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41-401: The hominid fossils at Jinniushan all belong to one individual. Initially, the fossils were believed to have belonged to a male specimen, since the fossils were so big. Later analysis shows that the fossil remains actually come from a female specimen. The Jinniushan specimen's body mass is estimated to be around 78.6 kg (173 lb), making it the largest female specimen ever discovered in

82-542: A certain temperature threshold, and below about 9–14 °C (48–57 °F), the honey bee reverts to ectothermy. Facultative endothermy can also be seen in multiple snake species that use their metabolic heat to warm their eggs. Python molurus and Morelia spilota are two python species where females surround their eggs and shiver in order to incubate them. Some ectotherms , including several species of fish and reptiles , have been shown to make use of regional endothermy, where muscle activity causes certain parts of

123-516: A drop. The resting human body generates about two-thirds of its heat through metabolism in internal organs in the thorax and abdomen, as well as in the brain. The brain generates about 16% of the total heat produced by the body. Heat loss is a major threat to smaller creatures, as they have a larger ratio of surface area to volume . Small warm-blooded animals have insulation in the form of fur or feathers . Aquatic warm-blooded animals, such as seals , generally have deep layers of blubber under

164-540: A feature shared with the specimen from Hulu Cave, Nanjing; both also share some features with the Zhoukoudian and Yunxian specimens . Morphologically, archaic female and Neanderthal females are more similar to modern females than their male counterparts are to modern males. The similar morphologies imply that the birth mechanics of the Jinniushan hominin is probably similar to that of modern females. Fossils of

205-404: A given body mass. As a consequence they also need higher food intake rates, which may limit abundance of endotherms more than ectotherms. Because ectotherms depend on environmental conditions for body temperature regulation, they typically are more sluggish at night and in the morning when they emerge from their shelters to heat up in the first sunlight. Foraging activity is therefore restricted to

246-466: A heavier build for their height and people who originate from warmer regions have a lighter build for their height. Relative sitting height is also negatively correlated with temperature for indigenous human populations , meaning that people who originate from colder regions have proportionally shorter legs for their height and people who originate from warmer regions have proportionally longer legs for their height. In 1968, A.T. Steegman investigated

287-594: A high variability (both within and between species) in their diurnal activity patterns. It is thought that the evolution of endothermia was crucial in the development of eutherian mammalian species diversity in the Mesozoic period. Endothermia gave the early mammals the capacity to be active during nighttime while maintaining small body sizes. Adaptations in photoreception and the loss of UV protection characterizing modern eutherian mammals are understood as adaptations for an originally nocturnal lifestyle, suggesting that

328-503: A renewed interest in Allen's rule due to global warming and the " microevolutionary changes" that are predicted by the rule. Marked differences in limb lengths have been observed when different portions of a given human population reside at different altitudes. Environments at higher altitudes generally experience lower ambient temperatures. In Peru , individuals who lived at higher elevations tended to have shorter limbs, whereas those from

369-1024: A species can reproduce, as embryos are generally intolerant of thermal fluctuations that are easily tolerated by adults. Endothermy may also provide protection against fungal infection. While tens of thousands of fungal species infect insects, only a few hundred target mammals, and often only those with a compromised immune system . A recent study suggests fungi are fundamentally ill-equipped to thrive at mammalian temperatures. The high temperatures afforded by endothermy might have provided an evolutionary advantage. Ectotherms increase their body temperature mostly through external heat sources such as sunlight energy; therefore, they depend on environmental conditions to reach operational body temperatures. Endothermic animals mostly use internal heat production through metabolic active organs and tissues (liver, kidney, heart, brain, muscle) or specialized heat producing tissues like brown adipose tissue (BAT). In general, endotherms therefore have higher metabolic rates than ectotherms at

410-679: Is ectothermy , although in general, there is no absolute or clear separation between the nature of endotherms and ectotherms. Endothermy was thought to have originated towards the end of the Permian Period . One recent study claimed the origin of endothermy within Synapsida (the mammalian lineage) was among Mammaliamorpha , a node calibrated during the Late Triassic period, about 233 million years ago. Another study instead argued that endothermy only appeared later, during

