The Sámi people are a Finno-Ugric people inhabiting Sápmi, which today encompasses large parts of Norway and Sweden, northern parts of Finland, the Murmansk Oblast of Russia. The Sámi have been known in English as Lapps or Laplanders. Sámi ancestral lands are not well-defined, their traditional languages are the Sámi languages and are classified as a branch of the Uralic language family. Traditionally, the Sámi have pursued a variety of livelihoods, including coastal fishing, fur trapping, sheep herding, their best-known means of livelihood is semi-nomadic reindeer herding. About 10% of the Sámi are connected to reindeer herding, providing them with meat and transportation. 2,800 Sámi people are involved in reindeer herding on a full-time basis. For traditional, environmental and political reasons, reindeer herding is reserved for only Sami people in some regions of the Nordic countries. Sámi refer to themselves as Sámit or Sápmelaš, the word Sámi being inflected into various grammatical forms.
It has been proposed that Sámi, Häme, Suomi are of the same origin and borrowed from the Baltic word *žēmē, meaning'land'. The Baltic word is cognate with Slavic zemlja, which means'land'; the Sámi institutions – notably the parliaments, radio and TV stations, etc. – all use the term Sámi, including when addressing outsiders in Norwegian, Finnish, or English. In Norwegian, the Sámi are today referred to by the Norwegianized form Same; the first probable historical mention of the Sámi, naming them Fenni, was by Tacitus, about 98 A. D. Variants of Finn or Fenni were in wide use in ancient times, judging from the names Fenni and Phinnoi in classical Roman and Greek works. Finn was the name used by Norse speakers to refer to the Sámi, as attested in the Icelandic Eddas and Norse sagas; the etymology is somewhat uncertain, but the consensus seems to be that it is related to Old Norse finna, from proto-Germanic *finthanan, the logic being that the Sámi, as hunter-gatherers "found" their food, rather than grew it.
As Old Norse developed into the separate Scandinavian languages, Swedes took to using Finn to refer to inhabitants of what is now Finland, while the Sámi came to be called Lapps. In Norway, however, Sámi were still called Finns at least until the modern era, some northern Norwegians will still use Finn to refer to Sámi people, although the Sámi themselves now consider this to be an inappropriate term. Finnish immigrants to Northern Norway in the 18th and 19th centuries were referred to as Kvens to distinguish them from the Sámi "Finns". Ethnic Finns are a distinct group from Sámi; the word Lapp can be traced to Icelandic lappir of Finnish origin. It is unknown how the word Lapp came into the Norse language, but one of the first written mentions of the term is in the Gesta Danorum by 12th-century Danish historian Saxo Grammaticus, who referred to the two Lappias, although he still referred to the Sami as Finns. In fact, Saxo never explicitly connects the Sami with the "two Laplands"; the term "Lapp" was popularized and became the standard terminology by the work of Johannes Schefferus, Acta Lapponica.
The Sámi are known in other languages by the exonyms Lap, Lapp, or Laplanders, although these are considered derogatory terms, while others accept at least the name Lappland. Variants of the name Lapp were used in Sweden and Finland and, through Swedish, adopted by many major European languages: English: Lapps. In Russian the corresponding term is лопари́ and in Ukrainian лопарі́. In Finland and Sweden, Lapp is common in place names, such as Lappi and Lapinlahti in Finland; as mentioned, Finn is a common element in Norwegian place names, whereas Lapp is exceedingly rare. Terminological issues in Finnish are somewhat different. Finns living in Finnish Lapland call themselves lappilainen, whereas the similar word for the Sámi people is lappalainen; this can be confusing for foreign visitors because of the similar lives Finns and Sámi people live today in Lapland. Lappalainen is a common family name in Finland. In the Scandinavian languages, the word saamelainen is used, at least in official contexts.
