Abstract
The interplay between the pectoral module (the pectoral girdle and limbs) and the pelvic module (the pelvic girdle and limbs) plays a key role in shaping avian evolution, but prior empirical studies on trait covariation between the two modules are limited. Here we empirically test whether (size-corrected) sternal keel length and ilium length are correlated during avian evolution using phylogenetic comparative methods. Our analyses on extant birds and Mesozoic birds both recover a significantly positive correlation. The results provide new evidence regarding the integration between the pelvic and pectoral modules. The correlated evolution of sternal keel length and ilium length may serve as a mechanism to cope with the effect on performance caused by a tradeoff in muscle mass between the pectoral and pelvic modules, via changing moment arms of muscles that function in flight and in terrestrial locomotion.
Highlights
The pectoral module and the pelvic module of birds are specialized for different functions, they are likely to be linked during evolution (Allen et al, 2013; Gatesy & Dial, 1996; Heers & Dial, 2015)
In the morphospace defined by sternal keel length and ilium length (Fig. 2), several outliers are identifiable in these extant birds
Gavia stellata deviates from others, but it largely follows the pattern of a positive correlation between sternal keel length and ilium length
Summary
The pectoral module (the pectoral girdle and limbs) and the pelvic module (the pelvic girdle and limbs) of birds are specialized for different functions, they are likely to be linked during evolution (Allen et al, 2013; Gatesy & Dial, 1996; Heers & Dial, 2015). The functional specialization could weaken the integration between the pectoral and pelvic limbs, as suggested by morphometric analyses of avian and mammalian limbs (Bell, Andres & Goswami, 2011; Schmidt & Fischer, 2009; Young, Hallgrímsson & Janis, 2005).
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