451-595: Is 1,330 cm (81 cu in). The encephalization quotient (EQ) is estimated to be around 4.150. Both are typical of the rapidly increasing brain capacity and EQ found in other specimens from the Middle Pleistocene. Neanderthals Homo antecessor Jinniushan Hualong Dali Xiahe Harbin Modern humans The Jinniushan specimen belongs to an archaic human with mixed Homo erectus and Homo sapiens features. The Jinniushan specimen

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492-1130: Is an organism that maintains its body at a metabolically favorable temperature, largely by the use of heat released by its internal bodily functions instead of relying almost purely on ambient heat. Such internally generated heat is mainly an incidental product of the animal's routine metabolism , but under conditions of excessive cold or low activity an endotherm might apply special mechanisms adapted specifically to heat production. Examples include special-function muscular exertion such as shivering , and uncoupled oxidative metabolism, such as within brown adipose tissue . Only birds and mammals are considered truly endothermic groups of animals. However, Argentine black and white tegu , leatherback sea turtles , lamnid sharks, tuna and billfishes , cicadas , and winter moths are mesothermic . Unlike mammals and birds, some reptiles, particularly some species of python and tegu , possess seasonal reproductive endothermy in which they are endothermic only during their reproductive season . In common parlance, endotherms are characterized as " warm-blooded ". The opposite of endothermy

533-525: Is similar to the Dali specimen, but more gracile, which can be explained by sexual dimorphism . The cranial vault and supraorbitals of the Jinniushan specimen are thinner than those of the Dali specimen. Jinniushan's external cranium is the same size as Dali's, but Jinniushan's bones are thinner, so the Jinniushan specimen has a larger brain capacity than the Dali specimen. Both specimens have flat and broad faces,

574-475: The Middle Jurassic , among crown-group mammals. Evidence for endothermy has been found in basal synapsids (" pelycosaurs "), pareiasaurs , ichthyosaurs , plesiosaurs , mosasaurs , and basal archosauromorphs . Even the earliest amniotes might have been endotherms. Many endotherms have a larger amount of mitochondria per cell than ectotherms. This enables them to generate heat by increasing

615-482: The predictions of Allen's rule. J.S. Alho and colleagues argued in 2011 that, although Allen's rule was originally formulated for endotherms , it can also be applied to ectotherms , which derive body temperature from the environment. In their view, ectotherms with lower surface area-to-volume ratios would heat up and cool down more slowly, and this resistance to temperature change might be adaptive in "thermally heterogeneous environments". Alho said that there has been

656-459: The skin and any pelage (fur) that they might have; both contribute to their insulation. Penguins have both feathers and blubber. Penguin feathers are scale-like and serve both for insulation and streamlining. Endotherms that live in very cold circumstances or conditions predisposing to heat loss, such as polar waters, tend to have specialised structures of blood vessels in their extremities that act as heat exchangers . The veins are adjacent to

697-546: The Arctic Mongoloids, particularly the Eskimo and Aleut , because these have similar morphological features in accordance with Allen's rule: a narrow nasal passage, relatively large heads, long to round heads, large jaws, relatively large bodies, and short limbs. Allen's rule may have also resulted in wide noses and alveolar and/or maxillary prognathism being more common in human populations in warmer regions, and

738-430: The arteries full of warm blood. Some of the arterial heat is conducted to the cold blood and recycled back into the trunk. Birds, especially waders , often have very well-developed heat exchange mechanisms in their legs—those in the legs of emperor penguins are part of the adaptations that enable them to spend months on Antarctic winter ice. In response to cold, many warm-blooded animals also reduce blood flow to

779-403: The assumption that Allen's rule caused the structural configuration of the face of human populations adapted to polar climate. Steegman did an experiment that involved the survival of rats in the cold. Steegman said that the rats with narrow nasal passages, broader faces, shorter tails and shorter legs survived the best in the cold. Steegman said that the experimental results had similarities with

820-583: The blood to cool, which reduces their core body temperature when the blood moves through the rest of the circulatory system. The major advantage of endothermy over ectothermy is decreased vulnerability to fluctuations in external temperature. Regardless of location (and hence external temperature), endothermy maintains a constant core temperature for optimal enzyme activity. Endotherms control body temperature by internal homeostatic mechanisms. In mammals, two separate homeostatic mechanisms are involved in thermoregulation—one mechanism increases body temperature, while

