FLIGHTLESSNESS IN GREBES (AVES, PODICIPEDIDAE): ITS INDEPENDENT EVOLUTION IN THREE GENERA

Area of Science:

• Evolutionary Biology
• Ornithology
• Comparative Anatomy

Background:

• Flightlessness in birds is a fascinating evolutionary adaptation.
• Grebes (family Podicipedidae) exhibit diverse adaptations to aquatic environments.
• Understanding the morphological underpinnings of flightlessness provides insights into evolutionary processes.

Purpose of the Study:

• To investigate the morphological characteristics associated with flightlessness in three grebe species.
• To compare the anatomical changes related to flightlessness across different grebe genera.
• To determine if these species are truly flightless or merely flight-impaired.

Main Methods:

• Analysis of 790 study skins and 322 skeletons.
• Inclusion of myological data from 40 anatomical specimens.
• Examination of ancillary data on wing-loadings and morphometric measurements.

Main Results:

• Convergent evolution of flightlessness in three grebe species (Rollandia microptera, Podilymbus gigas, Podiceps taczanowskii) characterized by increased body mass, reduced wing/tail lengths, and smaller pectoral muscles.
• Rollandia microptera showed the most significant divergence, with substantial changes in body mass, wing elements, sternum, and pectoral musculature.
• Podilymbus gigas exhibited minor differences, suggesting flight impairment due to allometric size increase and reduced muscle mass.
• Podiceps taczanowskii was intermediate, with reduced wing length and skeletal element widths; wing-loading data suggests it may be flight-impaired.

Conclusions:

• Morphological changes associated with flightlessness in grebes are largely paedomorphic, potentially involving delayed development of flight structures.
• Flightlessness in these species is linked to year-round residency in high-altitude lakes, offering advantages like thermodynamic benefits of larger size and feeding specialization.
• The evolution of flightlessness may also be driven by reduced niche overlap via increased sexual dimorphism and economy in pectoral-alar development.