From slenderness to robustness: Understanding long bone shape in sigmodontine rodents

Area of Science:

• Evolutionary biology
• Comparative anatomy
• Biomechanics

Background:

• Appendicular skeleton morphology reflects environmental pressures, phylogeny, and allometry.
• Covariation in bone shapes indicates morphological integration driven by ecological specialization.
• Geometric morphometrics offers a more detailed approach than classical linear morphometry for studying bone shape diversity.

Purpose of the Study:

• To explore morphological diversity of sigmodontine rodent long bones using geometric morphometrics.
• To investigate the influence of locomotor type and phylogeny on limb bone shape and integration.
• To examine relationships between ecological categories and morphological characters in rodents.

Main Methods:

• Geometric morphometric analysis of humerus, ulna, femur, and tibiofibula from 19 sigmodontine species.
• Categorization of species based on locomotor types: ambulatory, quadrupedal-saltatorial, natatorial, semifossorial, and scansorial.
• Analysis of bone shape variation and covariation within an ecomorphological framework.

Main Results:

• Functional demands and evolutionary history jointly shape forelimb and hindlimb bone morphology.
• A slenderness-robustness gradient in bone shape correlates with ecological categories.
• Quadrupedal-saltatorial species have slender, elongated limbs; natatorial and semifossorial species have shorter, robust bones.
• Functional and anatomical covariation observed between specific limb bones (e.g., ulna-tibiofibula, humerus-ulna).

Conclusions:

• The versatile morphology of sigmodontine long bones is crucial for their ecological and phylogenetic diversification.
• Limb bone shape and integration are influenced by a combination of locomotor demands and evolutionary history.
• Geometric morphometrics provides valuable insights into the relationship between form, function, and adaptation in skeletal morphology.