question: Why are animals smaller near the equator? Shouldn’t it be the other way around, since resources are more abundant in warmer climates?
comment: On the other side of that reasoning, small animals are far less effective at retaining heat. Larger bodies are better able to function in colder climes.
passages: Section::::Explanations.
  
  The earliest explanation, given by Bergmann when originally 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, and therefore stay warmer in cold climates. Warmer climates impose the opposite problem: body heat generated by metabolism needs to be dissipated quickly rather than stored within.
   Gigantothermy
  
  Gigantothermy (sometimes called ectothermic homeothermy or inertial homeothermy) is a phenomenon with significance in biology and paleontology, whereby large, bulky ectothermic animals are more easily able to maintain a constant, relatively high body temperature than smaller animals by virtue of their smaller surface area to volume ratio. A bigger animal has proportionately less of its body close to the outside environment than a smaller animal of otherwise similar shape, and so it gains heat from, or loses heat to, the environment much more slowly.
   Ectotherms occupying colder environments, such as mountain ranges or other areas of higher elevation, have been observed to invest in reproduction at larger adult body sizes due to a prolonged growth period. These populations of ectotherms are characterized as having smaller clutches of larger eggs, favoring a greater reproductive investment per egg and enhances offspring survival rates. Individuals occupying warmer environments experience a trade-off between body size and overall reproductive success that many individuals occupying colder environments do not, hence, prolonging growth to yield greater reproductive success in colder environments could potentially be an underlying mechanism for why a large percentage of ectotherms exhibit greater body sizes in colder environments. However, a sufficient explanation for this observable pattern has yet to be produced. 
   The surface area of an organism is important in several considerations, such as regulation of body temperature and digestion. Animals use their teeth to grind food down into smaller particles, increasing the surface area available for digestion. The epithelial tissue lining the digestive tract contains microvilli, greatly increasing the area available for absorption. Elephants have large ears, allowing them to regulate their own body temperature. In other instances, animals will need to minimize surface area; for example, people will fold their arms over their chest when cold to minimize heat loss.
   More recently, it has been proposed that an accurate prediction of the energy cost of running at a given speed can be made from the time available to generate force to support body weight. This theory suggests that smaller animals must take shorter, quicker steps to travel a given distance than larger animals. As a result, they have shorter foot ground contact times and less time to produce force on the ground. Due to this decreased amount of time to produce force, smaller animals must rely more heavily on metabolically costly fast muscle fibers to produce force to run at a given speed. Conversely, larger animals take slower and longer steps, contributing to an increase in the amount of time the foot is in contact with the ground during running. This longer contact time allows larger animals a greater amount of time to produce force. As a result, larger animals do not recruit as many metabolically costly fast muscle fibers in order to run a given speed. All of these factors result in a greater COT in smaller animals in comparison to larger animals. 
   Environmental temperature is one of the most important proximate factors affecting ectotherm body size because of their need to thermoregulate. Individuals that have been observed to follow the temperature-size rule have slower growth rates in colder environments, yet they enter a period of prolonged growth that yields larger adult body sizes. One proposed explanation for this involves a trade-off in life history traits. Ectotherms experience longer daily and seasonal activity times in warmer climates versus cooler climates, however, the increase in daily activity time is accompanied by higher infant and adult mortality rates due to predation. Under these environmental conditions, some individuals occupying these warmer climate environments will mature at smaller body sizes and undergo a shift in energy allocation of all acquired energy resources to reproduction. In doing so, these individuals sacrifice growth to larger adult body sizes to ensure reproductive success, even if the trade-off results in smaller offspring that have increased mortality rates. 
   At one level, it is possible that the clade's increased vulnerability to extinction, as its members become larger, means that no taxon survives long enough for individuals to reach huge sizes. There are probably also physically imposed limits to the size of some organisms; for instance, insects must be small enough for oxygen to diffuse to all parts of their bodies, flying birds must be light enough to fly, and the length of giraffes' necks may be limited by the blood pressure it is possible for their hearts to generate. Finally, there may be a competitive element, in that changes in size are necessarily accompanied by changes in ecological niche. For example, terrestrial carnivores over 21 kg almost always prey on organisms larger, not smaller, than themselves. If such a niche is already occupied, competitive pressure may oppose the directional selection. The three Canidae clades show a trend towards larger size before becoming extinct.
   Many predators are powerfully built and can catch and kill animals larger than themselves; this applies as much to small predators such as ants and shrews as to big and visibly muscular carnivores like the cougar and lion. 
  
