Alice Leavey: Comparative muscle anatomy of the anuran pelvis and hindlimb in relation to locomotor mode

For example, the behaviour and ecology of extinct animals must be inferred from limited fossilised remains; thus, a tight correlation between bone and soft-tissue anatomy, and how this relates to function, is often assumed (Bates et al., 2021). To better understand the strength of the relationship between form and function in extinct taxa, anatomical characteristics must be measured in living species for which behaviour and ecology are known (Perry & Prufrock, 2018). However, even this approach faces difficulty due to the ability of one trait to influence multiple functions (i.e. ‘one-to-many mapping’) and multiple morphological configurations to enable the same function (i.e. ‘many-to-one mapping’; Bergmann & McElroy, 2014; Holzman et al., 2011; Moen, 2019; Wainwright et al., 2005). Therefore, both detailed descriptions of how musculoskeletal anatomy varies among species and quantitative tests of how this can impact function must be carried out to fully understand the evolutionary origins of biological niches.

Anura, part of the class Amphibia, are ideal model organisms for tackling this fundamental challenge. Relatively small anatomical differences in their largely conserved body plan enable frogs to respond to various mechanical challenges and inhabit numerous ecological niches (Citadini et al., 2018; Gomes et al., 2009; Leavey et al., 2023; Lires et al., 2016; Moen et al., 2013; Soliz et al., 2017; Tulli et al., 2016; Vidal-García et al., 2014). 

For example, frogs use a range of locomotor modes including walking, hopping, jumping, swimming, burrowing and climbing to traverse different terrestrial, aquatic, arboreal and subterranean environments (Wells, 2007). Interspecific variation in pelvic and hindlimb myology has been long assumed to indicate differences in anuran locomotor behaviour (Collings & Richards, 2019; Fabrezi et al., 2014; Nauwelaerts et al., 2007; Ponssa et al., 2018), where a larger muscle indicates higher functional importance, as more energy has been invested into its growth despite the associated physiological and anatomical costs (e.g. daily energy expenditure) (Perry & Prufrock, 2018). 

Due to similar locomotor and microhabitat requirements, frogs also show repeated independent evolution of similar phenotypes on a global scale, suggesting that there are a limited number of ways in which frogs can respond to selection (Moen, 2019; Moen et al., 2016). This makes them ideal organisms for investigating the relationship between morphology, function, ecology and evolutionary history. However, many gaps in our knowledge of the anuran musculoskeletal system and its relationship to locomotor function remain, as a detailed comparative analysis of muscle anatomy has not been performed across multiple representative species for each primary locomotor mode.

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