Since prehistoric times, the Sami people of Arctic Europe have lived and worked in an area that stretches over the northern parts of the regions now known as Norway, Sweden and the Russian Kola Peninsula. They have inhabited the northern arctic and sub-arctic regions of Fenno-Scandinavia and Russia for at least 5,000 years; the Sami are counted among the Arctic peoples and are members of circumpolar groups such as the Arctic Council Indigeno
Brown adipose tissue
Brown adipose tissue or brown fat makes up the adipose organ together with white adipose tissue. Brown adipose tissue is found in all mammals. Classification of brown fat refers to two distinct cell populations with similar functions; the first shares a common embryological origin with muscle cells, found in larger "classic" deposits. The second develops from white adipocytes; these adipocytes are found interspersed in white adipose tissue and are named'beige' or'brite'. Brown adipose tissue is abundant in newborns and in hibernating mammals, it is present and metabolically active in adult humans, but its prevalence decreases as humans age. Its primary function is thermoregulation. In addition to heat produced by shivering muscle, brown adipose tissue produces heat by non-shivering thermogenesis. In contrast to white adipocytes, which contain a single lipid droplet, brown adipocytes contain numerous smaller droplets and a much higher number of mitochondria, which gives the tissue its color. Brown fat contains more capillaries than white fat.
These supply the tissue with oxygen and distribute the produced heat throughout the body. The presence of brown adipose tissue in adult humans was discovered during FDG-PET scans to detect metastatic cancers. Using these scans and data from human autopsies, several brown adipose tissue depots have been identified. In infants, brown adipose tissue depots include, but are not limited to: interscapular, suprarenal, para-aortic and around the pancreas and trachea; these deposits get more white fat-like during adulthood. In adults, the depots that are most detected in FDG-PET scans are the supraclavicular, mediastinal, para-aortic and suprarenal ones, it remains to be determined whether these depots are'classical' brown adipose tissue or beige/brite fat. Brown fat in humans in the scientific and popular literature refers to two cell populations defined by both anatomical location and cellular morphology. Both share the presence of small lipid droplets and numerous iron-rich mitochondria, giving the brown appearance.
"Classical" brown fat is found in vascularized deposits in somewhat consistent anatomical locations, such as between the shoulder blades, surrounding the kidneys, the neck, supraclavicular area, along the spinal cord. This has numerous small lipid droplets. Beige fat is the adrenergically inducible cell type, dispersed throughout adipose tissue, it has greater variability in lipid droplet size and a greater proportion of lipid droplets to mitochondria giving it a light brown appearance. Brown fat cells come from the middle embryo layer, mesoderm the source of myocytes and chondrocytes; the classic population of brown fat cells and muscle cells both seem to be derived from the same population of stem cells in the mesoderm, paraxial mesoderm. Both have the intrinsic capacity to activate the myogenic factor 5 promoter, a trait only associated with myocytes and this population of brown fat. Progenitors of traditional white fat cells and adrenergically induced brown fat do not have the capacity to activate the Myf5 promoter.
Both adipocytes and brown adipocyte may be derived from pericytes, the cells which surround the blood vessels that run through white fat tissue. Notably, this is not the same as the presence of Myf5 protein, involved in the development of many tissues. Additionally, muscle cells that were cultured with the transcription factor PRDM16 were converted into brown fat cells, brown fat cells without PRDM16 were converted into muscle cells; the mitochondria in a eukaryotic cell utilize fuels to produce energy in the form of adenosine triphosphate. This process involves storing energy as a proton gradient known as the proton motive force, across the mitochondrial inner membrane; this energy is used to synthesize ATP when the protons flow across the membrane through the ATP synthase enzyme. In endotherms, body heat is maintained by signaling the mitochondria to allow protons to run back along the gradient without producing ATP; this can occur since an alternative return route for the protons exists through an uncoupling protein in the inner membrane.
