Abstract
BackgroundAnimals that live at higher latitudes/elevations would have a larger body size (Bergmann’s rule) and a smaller appendage size (Allen’s rule) for thermoregulatory reasons. According to the heat conservation hypothesis, large body size and small appendage size help animals retain heat in the cold, while small body size and large appendage size help them dissipate heat in the warm. For animals living in seasonal climates, the need for conserving heat in the winter may tradeoff with the need for dissipating heat in the summer. In this study, we tested Bergmann’s rule and Allen’s rule in two widely-distributed passerine birds, the Oriental Magpie (Pica serica) and the Oriental Tit (Parus minor), across geographic and climatic gradients in China.MethodsWe measured body size (body mass and wing length) and appendage size (bill length and tarsus length) of 165 Oriental Magpie and 410 Oriental Tit individuals collected from Chinese mainland. We used linear mixed-effect models to assess variation patterns of body size and appendage size along geographic and climatic gradients.ResultsOriental Magpies have a larger appendage size and Oriental Tits have a smaller body size in warmer environments. Appendage size in Oriental Magpies and body size in Oriental Tits of both sexes were more closely related to the climates in winter than in summer. Minimum temperature of coldest month is the most important factor related to bill length and tarsus length of male Oriental Magpies, and wing length of male and female Oriental Tits. Bill length and tarsus length in female Oriental Magpies were related to the annual mean temperature and mean temperature of coldest quarter, respectively.ConclusionsIn this study, Oriental Magpies and Oriental Tits followed Allen’s rule and Bergmann’ rule respectively. Temperatures in the winter, rather than temperatures in the summer, drove morphological measurements in Oriental Magpies and Oriental Tits in Chinese mainland, demonstrating that the morphological measurements reflect selection for heat conservation rather than for heat dissipation.
Highlights
Animals that live at higher latitudes/elevations would have a larger body size (Bergmann’s rule) and a smaller appendage size (Allen’s rule) for thermoregulatory reasons
The featherless legs of birds are reported to play a role in heat exchange (Martineau and Larochelle 1988; Arad et al 1989; Maloney and Dawson 1994) and several studies have revealed a profound effect of temperature on tarsus length (Laiolo and Rolando 2001; VanderWerf 2012)
We examined the geographical variations of several major morphological traits in Oriental Magpies and Oriental Tits to evaluate whether they support Bergmann’s rule and Allen’s rule
Summary
Animals that live at higher latitudes/elevations would have a larger body size (Bergmann’s rule) and a smaller appendage size (Allen’s rule) for thermoregulatory reasons. Bergmann’s rule (1847), one of the best-known ecogeographic patterns, states that populations or closely related species of endotherms typically have a larger body size in colder climates as a result of thermoregulation (James 1970; Salewski and Watt 2017). Allen’s rule (1877) is an extension of Bergmann’s rule, predicting that appendage size in endotherms, including limbs, tails and ears, become larger in warm climates for similar thermoregulatory reasons. Smaller bills with less surface area help birds keep a constant temperature in colder environments, while the larger surface area of larger bills in warmer climates raises the efficiency of heat dissipation (Symonds and Tattersall 2010). Nudds and Oswald (2007) found that terns and gulls showed interspecific geographic variation predicted by Allen’s rule in the length of exposed leg elements, but not in feathered element length
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