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Raegan's Sandbox

Research Topic: Bergmann's Rule and it's Application to Mammals

Three Improvements to the page "Bergmann's Rule": https://en.wikipedia.org/wiki/Bergmann%27s_rule#Explanations

A few suggestions for improvement: 1) This page is lacking examples of Bergmann's Rule and it's application for mammals other than humans. I believe if more detailed examples were provided it would be clearer for someone learning about the subject to understand what is going on. 2) The concept of heat dissipation or conservation of metabolic heat has been excluded from the page thus far. An example would be a mammal in a southern desert would want to dissipate heat more readily and therefore would have a smaller mass, or vice versa a larger animal in a northern cold climate would want to use its mass to conserve it's metabolic heat. 3) The criticism of Bergmann's Rule is not noted well on this article as well. Like any rule or hypothesis, in order to understand it better you must be versed in both the pros and cons of the statement

One Sentence Added: 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.

Final Paper Starts Here

Bergmann’s Rule

Bergmann’s Rule is an ecological principle that states that the body mass of a species increases as latitude increases. In other words, the colder the climate, the larger the body mass of the species, and likewise the warmer the climate, the smaller the body mass. This hypothesizes that the environment is a selective pressure on species and thus changes body mass. This principle also relies on studying species that are in different geothermic areas as well as have been separated for a long enough time to be genetically different. This brings the concept of migration into play as an additional method of evolutionary selection. Studying genotypic variation due to migration has to be meticulously evaluated due to many factors of a new environment that can change a genotype. This must also be considered in order to prove or reject Bergmann’s Rule. While investigating Bergmann’s Rule, there has been significant speculation on whether to evaluate species on an intraspecific or interspecific level. The term intraspecific refers to individuals within the same species being evaluated against one and other, while interspecific refers to different species that occupy the same geographic space. Most methods testing Bergmann’s Rule are of intra-specific intentions and some see this as a flaw in experimental design. However, Bergmann himself considered his mechanism to apply generally both across and within species. Controversially, it has been studied and accepted that the most coherent way to study Bergmann’s Rule is to study species of similar organization (Clauss 2013). When studying similar, interspecific species it was found that as latitude increased, body size most often increased as well. In contrast, it became hard to determine if Bergmann’s Rule held true on the interspecific level because it is possible that different species respond to similar environmental pressures without adapting on the genomic level (Clauss 2013). There is not much other supporting evidence for interspecific comparisons because few studies have tried to tackle this much more in depth concept. It has also been said that comparing extremely different species can be detrimental to the analysis of Bergmann’s Rule because there are to many other conflicting pressures that can cause selection on the individuals (Belk 2002). This then blurs the data and it becomes difficult to decide whether Bergmann’s Rule is the cause of increased body mass or if a different pressure is to blame. Both inter and intraspecific problems, whether being competition of genes of two different species or within a species, cause nonstop adaptation to keep up with each other, known as the Red Queen Hypothesis. In terms of migration, it is crucial when studying Bergmann’s rule that sufficient time has passed before collecting data. Premature data collection could provide false negatives for Bergmann’s Rule as there was not enough time for individuals to change on the genomic level. It is also important to study Bergmann’s Rule without constant gene flow from other environments because this could cause skewed results as well. If there is constant gene flow from an area of different latitude, there will be little to no adaptation with in a population to study in a short period of time (Brown 1969). If significant time is given before study, there is expected to be a body mass difference between the two populations of different latitude if Bergmann’s Rule holds true. Many hypothesize that the real cause of increased body mass with increased latitude is due to heat dissipation. Originally it was thought that Bergmann’s Rule was affected by the temperature budget of an organism, that is, the core temperature of a certain organism and how it was correlated to the organism’s environment. However, this explanation has been under debate and thus a significant point to try to test in nature. One alternative suggestion to the driving force of Bergmann’s rule is that the reduction of internal heat in warmer climates is more important than minimizing heat loss in cold climates (McNab 1971). This means that organisms in warm climates need to be able to dissipate heat more actively than those in cold climates need to store heat. Likewise, many physiologist believe that the overall body mass is not as crucial to analyze as the amount of insulation an organism possess (Scholander 1955). It is clear that many evolutionary concepts are at work here in analyzing the driving force of Bergmann’s Rule. This is evident by trying to determine whether the environmental selection on a species is causing adaptations due to heat dissipation or adaptations to heat conservation. An interesting point in the concept of heat conservation is that a separate contributor to Bergmann’s Rule involves food shortage. If an organism is adapted to tolerate extremely cold temperatures, it is assumed said organism could also tolerate periods of food shortage, as cold temperature and food scarcity go hand in hand (Ashton 2000). This means the organism is able to use it’s own fat storage to provide the critical nutrition necessary for survival as well as it’s ability to procreate. This is an easy concept to see, as it would be presumed that a large individual has the ability to fast for longer periods, giving it an advantage to survive in these cold climates. It therefore can be determined that in evolutionary terms, a small body size would be selected against in cold climates because it is not beneficial to the individual to be small. Average size of the individual is under scrutiny while evaluating Bergmann’s Rule as well. It is generally accepted that larger species follow Bergmann’s Rule more often than smaller species do. This could be due to many things, one being that smaller species have a higher critical temperature than larger species. The critical temperature is the temperature at which a resting individual can maintain its basal metabolic rate (Scholander 1955). Another study provided evidence that adapting to heat loss was more significant than maintaining the basal metabolic rate. These types of inconsistencies show that there is still much to learn about Bergmann’s rule. The findings of this study were similar though in that it was agreed that larger bodied individuals have a better ability to reduce heat loss than that of a smaller bodied individual (McNab 1971). It is important to note however, that small species may in fact not be evaluated fairly. Measurement errors are more numerous in smaller individuals than larger ones and can create a larger deviation from Bergmann’s Rule. Additionally, smaller bodied species tend to inhabit a smaller area, limiting their experience with large fluctuations in temperature and latitude, making this a hard factor to study (Freckleton 2003). Colder climates have been proven to drastically alter nearly all aspects of life for all species. In colder climates, many species produce more offspring than in warmer climates, and these offspring often take longer to reach maturity than warmer climates (Rypel 2014). This longer time to maturation tends to lead to a larger body mass than that of a shorter maturation time. These findings are proportional to Bergmann’s Rule in that a cold climate changes the overall body size of offspring. As species become adapted to the cold, their offspring are adapted as well and this is most generally seen in body mass changes. Because so many factors can affect body size, there are many critics of Bergmann’s Rule. This is because several people believe that latitude itself is a poor indicator on body mass as an entire explanation. Some examples of other selective factors that contribute to body mass changes are size of food particles available, the effects of body size and being a predator, likewise, the effects of body size and predation, and resource availability. Resources are a major factor for the overall success of many organisms. If resources in a habitat were plentiful, it would be safe to assume that the average body mass for the organisms in that habitat would be larger than that of a habitat with poor resources. If resources are scare in a habitat, it could cause a population control situation where the total number of organisms is reduced to a number that can be sustained within that habitat, or it could have a genomic effect where organisms adapt to live on smaller amounts of resources and become smaller in body size. Resource availability thus becomes a modifying restraint on Bergmann’s Rule (Clauss 2013). Bergmann’s Rule can also be contradicted in terms of predators and predation. In some cases body size can determine the efficiency in which an organism can hunt. One would think the pressure of hunting would be of greater selection than that of latitude (Freckleton 2003). On the other hand, a large body size could be disadvantageous to those organisms that rely on avoiding predators as a means of survival. This would mean selection against being large would be more advantageous. Over time, predation does cause natural selection in many populations so it cannot be discounted as a means of changing body mass. Bergmann’s Rule is currently stuck between being widely accepted by many, and scrutinized by others. While most data and studies support Bergmann’s Rule more often than not, the cause of large body size is the debatable aspect. Most organisms tend to be larger with an increase in latitude, but whether this large body size is due to the latitude itself, or one of the many other pressures has yet to be perfected. Bergmann’s Rule also must be studied under strict guidelines, such as long term in order to observe changes at the genomic level, and under little to no gene flow from other latitudes. This keeps the data in perspective and able to be compared to other data collections regardless of location. Evolution plays a large role in the study of Bergmann’s Rule as many evolutionary topics are hard at work in these species. The Red Queen Hypothesis, Migration, and many affects of Natural Selection can be seen through these studies and these all contribute to the evolution of the species studied. As more and more studies are being performed across all species barriers it should be expected that Bergmann’s Rule could become fact or discounted altogether for the species studied.