861-580: The body surface-area-to-volume ratio for homeothermic animals varies with the average temperature of the habitat to which they are adapted (i.e. the ratio is low in cold climates and high in hot climates). Allen's rule predicts that endothermic animals with the same body volume should have different surface areas that will either aid or impede their heat dissipation. Because animals living in cold climates need to conserve as much heat as possible, Allen's rule predicts that they should have evolved comparatively low surface area-to-volume ratios to minimize

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902-423: The body to remain at higher temperatures than the rest of the body. This allows for better locomotion and use of the senses in cold environments. Students encounter a source of possible confusion between the terminology of physics and biology. Whereas the thermodynamic terms " exothermic " and " endothermic " respectively refer to processes that give out heat energy and processes that absorb heat energy, in biology

943-498: The day in nocturnal animals, thus reducing the energy cost of maintaining body temperature. Less drastic intermittent reduction in body temperature also occurs in other larger endotherms; for example human metabolism also slows down during sleep, causing a drop in core temperature, commonly of the order of 1 degree Celsius. There may be other variations in temperature, usually smaller, either endogenous or in response to external circumstances or vigorous exertion, and either an increase or

984-415: The daytime (diurnal activity patterns) in most vertebrate ectotherms. In lizards, for instance, only a few species are known to be nocturnal (e.g. many geckos) and they mostly use 'sit and wait' foraging strategies that may not require body temperatures as high as those necessary for active foraging. Endothermic vertebrate species are, therefore, less dependent on the environmental conditions and have developed

1025-405: The diurnal cycle in places of sharp temperature variations between day and night and during more of the year in places of great seasonal differences of temperature. This is accompanied by the need to expend more energy to maintain the constant internal temperature and a greater food requirement. Endothermy may be important during reproduction, for example, in expanding the thermal range over which

1066-420: The exposed leg lengths were negatively correlated with Tm axdiff (body temperature minus minimum ambient temperature), supporting the predictions of Allen's rule. J.S. Alho and colleagues argued that tibia and femur lengths are highest in populations of the common frog that are indigenous to the middle latitudes, consistent with the predictions of Allen's rule for ectothermic organisms . Populations of

1107-643: The extinct macaque Macaca robustus , large beaver Trogontherium , deer Sinomegaceros pachyosteus , rhinoceros Stephanorhinus kirchbergensis , and the vole Microtus brandtioides fossils were found at Jinniushan. Several new species of extinct birds were also discovered, including Aegypius jinniushanensis and Leptoptilos lüi . Allen%27s rule Allen's rule is an ecogeographical rule formulated by Joel Asaph Allen in 1877, broadly stating that animals adapted to cold climates have shorter and thicker limbs and bodily appendages than animals adapted to warm climates. More specifically, it states that

1148-520: The fossil record. The next largest female specimen ever discovered, found at Grotte du Prince, early Late Pleistocene, from around 100,000 BP, has an estimated body mass of 74 kg (163 lb). Body size in Homo reached its maximum during the Middle Pleistocene, so the size of the Jinniushan specimen is not surprising, especially since the specimen was found at a high latitude, cold climate location. In accordance with Allen's rule and Bergmann's rule ,

1189-682: The group went through an evolutionary bottleneck (the nocturnal bottleneck hypothesis ). This could have avoided predator pressure from diurnal reptiles and dinosaurs, although some predatory dinosaurs, being equally endothermic, might have adapted a nocturnal lifestyle in order to prey on those mammals. Many insect species are able to maintain a thoracic temperature above the ambient temperature using exercise. These are known as facultative or exercise endotherms. The honey bee , for example, does so by contracting antagonistic flight muscles without moving its wings (see insect thermoregulation ). This form of thermogenesis is, however, only efficient above

1230-601: The large body, wide trunk, and short limbs of the Jinniushan female is to be expected, as the hominins from that time relied more on their physical body as a cold adaptation, as their technological culture was not yet as advanced as later hominins. The fossil elements at Jinniushan all belong to one female individual. The fossil remains consist of one cranium , six vertebrae (one cervical, five thoracic), one complete left os coxae , one complete left ulna , one complete left patella , two left ribs, and several hand and feet bones. The Jinniushan specimen's estimated cranial capacity

