Bergmann's rule

The rule derives from the relationship between size in linear dimensions meaning that both height and volume will increase in colder environments.

[7] In addition to being a general pattern across space, Bergmann's rule has been reported in populations over historical and evolutionary time when exposed to varying thermal regimes.

[14] According to Marshall T. Newman in 1953, Native American populations are generally consistent with Bergmann's rule although the cold climate and small body size combination of the Eastern Inuit, Canoe Nation, Yuki people, Andes natives and Harrison Lake Lillooet runs contrary to the expectations of Bergmann's rule.

These populations show a shorter stature and smaller body size due to an adaptation to hotter and more humid environments.

The length of birds' lower leg bones (an indicator of body size) shortened by an average of 2.4% and their wings lengthened by 1.3%.

A similar study published in 2021 used measurements of 77 nonmigratory bird species captured live for banding in lowland Amazon rainforest.

[27] Regarding Cactaceae, the case of the saguaro (Carnegiea gigantea), once described as "a botanical Bergmann trend",[28] has instead been shown to depend on rainfall, particularly winter precipitation, and not temperature.

However, since Rapicactus grow in a distributional area in which average precipitation tends to diminish at higher latitudes, and their body size is not conditioned by climatic variables, this could suggest a possible Bergmann trend.

Warmer climates impose the opposite problem: body heat generated by metabolism needs to be dissipated quickly rather than stored within.

[31] 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.

[5] 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.

[37] Further, a 2024 study found the size of dinosaurs did not increase at northern Arctic latitudes, and that the rule was "only applicable to a subset of homeothermic animals" with regard to temperature when all other climatic variables are ignored.