Foot shape in arboreal birds: two morphological patterns for the same pincer-like tool

Area of Science:

• Evolutionary biology
• Comparative anatomy
• Biomechanics

Background:

• The feet serve as the primary interface between limbed animals and their environment.
• Complex environments, particularly for bipedal and arboreal species, exert strong selective pressures on foot morphology.
• Avian foot morphology exhibits remarkable diversity in toe orientation, making it an excellent model for studying evolutionary constraints.

Purpose of the Study:

• To investigate the evolutionary patterns and functional significance of avian foot morphology.
• To determine the influence of habitat, toe orientation, and phylogenetic constraints on foot structure.
• To identify distinct morphological patterns and their underlying developmental and functional controls.

Main Methods:

• Phalangeal proportion analysis within a phylogenetic framework.
• Comparative analysis of foot morphology across different avian species and habitats.
• Examination of skeletal development and muscle-morphogenetic control.

Main Results:

• Two primary avian foot morphological patterns were identified, correlating with habitat and toe orientation.
• In anisodactyl feet, hallux size varies with terrestriality, and a proximo-distal gradient in phalanx size is observed.
• In heterodactyl and zygodactyl feet, the hallux is reduced while another backward-pointing toe is enlarged, a pattern unique to birds.

Conclusions:

• Avian foot evolution demonstrates convergent patterns related to skeletal development and unique patterns influenced by muscle-morphogenetic control.
• Functional adaptations lead to a consistent pincer-like foot structure across diverse avian lineages.
• Foot morphology is a key adaptation for navigating complex three-dimensional environments.