This protein, known as uncoupling protein 1, facilitates the return of the protons after they have been pumped out of the mitochondria by the electron transport chain. This alternative route for protons uncouples oxidative phosphorylation and the energy in the PMF is instead released as heat. To some degree, all cells of endotherms give off heat when body temperature is below a regulatory threshold. However, brown adipose tissue is specialized for this non-shivering thermogenesis. First, each cell has a higher number of mitochondria compared to more typical cells. Second, these mitochondria have a higher-than-normal concentration of thermogenin in the inner membrane. In neonates, brown fat makes up about 5% of the body mass and is located on the back, along the upper half of the spine and toward the shoulders, it is of great importance to avoid hypothermia, as lethal cold is a major death risk for premature neonates. Numerous factors make infants more susceptible to cold than adults: A higher ratio of body surface area to body volume A higher proportional surface area of the head A low amount of musculature and the inability to shiver A lack of thermal insulation, e.g. subcutaneous fat and fine body hair (especially
Humidity is the amount of water vapour present in air. Water vapour, the gaseous state of water, is invisible to the human eye. Humidity indicates the likelihood for dew, or fog to be present; the amount of water vapour needed to achieve saturation increases as the temperature increases. As the temperature of a parcel of air decreases it will reach the saturation point without adding or losing water mass; the amount of water vapour contained within a parcel of air can vary significantly. For example, a parcel of air near saturation may contain 28 grams of water per cubic metre of air at 30 °C, but only 8 grams of water per cubic metre of air at 8 °C. Three primary measurements of humidity are employed: absolute and specific. Absolute humidity describes the water content of air and is expressed in either grams per cubic metre or grams per kilogram. Relative humidity, expressed as a percentage, indicates a present state of absolute humidity relative to a maximum humidity given the same temperature.
Specific humidity is the ratio of water vapor mass to total moist air parcel mass. Humidity plays an important role for surface life. For animal life dependent on perspiration to regulate internal body temperature, high humidity impairs heat exchange efficiency by reducing the rate of moisture evaporation from skin surfaces; this effect can be calculated using a heat index table known as a humidex. Absolute humidity is the total mass of water vapor present in mass of air, it does not take temperature into consideration. Absolute humidity in the atmosphere ranges from near zero to 30 grams per cubic metre when the air is saturated at 30 °C. Absolute humidity is the mass of the water vapor, divided by the volume of the air and water vapor mixture, which can be expressed as: A H = m H 2 O V n e t; the absolute humidity changes as air pressure changes, if the volume is not fixed. This makes it unsuitable for chemical engineering calculations, e.g. in drying, where temperature can vary considerably.
As a result, absolute humidity in chemical engineering may refer to mass of water vapor per unit mass of dry air known as the humidity ratio or mass mixing ratio, better suited for heat and mass balance calculations. Mass of water per unit volume as in the equation above is defined as volumetric humidity; because of the potential confusion, British Standard BS 1339 suggests avoiding the term "absolute humidity". Units should always be checked. Many humidity charts are given in g/kg or kg/kg; the field concerned with the study of physical and thermodynamic properties of gas–vapor mixtures is named psychrometrics. The relative humidity of an air-water mixture is defined as the ratio of the partial pressure of water vapor in the mixture to the equilibrium vapor pressure of water over a flat surface of pure water at a given temperature: ϕ = p H 2 O p H 2 O ∗ Relative humidity is expressed as a percentage. Relative humidity is an important metric used in weather forecasts and reports, as it is an indicator of the likelihood of precipitation, dew, or fog.
In hot summer weather, a rise in relative humidity increases the apparent temperature to humans by hindering the evaporation of perspiration from the skin. For example, according to the Heat Index, a relative humidity of 75% at air temperature of 80.0 °F would feel like 83.6 °F ±1.3 °F. Specific humidity is the ratio of the mass of water vapor to the total mass of the moist air parcel. Specific humidity is equal to the mixing ratio, defined as the ratio of the mass of water vapor in an air parcel to the mass of dry air for the same parcel; as temperature decreases, the amount of water vapor needed to reach saturation decreases. As the temperature of a parcel of air becomes lower it will reach the point of saturation without adding or losing water mass. A device used to measure humidity is called a hygrometer. A humidistat is a humidity-triggered switch used to control a dehumidifier. There are various devices used to regulate humidity. Calibration standards for the most accurate measurement include the gravimetric hygrometer, chilled mirror hygrometer, electrolytic hygrometer.
The gravimetric method, while the most accurate, is cumbersome. For fast and accurate measurement the chilled mirror method is effective. For process on-line measurements, the most used sensors nowadays are based on capacitance measurements to measure relative humidity with internal conversions to d
The Sahara is a desert located on the African continent. It is the largest hot desert in the world, the third largest desert overall after Antarctica and the Arctic, its area of 9,200,000 square kilometres is comparable to the area of the United States. The name'Sahara' is derived from a dialectal Arabic word for ṣaḥra; the desert comprises much of North Africa, excluding the fertile region on the Mediterranean Sea coast, the Atlas Mountains of the Maghreb, the Nile Valley in Egypt and Sudan. It stretches from the Red Sea in the east and the Mediterranean in the north to the Atlantic Ocean in the west, where the landscape changes from desert to coastal plains. To the south, it is bounded by the Sahel, a belt of semi-arid tropical savanna around the Niger River valley and the Sudan Region of Sub-Saharan Africa; the Sahara can be divided into several regions including: the western Sahara, the central Ahaggar Mountains, the Tibesti Mountains, the Aïr Mountains, the Ténéré desert, the Libyan Desert.
