Potential non-thermal factors in the Bergmann and Allen rules include general models related to hunger risk, predator risk, migration distance, and foraging ecology. The risk of starvation is higher in cold and seasonal environments and should favour larger animals with greater energy reserves at high latitudes; Larger animals may also be preferred at higher latitudes if they consume larger prey or a wider variety of prey sizes (“hunger risk hypothesis” 39,40,41,42,43). Meanwhile, the risk of starvation decreases and the risk of predators generally increases closer to the tropics.44,45 This should be chosen for lower body mass in animals and shorter wings in birds to improve mobility and escape ability (“predation risk hypothesis”40,46,47). Latitude lines in body size could refer to differences in migration behavior, where migratory birds flying longer distances are selected to be larger and have longer wings to increase the efficiency of flight48.49, or different age groups or sexes migrate to different locations. For example, strong selection pressure for males to arrive early at breeding sites can favor males flying shorter distances to closer non-breeding sites, resulting in patterns of latitude in body size for sexually dimorphic species (“differential migration hypothesis” 50:51). Finally, Allen`s rule can be related to latitude patterns in terms of food size or foraging behavior.12,52 For example, larger insects in tropical regions may choose longer beaks in insectivorous birds to improve the handling of larger prey.53 Similarly, shorebirds may have developed longer beaks in tropical regions, as benthic invertebrates are buried deeper below the surface in warmer climates (“food ecology hypothesis” 54,55,56). Mccollin D, Hodgson J, Crockett R. Do British birds follow Bergmann and Allen`s rules? An analysis of the variation in body size with latitude for four species. Study of birds. 2015;62:404–10. Ray, C.
(1960). The application of Bergmann and Allen`s rules to the Poikilothermen. J. Morphol. 106, 85–108. doi: 10.1002/jmor.1051060104 We suggest that the evidence for Bergmann`s rule and Allen`s dominance over ecologically diverse species with different migration behaviors and foraging strategies favors the sole overall explanation of the thermoregulatory hypothesis. While the models we observe could be explained by several alternative explanations that work in combination (e.g., developmental plasticity, famine risk, predation risk, differential migration, food ecology), the thermoregulatory hypothesis provides the most economical explanation for the high consistency of Bergmann and Allen rules in Australian shorebirds. We tested the evidence for Bergmann`s and Allen`s rules by comparing the beak length and body size of con-specific shorebirds in northern and southern Australia. Comparing shorebirds in two different regions allows us to test whether the latitude effects differ from an ecologically diverse species (Fig. 1).
In the Bergmann rule test, we estimated body size based on wing length and body mass. Wing length and body mass are strongly correlated (ra = â0.82, Nâ = â118,725 for all species). Wing length is a good indicator of body size88, is less sensitive to seasonal fluctuations than body mass89, and is commonly used in previous research, including Bergmann`s original observation of latitude trends4. Meanwhile, body mass directly affects the surface-to-mass ratio, which is relevant for a thermal explanation of Bergmann`s rule17, as most of birds` heat exchanges take place through their bodies.90 Differences in wing width can also be explained by other factors, whether wing length is an approximation of body size, but relevant to the assumptions described in Box 2, such as risk of predation47 or migration distance.91 In testing Allen`s rule, we examined whether birds in northern Australia have a longer beak relative to their body size (“relative beak length, which controls the variation in body size by including wing length as covariates in models”) and longer beaks, regardless of the variation in body size (“absolute beak length”). By assessing the relative length of the beak, we can determine whether northern birds have a longer beak relative to their body to promote heat loss. However, changes in the relative length of the beak may be the result of choosing a smaller body size rather than a larger beak size, making it difficult to disentangle the answers in our hypothetical framework; The evaluation of the absolute length of the invoice allows us to check if the invoices are longer, regardless of the drivers who may affect the size of the body. Olalla-Tárraga, M. Á. (2011). “Nullius in Bergmann” or the pluralistic approach to ecogeographical rules: a response to Watt et al. (2010). Oikos 120, 1441–1444.
doi: 10.1111/j.1600-0706.2011.19319.x Cardilini, A. P. A., Buchanan, K. L., Sherman, C. D. H., Cassey, P. & Symonds, M. R. E. Tests of ecogeographical relationships in a non-native species: What regulates bird morphology? Oecologia 181, 783â793 (2016). According to Bergmann and Allen`s rules, shorebirds in australia`s tropical north are generally smaller (shorter wings, lighter body mass) and have a longer beak than temperate South Australian congeners. This trend is evident in five ecologically diverse families, including migratory species that spend their first life in the Arctic or cold deserts of Central Asia, and non-migratory species.
It is likely that many factors influence the variation in width in the size and shape of animals. However, when considered in their entirety, our results best support the thermoregulatory hypothesis of Bergmann`s and Allen`s rules and further suggest that genetic adaptation to thermoregulation in warm climates affects the size and shape of shorebirds (Boxes 2 and Fig. 2). Gutierrez-Pinto N, McCracken KG, Alza L, Tubaro P, Kopuchian C, Astie A, et al. The validity of ecogeographical rules depends on context: examination of Bergmann and Allen rules in terms of latitude and altitude in a widely used Andean duck. Biol J Linn Soc. 2014;111:850–62. Tian, L. & Benton, M. J.
Predicting biotic responses to future climate warming with classic ecogeographic rules. Curr. 30, R744âR749 (2020). Our results are best explained by the thermoregulatory hypothesis, as birds in Northern Australia have both smaller bodies and a longer beak than their Counterparts in South Australia (Boxes 2 and Fig. 2). Differences in body size and beak length are more pronounced in resident species that experience local climatic conditions throughout the year (Fig. 3BâD). We interpret these models in terms of selection to support heat loss in Australia`s tropical north: it is unlikely that the difference is fully explained by adaptation to the heat economy in South Australia, as there is a pattern for migratory and resident species and migrants from northern and southern Australia travel to the cold northern hemisphere. to reproduce58. Researchers have suggested that animal size and shape choices in the tropics should be relaxed, and research on migratory birds focuses on morphological adaptations for thermal insulation in the northern hemisphere.8,20 However, non-morphological adaptations could allow migratory birds in northern Australia to develop smaller bodies and longer beaks while saving heat in their breeding grounds.