What is Bergmann's rule?
'''Bergmann's rule''' is an [[biogeography|ecogeographic]] principle. It says that, within a widely distributed group of animals, the larger ones are found in colder environments, and the smaller ones are found in warmer regions. So, for example, with [[bears]] the largest member is the [[polar bear]], and the smallest members are found in [[subtropical]] areas ([[panda]]). The largest [[tiger]] is the [[Siberian tiger]]. Large mammals of the late [[ice age]] were in general larger than their descendants today. The rule is named after nineteenth-century [[Germany|German]] biologist [[Carl Bergmann (anatomist)|Carl Bergmann]], who described the [[pattern]] in 1847, though he was not the first to notice it. Bergmann's rule appears to hold true for many mammals and birds, but there are exceptions.<ref>Meiri S. & Dayan T. 2003. On the validity of Bergmann’s rule. ''J. Biogeography '''30''', 331–351.</ref><ref>Ashton K.G Tracy M.C. & de Queiroz A. 2000. Is Bergmann's rule valid for mammals? ''The American Naturalist'', '''156''', 390–415.</ref><ref>Millien V. ''et al'' 2006. Ecotypic variation in the context of global climate change: revisiting the rules. ''Ecology Letters'' '''9''': 853–869.</ref><ref>Freckleton R.P.; Harvey, P.H. & Pagel M. 2003. Bergmann's rule and body size in mammals. ''American Naturalist'' '''161''' (5): 821–825. [https://www.jstor.org/discover/10.1086/374346?uid=3738032&uid=2&uid=4&sid=21103774851461]</ref> Although originally put in terms of [[species]], it seems to apply to populations within a species. Bergmann's rule is most often applied to mammals and birds which are [[endothermic|endotherms]], but some researchers have also found evidence for the rule in studies of [[ectotherm]]ic species.<ref>Olalla-Tárraga, Miguel Á; Rodríguez M.Á. & Hawkins B.A. 2006. Broad-scale patterns of body size in squamate reptiles of Europe and North America. ''Journal of Biogeography'' '''33''' (5): 781–793. [http://onlinelibrary.wiley.com/doi/10.1111/j.1365-2699.2006.01435.x/abstract;jsessionid=A8F7A2