For several hundred thousand years, the Sahara has alternated between desert and savanna grassland in a 41,000 year cycle caused by the precession of the Earth's axis as it rotates around the Sun, which changes the location of the North African Monsoon. The area is next expected to become green in about 15,000 years. There is a suggestion that the last time that the Sahara was converted from savanna to desert it was due to overgrazing by the cattle of the local population; the Sahara covers large parts of Algeria, Egypt, Mali, Niger, Western Sahara and Tunisia. It covers 9 million square kilometres, amounting to 31% of Africa. If all areas with a mean annual precipitation of less than 250 mm were included, the Sahara would be 11 million square kilometres, it is one of three distinct physiographic provinces of the African massive physiographic division. The Sahara is rocky hamada. Wind or rare rainfall shape the desert features: sand dunes, dune fields, sand seas, stone plateaus, gravel plains, dry valleys, dry lakes, salt flats.
Unusual landforms include the Richat Structure in Mauritania. Several dissected mountains, many volcanic, rise from the desert, including the Aïr Mountains, Ahaggar Mountains, Saharan Atlas, Tibesti Mountains, Adrar des Iforas, the Red Sea Hills; the highest peak in the Sahara is Emi Koussi, a shield volcano in the Tibesti range of northern Chad. The central Sahara is hyperarid, with sparse vegetation; the northern and southern reaches of the desert, along with the highlands, have areas of sparse grassland and desert shrub, with trees and taller shrubs in wadis, where moisture collects. In the central, hyperarid region, there are many subdivisions of the great desert: Tanezrouft, the Ténéré, the Libyan Desert, the Eastern Desert, the Nubian Desert and others; these arid areas receive no rain for years. To the north, the Sahara skirts the Mediterranean Sea in Egypt and portions of Libya, but in Cyrenaica and the Maghreb, the Sahara borders the Mediterranean forest and scrub eco-regions of northern Africa, all of which have a Mediterranean climate characterized by hot summers and cool and rainy winters.
According to the botanical criteria of Frank White and geographer Robert Capot-Rey, the northern limit of the Sahara corresponds to the northern limit of date palm cultivation and the southern limit of the range of esparto, a grass typical of the Mediterranean climate portion of the Maghreb and Iberia. The northern limit corresponds to the 100 mm isohyet of annual precipitation. To the south, the Sahara is bounded by the Sahel, a belt of dry tropical savanna with a summer rainy season that extends across Africa from east to west; the southern limit of the Sahara is indicated botanically by the southern limit of Cornulaca monacantha, or northern limit of Cenchrus biflorus, a grass typical of the Sahel. According to climatic criteria, the southern limit of the Sahara corresponds to the 150 mm isohyet of annual precipitation. Important cities located in the Sahara include the capital of Mauritania; the Sahara is the world's largest low-latitude hot desert. It is located in the horse latitudes under the subtropical ridge, a significant belt of semi-permanent subtropical warm-core high pressure where the air from upper levels of the troposphere tends to sink towards the ground.