Bibliography

Ashton, Kyle G., Mark C. Tracy, and Alan De Queiroz. 2000. Is Bergmann’s Rule Valid for Mammals? The American Naturalist 156.4: 390-415. JSTOR. Web. 14 Sept. 2014. . Belk, Mark C., and Derek D. Houston. 2002. Bergmann's Rule in Ectotherms: A Test Using Freshwater Fishes :: Institutional Repository. Bergmann's Rule in Ectotherms: A Test Using Freshwater Fishes :: Institutional Repository. Web. 28 Oct. 2014. . Brown, James H., and Anthony K. Lee. 1969. Bergmann's Rule and Climatic Adaptation in Woodrats (Neotoma). Evolution 23.2: 329. JSTOR. Web. 14 Sept. 2014. . Clauss, Marcus, Marie T. Dittmann, Dennis W. H. Müller, Carlo Meloro, and Daryl Codron. 2013. Bergmann′s Rule in Mammals: A Cross-species Interspecific Pattern. Oikos: No. Web of Science. Web. 14 Sept. 2014. Freckleton, Robert P., Paul H. Harvey, and Mark Pagel. 2003. Bergmann’s Rule and Body Size in Mammals. The American Naturalist 161.5: 821-25. JSTOR. Web. 14 Sept. 2014. . Mcnab, Brian K. 1971. On the Ecological Significance of Bergmann's Rule. Ecology 52.5: 845. JSTOR. Web. 14 Sept. 2014. . Rypel, Andrew L. 2014. The Cold-Water Connection: Bergmann’s Rule in North American Freshwater Fishes. The American Naturalist 183.1: 147-56. JSTOR. Web. 28 Oct. 2014. . Scholander, P. F. 1955. Evolution of Climatic Adaptation in Homeotherms. JSTOR. Web. 28 Oct. 2014. .

