Temperature-size rule

Relation to Bergmann's rule In 1847, Carl Bergmann published his observations that endothermic body size (i.e. mammals) increased with increasing latitude, commonly known as Bergmann's rule. His rule postulated that selection favored within species individuals with larger body sizes in cooler temperatures because the total heat loss would be diminished through lower surface area to volume ratios. Despite this, ectotherms have largely been observed to still exhibit larger body sizes in colder environments. Formulation of the rule Ray (1960) originally examined body sizes in several species of ectotherms and discovered that around 80% of them exhibited larger body sizes in lower temperatures. A few decades later, Atkinson (1994) performed a similar review of temperature effects on body size in ectotherms. His study, which included 92 species of ectotherms ranging from animals and plants to protists and bacteria, concluded that a reduction in temperature resulted in an increase in organism size in 83.5% of cases. Atkinson's findings provided support for Ray's published works that ectotherms have an observable trend in body size when temperature is the primary environmental variable. The results of his study prompted him to name the increase in ectothermic body size in colder environments as the temperature-size rule. == 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. Size at maturity 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. 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. == Investigation ==

Supporting evidence • In the soil nematode, Caenorhabditis elegans, adult body size reared at 10°C was approximately 33% greater than individuals grown at 25°C. Although there was no mention of exhibiting body size patterns in accordance with the temperature-size rule, the faster growth rates and higher mortality imply that lizards in the high altitude enclosures had slower growth rates and lower hatching mortality, a routine pattern indicative of species that conform to the possible temperature-size rule tradeoff. • Eastern fence lizards, Sceloporus undulatus, exhibit delayed maturation at larger body sizes, a trend consistent with the temperature-size rule. Exceptions • The grasshopper, Chorthippus brunneus, is a high temperature specialist (or stenotherm) that matures to larger body sizes at high temperatures, making it an exception to the temperature-size rule. • Juvenile survivorship in Sceloporus graciosus is not found to be higher in cooler environments, leading the species to exhibit body size clines inconsistent with the temperature size rule. • 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. Notes The supporting evidence and the exceptions to the temperature-size rule listed above are only a few of the potential supporting/opposing evidence available for the temperature-size rule. Each was provided to support the claim that patterns of body size observed in variable environments are not 100% predictable and more research is required to identify and understand all of the mechanisms responsible. == References ==