This steady descending airflow causes a drying effect in the upper troposphere. The sinking air prevents evaporating water from rising, therefore prevents adiabatic cooling, which makes cloud formation difficult to nearly impossible; the permanent dissolution of clouds allows thermal radiation. The stability of the atmosphere above the desert prevents any convective overturning, thus making rainfall non-existent; as a consequence, the weather tends to be sunny and stable with a minimal chance of rainfall. Subsiding, dry air masses associated with subtropical high-pressure systems are unfavorable for the development of convectional showers; the subtropical ridge is the predominant factor that explains the hot desert climate (Köppen climate classifica
Bantu people are the speakers of Bantu languages, comprising several hundred indigenous ethnic groups in sub-Saharan Africa, spread over a vast area from Central Africa across the African Great Lakes to Southern Africa. Linguistically, Bantu languages belong to the Southern Bantoid branch of Benue–Congo, one of the language families grouped within the Niger–Congo phylum; the total number of Bantu languages ranges in the hundreds, depending on the definition of "language" vs. "dialect" estimated at between 440 and 680 distinct languages. The total number of Bantu speakers is in the hundreds of millions, ranging at 350 million in the mid-2010s. About 60 million Bantu speakers, divided into some 200 ethnic or tribal groups, are found in the Democratic Republic of Congo alone; the larger of the individual Bantu groups have populations of several million, e.g. the Shona of Zimbabwe, the Zulu of South Africa the Luba of the Democratic Republic of the Congo, the Sukuma of Tanzania, or the Kikuyu of Kenya.
The word Bantu for the language families and its speakers is an artificial term based on the reconstructed Proto-Bantu term for "people" or "humans". It was first introduced by Wilhelm Bleek in 1857 or 1858, popularised in his Comparative Grammar of 1862; the name was coined to represent the word for "people" in loosely reconstructed Proto-Bantu, from the plural noun class prefix *ba- categorizing "people", the root *ntʊ̀ - "some, any". There is no native term for the group, as populations refer to their languages by ethnic endonyms but did not have a concept for the larger ethno-linguistic phylum. Bleek's coinage was inspired by the anthropological observation of groups self-identifying as "people" or "the true people"; that is, idiomatically the reflexes of *bantʊ in the numerous languages have connotations of personal character traits as encompassed under the values system of ubuntu known as hunhu in Chishona or botho in Sesotho, rather than just referring to all human beings. The root in Proto-Bantu is reconstructed as *-ntʊ́.
Versions of the word Bantu occur in all Bantu languages: for example, as watu in Swahili. Bantu languages derive from a Proto-Bantu language, estimated to have been spoken about 4,000 to 3,000 years ago in West/Central Africa, they were spread across Central and Southern Africa in the Bantu expansion, a rapid succession of migrations during the 1st millennium BC, in one wave moving across the Congo basin towards East Africa, in another moving south along the African coast and the Congo River system towards Angola. The geographical origin of the Bantu expansion is somewhat open to debate. Two main scenarios are proposed, an early expansion to Central Africa, a single origin of the migration radiating from there, or an early separation into an eastward and a southward wave of migration. Genetic analysis shows a significant clustering of Bantu peoples by region, suggesting admixture from local populations, with the Eastern Bantu forming a separate ancestral cluster, the Southern Bantu showing derivation from Western Bantu by Khoisan admixture and low levels of Eastern Bantu admixture.
According to the early-split scenario described in the 1990s, the southward migration had reached the Central African rain forest by about 1500 BC, the southern Savannahs by 500 BC, while the eastward migration reached the Great Lakes by 1000 BC, expanding further from there, as the rich environment supported a dense population. Movements by small groups to the southeast from the Great Lakes region were more rapid, with initial settlements dispersed near the coast and near rivers, due to comparatively harsh farming conditions in areas farther from water. Pioneering groups had reached modern KwaZulu-Natal in South Africa by about AD 300 along the coast, the modern Northern Province by AD 500; the Bantu peoples assimilated and/or displaced a number of earlier inhabitants that they came across, such as Pygmy and Khoisan populations in the centre and south, respectively. They encountered some Afro-Asiatic outlier groups in the southeast, as well as Nilo-Saharan groups; as cattle terminology in use amongst the few modern Bantu pastoralist groups suggests, the Bantu migrants would acquire cattle from their new Cushitic neighbors.