1271-423: The need to avoid hyperthermia . Endothermy does not provide greater speed in movement than ectothermy (cold-bloodedness)—ectothermic animals can move as fast as warm-blooded animals of the same size and build when the ectotherm is near or at its optimal temperature, but often cannot maintain high metabolic activity for as long as endotherms. Endothermic/homeothermic animals can be optimally active at more times during

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1312-431: The opposite in colder regions. A contributing factor to Allen's rule in vertebrates may be that the growth of cartilage is at least partly dependent on temperature. Temperature can directly affect the growth of cartilage , providing a proximate biological explanation for this rule. Experimenters raised mice either at 7 degrees, 21 degrees or 27 degrees Celsius and then measured their tails and ears. They found that

1353-404: The other decreases it. The presence of two separate mechanisms provides a very high degree of control. This is important because the core temperature of mammals can be controlled to be as close as possible to the optimal temperature for enzyme activity. The overall rate of an animal's metabolism increases by a factor of about two for every 10 °C (18 °F) rise in temperature , limited by

1394-760: The rate at which they metabolize fats and sugars . Accordingly, to sustain their higher metabolism, endothermic animals typically require several times as much food as ectothermic animals do, and usually require a more sustained supply of metabolic fuel. In many endothermic animals, a controlled temporary state of hypothermia conserves energy by permitting the body temperature to drop nearly to ambient levels. Such states may be brief, regular circadian cycles called torpor , or they might occur in much longer, even seasonal, cycles called hibernation . The body temperatures of many small birds (e.g. hummingbirds ) and small mammals (e.g. tenrecs ) fall dramatically during daily inactivity, such as nightly in diurnal animals or during

1435-410: The same population who inhabited the more low-lying coastal areas generally had longer limbs and larger trunks. Katzmarzyk and Leonard similarly noted that human populations appear to follow the predictions of Allen's rule. There is a negative association between body mass index and mean annual temperature for indigenous human populations, meaning that people who originate from colder regions have

1476-448: The same species from different latitudes may also follow Allen's rule. R.L. Nudds and S.A. Oswald argued in 2007 that there is poor empirical support for Allen's rule, even if it is an "established ecological tenet". They said that the support for Allen's rule mainly draws from studies of single species, since studies of multiple species are "confounded" by the scaling effects of Bergmann's rule and alternative adaptations that counter

1517-404: The skin by vasoconstriction to reduce heat loss. As a result, they blanch (become paler). In equatorial climates and during temperate summers, overheating ( hyperthermia ) is as great a threat as cold. In hot conditions, many warm-blooded animals increase heat loss by panting, which cools the animal by increasing water evaporation in the breath, and/or flushing, increasing the blood flow to

1558-405: The skin so the heat will radiate into the environment. Hairless and short-haired mammals, including humans and horses, also sweat , since the evaporation of the water in sweat removes heat. Elephants keep cool by using their huge ears like radiators in automobiles. Their ears are thin and the blood vessels are close to the skin, and flapping their ears to increase the airflow over them causes

1599-416: The surface area by which they dissipate heat, allowing them to retain more heat. For animals living in warm climates, Allen's rule predicts the opposite: that they should have comparatively high ratios of surface area to volume. Because animals with low surface area-to-volume ratios would overheat quickly, animals in warm climates should, according to the rule, have high surface area-to-volume ratios to maximize

1640-414: The surface area through which they dissipate heat. Though there are numerous exceptions, many animal populations appear to conform to the predictions of Allen's rule. The polar bear has stocky limbs and very short ears that are in accordance with the predictions of Allen's rule, so does the snow leopard . In 2007, R.L. Nudds and S.A. Oswald studied the exposed lengths of seabirds ' legs and found that

1681-600: The tails and ears were significantly shorter in the mice raised in the cold in comparison to the mice raised at warmer temperatures, even though their overall body weights were the same. They also found that the mice raised in the cold had less blood flow in their extremities . When they tried growing bone samples at different temperatures, the researchers found that the samples grown in warmer temperatures had significantly more growth of cartilage than those grown in colder temperatures. Endotherm An endotherm (from Greek ἔνδον endon "within" and θέρμη thermē "heat")

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