific names are monophyletic, the sequence of scientific classification reflects the evolution