Evolution of gliding in squirrel‐related rodents (Mammalia: Sciuromorpha) did not induce a new optimum on the cortical thickness of the scapular glenoid fossa

Abstract

Many of the squirrel‐related rodents (i.e., Sciuromorpha) are tree‐dwelling species known to be very agile climbers. This taxon also includes the most diverse clade of gliding (aerial) mammals that likely descended from a non‐gliding arboreal ancestor and evolved a patagium (i.e., a gliding membrane) to increase gliding performance. Glides can cover distances of up to 150 m and landing is typically accomplished by stalling the patagium to reduce impact velocity. It remains unclear if this behavior suffices to keep stresses on the locomotor apparatus similar to those experienced by their arboreal relatives or whether gliding behavior increases landing forces and stresses. The sparsely available support reaction force data are ambiguous, but bone microstructure is highly adaptable to changes in loading regime and likely provides insights into this question. Using μCT scans, we compared the cortical thickness of the glenoid fossa of the shoulder joint between arboreal and aerial Sciuromorpha using evolutionary model comparison, while also accounting for regional differences of the glenoid fossa. We did not find any differences between these locomotor behaviors, irrespective of the glenoid region. These findings agree with previous analyses of the microstructure of the femur in Sciuromorpha. We discuss different aspects that could explain the similarity in cortical thickness. According to our analysis of glenoid cortical thickness the loading regime appears not to have changed after the evolution of gliding locomotion, likely due to adjustments in landing performance.

Ecomorphology and sensory biology of bats

“…Instead, the authors suggest behavioral changes, such as adjustments in landing posture, may help deal with these forces. The findings of Berghäuser et al (2023) also offer indirect support to the hypothesis that bats evolved from gliding mammals, as they demonstrate that gliding does not require major changes to bone architecture, such as increased robusticity. Thus, hypothetical gliding ancestors to bats could have had gracile, lightweight limb skeletons similar to extant flying squirrels, and would have been ideally positioned to evolve further modifications allowing flight (Rickman et al, 2023).…”

“…In the absence of fossils constituting stages that may presage flight adaptations, scenarios for flight evolution have been proposed and tested using extant mammals as models, often involving intermediate stages resembling gliding locomotion (e.g., Bishop, 2008; Burtner et al, 2023). In this issue, Berghäuser et al (2023) examined cortical bone thickness of the glenoid fossa in arboreal climbing versus aerial (gliding) squirrels. They found no significant difference in thickness between locomotor groups, suggesting that no biomechanical adaptation in terms of bone robusticity has evolved to resolve landing forces at the end of a glide.…”