  Section::::Specialization.:Diet and behaviour.
   Section::::Hesse's rule.
  
  In 1937 German zoologist and ecologist Richard Hesse proposed an extension of Bergmann's rule. Hesse's rule, also known as the heart–weight rule, states that species inhabiting colder climates have a larger heart in relation to body weight than closely related species inhabiting warmer climates.
  
  Section::::Criticism.
  
  In a 1986 study, Valerius Geist claimed Bergmann's rule to be false: the correlation with temperature is spurious; instead, Geist found that body size is proportional to the duration of the annual productivity pulse, or food availability per animal during the growing season.
   BULLET::::- In the same study that Ashton & Feldman provided evidence that chelonians exhibit body size clines consistent with the temperature-size rule, they also provided evidence that squamates (lizards and snakes) trend towards larger body sizes in warmer environments (40 out of 56 species increased in size with temperature). This was the first study to show a major group of ectotherms that show the converse to the temperature-size rule.
  
  Section::::Investigation.:Notes.
   Section::::Investigation.
  
  Section::::Investigation.:Supporting evidence.
  
  BULLET::::- In the soil nematode, "Caenorhabditis elegans", adult body size reared at 10°C was approximately 33% greater than individuals grown at 25°C.
  
  BULLET::::- Ashton & Feldman's (2003) study concluded that chelonians (turtles) follow the temperature-size rule with 14 of 15 sp. decreasing in size with increasing temperature.
  
  BULLET::::- Body size in the larval ant lion, "Myrmeleon immaculatus", has been observed to follow Bergmann's size clines in response to latitudinal changes. However, when reared in high and low temperatures, body size was not affected. Food availability was the driving mechanism behind recorded body size variation.
   Section::::Ultimate mechanisms.:Phylogeny.
  
  Similar magnitudes of body weight dimorphism have been observed in all species within several taxonomic groups such as callitrichids, hylobatids, "Cercopithecus", and "Macaca". Such correlation between phylogenetic relatedness and sexual dimorphism across different groups reflects similarities in their behaviors and ecological conditions, but not in independent adaptations. This idea is referred to as “phylogenetic niche conservatism."
  
  Section::::Ultimate mechanisms.:Terrestriality.
   It has been hypothesized that as temperature increases, body size would decrease. Smaller body size would dissipate heat more efficiently, so in an increased temperature environment one would expect an animal to be smaller. The opposite also holds true, when temperature decreases, studies have shown with great correlation that body size increases. This was shown.
  
  Climate change has been associated with changes in plant as well as animal size.
  
  Section::::Adaptation.:Factors contributing to adaptation.
  
  Short generational times as for many microbial disease organisms, small insects, common fisheries species and annual plants are thought to be highly more adaptive.
   Section::::Possible mechanisms.:Energetic constraints.
  
  A small bodied animal has a greater capacity to be more abundant than a large bodied one. Purely as a function of geometry many more small things can be packed into a given space than can large things into the same area. However, these limits are generally never reached in ecological systems as other resources become limiting long before the packing limits are reached. Additionally, smaller species may have many more ecological niches available to them and thus facilitating the diversification of life (Hutchinson and MacArthur, 1959).
   Larger organisms require more food overall (Blackburn and Gaston, 1997), however they require less energy per unit of body mass. As a result, larger species are able to survive on a lower quality diet than smaller species. For example, grazing animals make up for their poor quality diet by digesting food longer and are able to extract more energy from it (Maurer et al. 1992). Smaller species tend to specialize in a habitat that can provide them with a high quality diet.
   Section::::Evolution of large body size.:In marine mammals.
   Section::::Body size as a result of temperature.
  
