Modular adaptation of the ribcage in response to artificial selection for endurance running

The capability for sustained running has convergently evolved multiple times in mammals, and involves myriad anatomical, physiological, and behavioral adaptations. The ribcage plays a critical role in both respiration and locomotion but its adaptations to running are largely unexplored. Robustly testing adaptation in wild populations is challenging, so we use artificial selection for voluntary wheel-running behavior (i.e., High Runner or HR mice) to directly test form-function relationships associated with sustained running. We compared ribcage configuration and shape of HR (males: 52, females: 47) to control (males: 48, females: 48) mice using rib counts and 3D Geometric Morphometrics. Two of four HR lines had an additional rib and increased variation in the proportion of true to false ribs, suggesting that ribcage patterning has been impacted by selection. This variability among lines suggests that selection for wheel running has resulted in adaptations that are expressed variably among the selected lines, resulting in "multiple solutions" to selection. Total ribcage shape did not vary significantly between HR and Control mice. Instead, the effect of selection varied along the ribcage, with significant effects in the caudal ribs. Further, the caudal ribcage of HR mice showed increased disparity, within-rib, and among-rib integration compared to controls. The strong response of caudal ribs indicates a modular pattern of adaptation, with cranial ribs possibly constrained by their role in ventilation. This study demonstrates that adaptation in the mammalian ribcage is likely shaped by a complex interplay of selection and craniocaudal integration and may result in variation at multiple anatomical levels (count, shape, modularity).