Addition to Bergmann's Rule Wikipedia Page

Because so many factors can affect body size, there are many critics of Bergmann’s Rule. This is because several people believe that latitude itself is a poor indicator on body mass as an entire explanation. Some examples of other selective factors that contribute to body mass changes are size of food particles available, the effects of body size and being a predator, likewise, the effects of body size and predation, and resource availability. For example, if an organism is adapted to tolerate extremely cold temperatures, it is assumed said organism could also tolerate periods of food shortage, as cold temperature and food scarcity go hand in hand (Ashton 2000). This means the organism is able to use it’s own fat storage to provide the critical nutrition necessary for survival as well as it’s ability to procreate. This is an easy concept to see, as it would be presumed that a large individual has the ability to fast for longer periods, giving it an advantage to survive in these cold climates. It therefore can be determined that in evolutionary terms, a small body size would be selected against in cold climates because it is not beneficial to the individual to be small. Resources are a major factor for the overall success of many organisms. If resources in a habitat were plentiful, it would be safe to assume that the average body mass for the organisms in that habitat would be larger than that of a habitat with poor resources. If resources are scare in a habitat, it could cause a population control situation where the total number of organisms is reduced to a number that can be sustained within that habitat, or it could have a genomic effect where organisms adapt to live on smaller amounts of resources and become smaller in body size. Resource availability thus becomes a modifying restraint on Bergmann’s Rule (Clauss 2013).

Annotated Bibliography:

Ashton, Kyle G., Mark C. Tracy, and Alan De Queiroz. "Is Bergmann’s Rule Valid for Mammals?" The American Naturalist 156.4 (2000): 390-415. JSTOR. Web. 14 Sept. 2014. . This journal by Ashton, Tracy, and Queiroz focused on whether the hypothesis that smaller mammals tend to conform to Bergmann's Rule more closely than larger mammals is accurate. They found no correlation that supports this statement. They found that Bergmann's Rule was a general trend supported by many different mammal species and that heat conservation in smaller mammals had no bearing on the extent to which the mammal applied to Bergmann's Rule.

Brown, James H., and Anthony K. Lee. "Bergmann's Rule and Climatic Adaptation in Woodrats (Neotoma)." Evolution 23.2 (1969): 329. JSTOR. Web. 14 Sept. 2014. . This article takes a look at a specific mammal of the genus Neotoma and how different species in this genus apply to Bergmann's Rule. The research here provides that these mammals directly follow Bergmann's Rule on an infraspecific as well as intrageneric variation level. This article also refutes work done by Scholander stating that body sizes of mammals are not affected by adaptations to temperature.

Clauss, Marcus, Marie T. Dittmann, Dennis W. H. Müller, Carlo Meloro, and Daryl Codron. "Bergmann′s Rule in Mammals: A Cross-species Interspecific Pattern." Oikos (2013): No. Web of Science. Web. 14 Sept. 2014. . This journal claims that evaluations of Bergmann's Rule have only been studied on an intra-specific and not a inter-specific level. They found that as far as conventional methods go that there are no significant correlations between latitude and body mass in mammals. However there is a significant correlation when said mammals are closely related instead. They studied 3561 mammal species from 26 orders to evaluate this concept.

Freckleton, Robert P., Paul H. Harvey, and Mark Pagel. "Bergmann’s Rule and Body Size in Mammals." The American Naturalist 161.5 (2003): 821-25. JSTOR. Web. 14 Sept. 2014. . This journal takes a statistical approach to the correlations between Bergmann's Rule and physical data collected on different species of mammals. They found a negative correlation between mammal body length and mass to latitude, which says a lot about Bergmann's Rule as a whole. However they say that phenotypically the rule seems favorable to most of the species studied.

Mcnab, Brian K. "On the Ecological Significance of Bergmann's Rule." Ecology 52.5 (1971): 845. JSTOR. Web. 14 Sept. 2014. . This article blatantly states that most mammals latitudinally widespread in North America do not follow Bergmann's rule. Likewise the positive correlation between weight and latitude cannot be based on heat exchange from their findings. They also make an interesting point saying that most large mammals are that way not do to their location but due to a larger prey instead.