Data from: Testing the CREA ‘rule’ in Galliformes

Recent comparative studies have indicated the existence of a common cranial evolutionary allometric (CREA) pattern in mammals and birds, in which smaller species have relatively smaller faces and bigger braincases than larger species. In these studies, cranial allometry was tested using a multivariate regression between shape (described using landmarks coordinates) and size (i.e. centroid size), after accounting for phylogenetic relatedness. Alternatively, cranial allometry can be determined by comparing the sizes of two anatomical parts using a bivariate regression analysis. In this analysis, a slope higher or lower than 1 indicates the existence of positive or negative allometry, respectively. Thus, in those species that support the CREA 'rule', positive allometry is expected for the association between face size and braincase size, which would indicate that larger species have disproportionally larger faces. In the present study, I applied these two approaches to explore cranial allometry in 83 Galliformes (Aves, Galloanserae), ranging in body weight from 30 g to 2.5 kg. The multivariate regression between shape and centroid size revealed the existence of a significant allometric pattern resembling CREA, whereas the second analysis revealed a negative allometry for beak size and braincase size (i.e. contrary to the CREA 'rule', larger galliform species have disproportionally shorter beaks than smaller galliform species). This study suggests that the presence of CREA may be overestimated when using cranium size as the standard measurement.

近期的比较研究揭示，哺乳类与鸟类均存在一套共享的颅部演化异速生长（cranial evolutionary allometric, CREA）模式：相较于体型较大的类群，体型更小的物种拥有相对更短的面部与更大的颅腔。此类研究中，研究者会在校正系统发育亲缘关系后，通过以形态（通过地标坐标描述）与体型（即质心尺寸）间的多元回归检验颅部异速生长现象。此外，也可通过双变量回归分析比对两个解剖结构的尺寸，以此判定颅部异速生长：该分析中，若回归斜率高于或低于1，则分别对应正异速生长与负异速生长。因此，在符合CREA“法则”的类群中，面部尺寸与颅腔尺寸的关联应呈现正异速生长，这意味着体型更大的物种其面部比例会相对更长。本研究采用上述两种方法，探究了83种鸡形目（Aves, Galloanserae，鸡雁小纲）物种的颅部异速生长情况，其体重跨度为30克至2.5千克。形态与质心尺寸间的多元回归分析显示，存在与CREA相符的显著异速生长模式；而第二项分析则显示喙部尺寸与颅腔尺寸间呈现负异速生长——即与CREA“法则”相悖，体型更大的鸡形目物种其喙部比例相对更短。本研究提示，若以颅部尺寸作为标准化测量指标，CREA的存在性可能被高估。