New perspectives on head and neck allometry and ecomorphology in tetrapods

The skull and neck are vital parts of the body, influencing feeding ecology, habitat exploitation and locomotion. Numerous studies have therefore sought to understand how the size of these segments vary with ecology and scale with overall body size. However, across past literature many different metrics have been used to represent both head and neck size, alongside disparate methods for body size normalisation and varied statistical approaches to analysing patterns. Furthermore, while several studies have examined allometric patterns across species of birds and dinosaurs, there are relatively few studies of other groups like mammals, non-avian reptiles and amphibians. It is therefore currently difficult to combine or compare analyses from past studies to arrive at a clear picture of ecological and taxonomic trends in tetrapod head and neck allometry and evolution. To address these issues, we present a new analysis of head and neck proportions using a data set of 410 three-dimensional digital skeletons that samples a wide taxonomic breadth of extinct and extant terrestrial tetrapods. Allometric and ecological patterns in head and neck size were analysed using phylogenetically informed approaches, with head and neck size quantified using multiple metrics representative of a range of methods used across previous studies. We find that different measurements used in the literature to represent head and neck size do not always yield qualitatively consistent results in terms of allometric patterns within and between major taxonomic and ecological groups. For example, across tetrapods, all metrics suggest negative allometry in skull size, whereas the pattern of allometry seen in the neck is influenced by the metric used (length versus volume). We also find that allometric patterns in linear metrics for head and neck size are better described by a linear model, whilst volumetric measurements are better fitted by a quadratic model for both the head and neck. Statistical support for quadratic models appears to be driven by species over 100 kg tending to show greater negative allometry in skull volume, whilst the neck shows strong positive allometry above this approximate size threshold. The disparate allometric patterns given by different metrics typically result from systematic variation in segment shape, which may often have adaptive significance. For example, distinct allometric trends in skull length and width are recovered across taxonomic and trophic groups, which may represent mechanical interactions between bite force and velocity in different feeding modes, particularly carnivory, insectivory and piscivory. Potentially adaptively significant patterns were also recovered in neck allometry for piscivores, and in the neck-head scaling seen in carnivores and herbivores, where a larger head in bigger carnivores enables capture of large prey but necessitates a reduced neck, while relatively smaller head sizes in herbivores (as food processing shifts to the gut) allow longer necks to increase the three-dimensional volume ('feeding envelope') accessible to the head-neck system. The disparate qualitative and quantitative allometric relationships given by different metrics suggests that future work should carefully consider the choice of parameter used to represent skull and neck size when comparing trophic and taxonomic groups and making ecological and macroevolutionary inferences.