  Temperature affects the size of offspring in "Halictus rubicundus", but there are two hypotheses that offer explanations as to why such is the case:
  
  BULLET::::- At optimum temperatures, the maximum offspring size will be produced. Smaller sizes will be the result of both higher and lower temperatures based on differing stresses. So varying conditions around one optimum temperature will lead to different offspring sizes.
   A study of the estimated bite force at the canine teeth of a large sample of living and fossil mammalian predators, when adjusted for their body mass, found that for placental mammals the bite force at the canines (in Newtons/kilogram of body weight) was greatest in the extinct dire wolf (163), followed among the modern canids by the four hypercarnivores that often prey on animals larger than themselves: the African hunting dog (142), the gray wolf (136), the dhole (112), and the dingo (108). The bite force at the carnassials showed a similar trend to the canines. A predator's largest prey size is strongly influenced by its biomechanical limits.
   Sexual dimorphism may also influence differences in parental investment during times of food scarcity. For example, in the blue-footed booby, the female chicks grow faster than the males, resulting in booby parents producing the smaller sex, the males, during times of food shortage. This then results in the maximization of parental lifetime reproductive success. In Black-tailed Godwits "Limosa limosa limosa" females are also the larger sex, and the growth rates of female chicks are more susceptible to limited environmental conditions.
   Other studies conducted by Dennis, D. and Lavigne,(1975) have shown that the ratio between the size of the prey and the asilid varies from 1.8:1 to 3.7:1, with an average of 2.6:1. The ratio tends to increase with decreasing size of the predator.
   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, and also have been separated long enough to genetically differentiate in response to these thermal conditions.
   The term "mesopredator release" was first used by Soulé and colleagues in 1988 to describe a process whereby mid-sized carnivorous mammals became far more abundant after being "released" from the control of a larger carnivore. This, in turn, resulted in decreased populations of still smaller prey species, such as birds. This may lead to dramatic prey population decline, or even extinction, especially on islands. This process arises when mammalian top predators are considered to be the most influential factor on trophic structure and biodiversity in terrestrial ecosystems. Top predators may feed on herbivores and kill predators in lower trophic levels as well. Thus, reduction in the abundance of top predators may cause the medium-sized predator population to increase, therefore having a negative effect on the underlying prey community. The mesopredator release hypothesis offers an explanation for the abnormally high numbers of mesopredators and the decline in prey abundance and diversity. The hypothesis supports the argument for conservation of top predators because they protect smaller prey species that are in danger of extinction. This argument has been a subject of interest within conservation biology for years, but few studies have adequately documented the phenomenon.
   Because many factors can affect body size, there are many critics of Bergmann's Rule. Some believe that latitude itself is a poor predictor of body mass. 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, and resource availability. For example, if an organism is adapted to tolerate cold temperatures, it may also 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.
   Section::::Tradeoffs as possible underlying mechanisms.
  
  Section::::Tradeoffs as possible underlying mechanisms.:Life history model.
  
  Life history models highlighting optimal growth patterns suggest that individuals assess the environment for potential resources and other proximate factors and mature at a body size that yields the greatest reproductive success, or highest percentage of offspring surviving to reach reproductive maturity. 
  
  Section::::Tradeoffs as possible underlying mechanisms.:Size at maturity.
   MTE is based on an interpretation of the relationships between body size, body temperature, and metabolic rate across all organisms. Small-bodied organisms tend to have higher mass-specific metabolic rates than larger-bodied organisms. Furthermore, organisms that operate at warm temperatures through endothermy or by living in warm environments tend towards higher metabolic rates than organisms that operate at colder temperatures. This pattern is consistent from the unicellular level up to the level of the largest animals and plants on the planet.
  
raw_presuppositions: Animals near the equator should be larger since there are more resources.
raw_corrections: Larger animals are better at functioning in colder climates.