Linguistic evidence indicates that Bantus borrowed the custom of milking cattle directly from Cushitic peoples in the area. Interactions between Bantu and Cushitic peoples resulted in Bantu groups with significant Cushitic ethnic admixture, such as the Tutsi of the African Great Lakes region. Between the 14th and 15th centuries, Bantu-speaking states began to emerge in the Great Lakes region and in the savannah south of the Central African rain forest. On the Zambezi river, the Monomatapa kings built the Great
Culture is the social behavior and norms found in human societies. Culture is considered a central concept in anthropology, encompassing the range of phenomena that are transmitted through social learning in human societies. Cultural universals are found in all human societies; the concept of material culture covers the physical expressions of culture, such as technology and art, whereas the immaterial aspects of culture such as principles of social organization, philosophy and science comprise the intangible cultural heritage of a society. In the humanities, one sense of culture as an attribute of the individual has been the degree to which they have cultivated a particular level of sophistication in the arts, education, or manners; the level of cultural sophistication has sometimes been seen to distinguish civilizations from less complex societies. Such hierarchical perspectives on culture are found in class-based distinctions between a high culture of the social elite and a low culture, popular culture, or folk culture of the lower classes, distinguished by the stratified access to cultural capital.
In common parlance, culture is used to refer to the symbolic markers used by ethnic groups to distinguish themselves visibly from each other such as body modification, clothing or jewelry. Mass culture refers to the mass-produced and mass mediated forms of consumer culture that emerged in the 20th century; some schools of philosophy, such as Marxism and critical theory, have argued that culture is used politically as a tool of the elites to manipulate the lower classes and create a false consciousness, such perspectives are common in the discipline of cultural studies. In the wider social sciences, the theoretical perspective of cultural materialism holds that human symbolic culture arises from the material conditions of human life, as humans create the conditions for physical survival, that the basis of culture is found in evolved biological dispositions; when used as a count noun, a "culture" is the set of customs and values of a society or community, such as an ethnic group or nation. Culture is the set of knowledge acquired over time.
In this sense, multiculturalism values the peaceful coexistence and mutual respect between different cultures inhabiting the same planet. Sometimes "culture" is used to describe specific practices within a subgroup of a society, a subculture, or a counterculture. Within cultural anthropology, the ideology and analytical stance of cultural relativism holds that cultures cannot be objectively ranked or evaluated because any evaluation is situated within the value system of a given culture; the modern term "culture" is based on a term used by the Ancient Roman orator Cicero in his Tusculanae Disputationes, where he wrote of a cultivation of the soul or "cultura animi," using an agricultural metaphor for the development of a philosophical soul, understood teleologically as the highest possible ideal for human development. Samuel Pufendorf took over this metaphor in a modern context, meaning something similar, but no longer assuming that philosophy was man's natural perfection, his use, that of many writers after him, "refers to all the ways in which human beings overcome their original barbarism, through artifice, become human."In 1986, philosopher Edward S.
Casey wrote, "The word culture meant'place tilled' in Middle English, the same word goes back to Latin colere,'to inhabit, care for, worship' and cultus,'A cult a religious one.' To be cultural, to have a culture, is to inhabit a place sufficiently intensive to cultivate it—to be responsible for it, to respond to it, to attend to it caringly." Culture described by Richard Velkley:... meant the cultivation of the soul or mind, acquires most of its modern meaning in the writings of the 18th-century German thinkers, who were on various levels developing Rousseau's criticism of "modern liberalism and Enlightenment". Thus a contrast between "culture" and "civilization" is implied in these authors when not expressed as such. In the words of anthropologist E. B. Tylor, it is "that complex whole which includes knowledge, art, law and any other capabilities and habits acquired by man as a member of society." Alternatively, in a contemporary variant, "Culture is defined as a social domain that emphasizes the practices and material expressions, over time, express the continuities and discontinuities of social meaning of a life held in common.
The Cambridge English Dictionary states that culture is "the way of life the general customs and beliefs, of a particular group of people at a particular time." Terror management theory posits that culture is a series of activities and worldviews that provide humans with the basis for perceiving themselves as "person of worth within the world of meaning"—raising themselves above the physical aspects of existence, in order to deny the animal insignificance and death that Homo sapiens became aware of when they acquired a larger brain. The word is used in a general sense as the evolved ability to categorize and represent experiences with symbols and to act imaginatively and creatively; this ability arose with the evolution of behavioral modernity in humans around 50,000 years ago, is thought to be unique to humans, although some other species have demonstrated similar, though much less complex, abilities for social learning. It is used to denote the co
Bergmann's rule is an ecogeographical rule that states that within a broadly distributed taxonomic clade and species of larger size are found in colder environments, species of smaller size are found in warmer regions. Although formulated in terms of species within a genus, it has been recast in terms of populations within a species, it is often cast in terms of latitude. It is possible that the rule applies to some plants, such as Rapicactus; the rule is named after nineteenth century German biologist Carl Bergmann, who described the pattern in 1847, although he was not the first to notice it. Bergmann's rule is most applied to mammals and birds which are endotherms, but some researchers have found evidence for the rule in studies of ectothermic species; such as the ant Leptothorax acervorum. While Bergmann's rule appears to hold true for many mammals and birds, there are exceptions. Larger-bodied animals tend to conform more to Bergmann's rule than smaller-bodied animals, at least up to certain latitudes.
