Origins and the logic of heat exchange
Observing North American mammals and birds, Allen (1877) recorded that ears, tails and limbs were shorter in cold northern populations than in southern ones. Protruding body parts have a large surface area relative to their volume and so lose much heat; short appendages therefore help conserve heat in cold places, while long appendages help shed heat in warm places. The rule is regarded as a companion to Bergmann's rule (1847) on overall body size. Scholander (1955) criticized the physiological significance of such rules, arguing that insulation such as fur may matter more, while Mayr (1956) replied that geographic character gradients are nonetheless widely observed.
Development and empirical studies
Roberts (1953) and Ruff (1994) reported that human populations in colder regions also tend to have relatively shorter limbs, and discussed the short limbs of Neanderthals in relation to cold adaptation. Serrat, King and Lovejoy (2008) showed experimentally that genetically similar mice raised at different temperatures differ in cartilage growth and hence in tail and limb length, indicating that part of the pattern arises from developmental plasticity. Nudds and Oswald (2007) found trends consistent with Allen's rule in the leg lengths of seabirds, and Symonds and Tattersall (2010) did so in the bill sizes of more than 200 bird species. Tattersall, Andrade and Abe (2009) showed that the large bill of the toucan acts as a radiator, shedding heat by controlling blood flow.
Recent research and climate change
Analyzing rodents worldwide, Alhajeri et al. (2020) reported that Allen's rule generally holds for tail and ear length, though the size of the effect differs among groups. Gardner et al. (2011) proposed that declining body size under climate warming may be a third universal response. Ryding et al. (2021) reviewed evidence of shape-shifting, in which appendages such as bills, ears and legs have lengthened with warming in many birds and mammals, and suggested that this may be Allen's rule playing out over time.
FAQ
- What is Allen's rule in simple terms?
- Animals in cold places tend to have short ears, legs and tails that lose less heat, while animals in hot places tend to have longer ones that shed heat more easily.
- How is Allen's rule related to climate change?
- Lengthening of bills, ears and legs in some birds and mammals has been reported alongside warming, and this is studied as a possible case of Allen's rule appearing over time.
Sources reviewed: 2026-10-26
References
- Allen, J. A. (1877). The influence of physical conditions in the genesis of species. Radical Review, 1, 108–140.
- Bergmann, C. (1847). Über die Verhältnisse der Wärmeökonomie der Thiere zu ihrer Grösse. Göttinger Studien, 3(1), 595–708.
- Scholander, P. F. (1955). Evolution of climatic adaptation in homeotherms. Evolution, 9(1), 15–26.
- Mayr, E. (1956). Geographical character gradients and climatic adaptation. Evolution, 10(1), 105–108.
- Roberts, D. F. (1953). Body weight, race and climate. American Journal of Physical Anthropology, 11(4), 533–558.
- Ruff, C. B. (1994). Morphological adaptation to climate in modern and fossil hominids. Yearbook of Physical Anthropology, 37, 65–107.
- Serrat, M. A., King, D., & Lovejoy, C. O. (2008). Temperature regulates limb length in homeotherms by directly modulating cartilage growth. Proceedings of the National Academy of Sciences, 105(49), 19348–19353.
- Nudds, R. L., & Oswald, S. A. (2007). An interspecific test of Allen's rule: Evolutionary implications for endothermic species. Evolution, 61(12), 2839–2848.
- Symonds, M. R. E., & Tattersall, G. J. (2010). Geographical variation in bill size across bird species provides evidence for Allen's rule. American Naturalist, 176(2), 188–197.
- Tattersall, G. J., Andrade, D. V., & Abe, A. S. (2009). Heat exchange from the toucan bill reveals a controllable vascular thermal radiator. Science, 325(5939), 468–470.
- Alhajeri, B. H., Fourcade, Y., Upham, N. S., & Alhaddad, H. (2020). A global test of Allen's rule in rodents. Global Ecology and Biogeography, 29(12), 2248–2260.
- Ryding, S., Klaassen, M., Tattersall, G. J., Gardner, J. L., & Symonds, M. R. E. (2021). Shape-shifting: Changing animal morphologies as a response to climatic warming. Trends in Ecology & Evolution, 36(11), 1036–1048.
- Gardner, J. L., Peters, A., Kearney, M. R., Joseph, L., & Heinsohn, R. (2011). Declining body size: A third universal response to warming? Trends in Ecology & Evolution, 26(6), 285–291.