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Acclimation of the thermal reaction norm might also be favored if the environment varies spatially or temporally Gabriel and Lynch, Because the pattern of environmental variation determines how natural selection acts on the thermal reaction norm for growth rate, an important task for biologists is to characterize these patterns and incorporate them into theories designed to understand temperature-size relationships.
The covariation between temperature and other environmental variables can also shape the developmental reaction norm. A higher temperature could be a reliable cue for increasing resources and hence signal the opportunity for population growth Atkinson et al. On the other hand, if higher temperatures are usually associated with a scarcity of resources, the potential for growth observed in the laboratory might not be realized in nature.
Higher temperatures could also be associated with greater risks of mortality through changes in the density and activity of predators e. If temperature covaries with the abundance of prey or predators in a particular manner, natural selection will favor reaction norms that have the greatest fitness under those conditions.
This point is especially important because theory predicts that increased predation should have direct and indirect effects on size at maturity Abrams and Rowe, ; the direct effect is a reduction in size because higher rates of mortality favor earlier maturation, whereas the indirect effect is an increase in size because predation decreases intraspecific competition for resources. Because resources are often limiting in nature, one might expect developmental reaction norms to be shaped by both direct and indirect effects of predation.
Breaking the natural covariation among temperature, food availability, and predation risk can create a condition that never occurs in nature, which would lead to an erroneous interpretation of results Bernardo, The way in which these variables interact to determine thermal reaction norms for growth rate and size at maturity e. Presently, we know very little about this covariation in most populations of ectotherms suggesting an obvious need to pay more attention to the natural context in which temperature-size relationships have evolved.
Current theories of nonadaptive or adaptive plasticity of body size in response to temperature are relatively simple, in that each focuses on only one or two of the mechanisms by which temperature can influence the life history. In reality, most variables are affected by temperature, and optimal reaction norms for age and size at maturity depend on the relative strengths of these thermal effects.
Atkinson suggested that three thermal effects in particular were key to understanding temperature-size relationships: thermal constraints on maximal body size, thermal sensitivities of growth rate, and thermal sensitivities of juvenile survivorship. To this list, we add thermal effects on the frequency of reproduction and the survivorship of adults, which have not received serious consideration from life historians but see Charnov and Gillooly, Because reproduction is typically less frequent in colder environments, natural selection could favor a larger body size to enhance fecundity at each reproductive episode.
For similar reasons, a larger size at maturity might be adaptive if the survivorship of adults is lower in colder environments Stearns and Koella, Finally, a larger body size could enable individuals to produce larger offspring or to provide better parental care, which are thought to be adaptive in colder environments Perrin, ; Yampolsky and Scheiner, Like many hypotheses in evolutionary ecology Quinn and Dunham, , these mechanisms are not mutually exclusive; therefore, some or all of them could contribute to an explanation for the temperature-size rule.
Moreover, the relative importance of each mechanism probably varies among species. By combining these mechanisms in a single theory, one can achieve a deeper understanding of the relationships among temperature, growth rate and body size.
When developing a multivariate theory of temperature-size relationships, biologists should focus on the developmental reaction norm because this approach forces one to consider the coevolution of thermal reaction norms for growth rate and size at maturity. Allometric and thermal effects on growth rate can be modeled by including well-established functional constraints, such as tradeoffs associated with acquisition, allocation, and specialization.
The natural variation in temperature and the covariations between temperature and other environmental variables must be considered because they play important roles in the coevolution of growth rate and body size. Because current theory describes how the thermal environment shapes the optimal reaction norm for growth rate, modeling the developmental reaction norm could reveal why specific temperature-size relationships have evolved in specific environments. Such a breakthrough is needed if we are to understand not only why most species follow the temperature-size rule, but also why certain species do not.
Table 1. Thermal sensitivities of anabolism and catabolism in ectotherms. In three species of isopods, the optimal temperature for growth rate decreased with increasing body size.
Adapted from Panov and McQueen In contrast to the assumption of Perrin , the relationship between environmental temperature and the growth efficiency of an ectotherm is usually positive and is rarely negative. Relationships between temperature and growth efficiency gross or net were characterized as positive, negative, unimodal, or statistically insignificant. Data on gross and net growth efficiencies represent 89 populations of 53 species and 24 populations of 20 species, respectively.
Adapted from Angilletta and Dunham The thermal sensitivity of juvenile survivorship needed to explain the temperature-size rule depends on the thermal sensitivities of anabolism and catabolism see Appendix. The upper plot depicts the case in which the allometries of anabolism and catabolism are identical.
The lower plot depicts the case in which anabolism scales allometrically with body mass the exact relationship does not matter and catabolism scales isometrically.
The lines are isoclines for realistic combinations of thermal sensitivities of anabolism and catabolism, which are listed in the margin as [Q 10 of anabolism, Q 10 of catabolism]. To the right of each isocline, the optimal size at maturity decreases with increasing temperature in accord with the temperature-size rule.
The points are thermal sensitivities of survivorship for populations of species of ectotherms see text for details. Relationships between temperature and the natural logarithm of survivorship for larval top and post-larval bottom fishes in natural environments.
Survivorships of larval and post-larval fishes were calculated from daily rates of mortality tabulated by Houde and Pepin , respectively. A hypothetical multivariate reaction norm depicting the influence of environmental temperature on body size throughout ontogeny. Growth rate at a particular temperature is equal to the slope of the relationship between age and body size. The bold line depicts the thermal reaction norm for age and size at maturity.
In this example, a lower temperature results in slower growth but a larger size at maturity, which accords with the temperature-size rule.
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This heat escapes the bodies of humans and other organisms and is what causes a room full of people to get uncomfortably hot. Heat generated by metabolism plays an important role in keeping the bodies of endothermic animals warm.
Endotherms, primarily birds and mammals, are animals that are able to regulate their own body temperature using the energy generated by metabolism. The cells of any given organism contain many different types of enzymes, each of which is responsible for a particular chemical reaction.
All of these enzymes require a similar range of temperature in order to function. The relationship between the rate of metabolism and temperature can be visualized as a hump-shaped curve. Enzyme activity, and therefore metabolism, is slow at the lower and upper ends of a given temperature range, and highest at some optimum point. The optimum temperature for the typical human enzyme is 37 degrees Celsius The human body therefore maintains a temperature of about 37 degrees Celsius to maximize metabolic rate.
Enzyme activity drops sharply at temperatures above Temperature in the surrounding environment directly affects the metabolic rate of ectothermic animals, animals that are unable to regulate their own body temperature.
For instance, the metabolic rate of lizards is low at cold temperatures and a high at hot temperatures.
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