This reflects a reduced ability to avoid stressful environments, such as by burrowing. In addition to being a general pattern across space, Bergmann's rule has been reported in populations over historical and evolutionary time when exposed to varying thermal regimes. In particular, reversible dwarfing of mammals has been noted during two brief upward excursions in temperature during the Paleogene: the Paleocene-Eocene thermal maximum and the Eocene Thermal Maximum 2. Human populations near the poles, including the Inuit and Sami people, are on average heavier than populations from mid-latitudes, consistent with Bergmann's rule, they tend to have shorter limbs and broader trunks, consistent with Allen's rule. According to Marshall T. Newman in 1953, Native American populations are consistent with Bergmann's rule although the cold climate and small body size combination of the Eastern Inuit, Canoe Nation, Yuki people, Andes natives and Harrison Lake Lillouet runs contrary to the expectations of Bergmann's rule.
Newman contends that Bergmann's rule holds for the populations of Eurasia, but it does not hold for those of sub-Saharan Africa. The earliest explanation, given by Bergmann when formulating the rule, is that larger animals have a lower surface area to volume ratio than smaller animals, so they radiate less body heat per unit of mass, therefore stay warmer in cold climates. Warmer climates impose the opposite problem: body heat generated by metabolism needs to be dissipated rather than stored within. Thus, the higher surface area-to-volume ratio of smaller animals in hot and dry climates facilitates heat loss through the skin and helps cool the body, it is important to note that when analyzing Bergmann's Rule in the field that groups of populations being studied are of different thermal environments, have been separated long enough to genetically differentiate in response to these thermal conditions. In marine crustaceans, it has been proposed that an increase in size with latitude is observed because decreasing temperature results in increased cell size and increased life span, both of which lead to an increase in maximum body size.
The size trend has been observed in hyperiid and gammarid amphipods, stomatopods and planktonic euphausiids, both in comparisons of related species as well as within distributed species. Deep-sea gigantism is observed in some of the same groups for the same reasons. In 1937 German zoologist and ecologist Richard Hesse proposed an extension of Bergmann's rule. Hesse's rule known as the heart–weight rule, states that species inhabiting colder climates have a larger heart in relation to body weight than related species inhabiting warmer climates. According to a 1986 study, Valerius Geist claimed Bergmann's rule to be false: the correlation with temperature is spurious; because many factors can affect body size, there are many critics of Bergmann's Rule. Some believe that latitude. Examples of other selective factors that may contribute to body mass changes are the size of food items available, effects of body size on success as a predator, effects of body size on vulnerability to predation, resource availability.
For example, if an organism is adapted to tolerate cold temperatures, it may tolerate periods of food shortage, due to correlation between cold temperature and food scarcity. A larger organism can rely on its greater fat stores to provide the energy needed for survival as well being able to procreate for longer periods. Resource availability is a major constraint on the overall success of many organisms. Resource scarcity can limit the total number of organisms in a habitat, over time can cause organisms to adapt by becoming smaller in body size. Resource availability thus becomes a modifying restraint on Bergmann’s Rule. Bergmann's rule cannot be applied to plants, above all for latitude. Regarding Cactaceae, the case of Carnegiea gigantea, described as "a botanical Bergmann trend" by Niering, Whittaker, & Lowe, has instead been shown to depend on rainfall winter precipitation, not temperature, so Bergmann's rule is not applicable to Carnegiea populations. Members of the genus Rapicactus are larger in cooler environments, as their stem diameter increases with altitude and, above all, latitude.
Since Rapicactus grow in a distributional area in which average precipitation tends to diminish at higher latitudes, their body size is not conditioned by climat