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Ecology and caudal skeletal morphology in birds: the convergent evolution of pygostyle shape in underwater foraging taxa.

Felice RN, O'Connor PM - PLoS ONE (2014)

Bottom Line: The evolution of this function for the tail contributed to the diversification of birds by allowing them to utilize a wider range of flight behaviors and thus exploit a greater range of ecological niches.This study explores whether differences in flight behavior are also associated with variation in caudal vertebra and pygostyle morphology.Thus, distinct locomotor behaviors influence not only feather attributes but also the underlying caudal skeleton, reinforcing the importance of the entire caudal locomotor module in avian ecological diversification.

View Article: PubMed Central - PubMed

Affiliation: Department of Biological Sciences, Ohio University, Athens, Ohio, United States of America ; Ohio Center for Ecology and Evolutionary Studies, Ohio University, Athens, Ohio, United States of America.

ABSTRACT
Birds exhibit a specialized tail that serves as an integral part of the flight apparatus, supplementing the role of the wings in facilitating high performance aerial locomotion. The evolution of this function for the tail contributed to the diversification of birds by allowing them to utilize a wider range of flight behaviors and thus exploit a greater range of ecological niches. The shape of the wings and the tail feathers influence the aerodynamic properties of a bird. Accordingly, taxa that habitually utilize different flight behaviors are characterized by different flight apparatus morphologies. This study explores whether differences in flight behavior are also associated with variation in caudal vertebra and pygostyle morphology. Details of the tail skeleton were characterized in 51 Aequornithes and Charadriiformes species. Free caudal vertebral morphology was measured using linear metrics. Variation in pygostyle morphology was characterized using Elliptical Fourier Analysis, a geometric morphometric method for the analysis of outline shapes. Each taxon was categorized based on flight style (flap, flap-glide, dynamic soar, etc.) and foraging style (aerial, terrestrial, plunge dive, etc.). Phylogenetic MANOVAs and Flexible Discriminant Analyses were used to test whether caudal skeletal morphology can be used to predict flight behavior. Foraging style groups differ significantly in pygostyle shape, and pygostyle shape predicts foraging style with less than 4% misclassification error. Four distinct lineages of underwater foraging birds exhibit an elongate, straight pygostyle, whereas aerial and terrestrial birds are characterized by a short, dorsally deflected pygostyle. Convergent evolution of a common pygostyle phenotype in diving birds suggests that this morphology is related to the mechanical demands of using the tail as a rudder during underwater foraging. Thus, distinct locomotor behaviors influence not only feather attributes but also the underlying caudal skeleton, reinforcing the importance of the entire caudal locomotor module in avian ecological diversification.

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Avian caudal skeleton in left lateral (A) and dorsal (B) views.Abbreviations: fcv, free caudal vertebra; pyg, pygostyle; syn, synsacrum. Scale bar equals 5 mm.
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pone-0089737-g001: Avian caudal skeleton in left lateral (A) and dorsal (B) views.Abbreviations: fcv, free caudal vertebra; pyg, pygostyle; syn, synsacrum. Scale bar equals 5 mm.

Mentions: The avian tail is one such highly variable structure, with modern birds using the tail as an integral component of the flight apparatus [16]–[18]. The role of the tail in flight is to supplement the lift produced by the wings during slow flight, reduce whole-body drag, and both stabilize and maneuver the bird during flight [19]–[22]. Bird tail morphology is specialized for its function as part of the locomotor apparatus and consists of an articulated fan of tail feathers, separate muscular systems for tail movements and tail fanning, and a modified, shortened tail skeleton [18]. The avian caudal skeleton consists of several (five to nine) free caudal vertebrae (Fig. 1). The terminal element of the caudal skeleton is the pygostyle, represented by a single, co-ossified unit consisting of the fused caudal-most vertebrae, ranging from three to seven in number [18], [23]. This serves as an attachment site for caudal musculature, tail feathers, and as an anchor for the tail fanning mechanism itself [16], [18].


Ecology and caudal skeletal morphology in birds: the convergent evolution of pygostyle shape in underwater foraging taxa.

Felice RN, O'Connor PM - PLoS ONE (2014)

Avian caudal skeleton in left lateral (A) and dorsal (B) views.Abbreviations: fcv, free caudal vertebra; pyg, pygostyle; syn, synsacrum. Scale bar equals 5 mm.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3935938&req=5

pone-0089737-g001: Avian caudal skeleton in left lateral (A) and dorsal (B) views.Abbreviations: fcv, free caudal vertebra; pyg, pygostyle; syn, synsacrum. Scale bar equals 5 mm.
Mentions: The avian tail is one such highly variable structure, with modern birds using the tail as an integral component of the flight apparatus [16]–[18]. The role of the tail in flight is to supplement the lift produced by the wings during slow flight, reduce whole-body drag, and both stabilize and maneuver the bird during flight [19]–[22]. Bird tail morphology is specialized for its function as part of the locomotor apparatus and consists of an articulated fan of tail feathers, separate muscular systems for tail movements and tail fanning, and a modified, shortened tail skeleton [18]. The avian caudal skeleton consists of several (five to nine) free caudal vertebrae (Fig. 1). The terminal element of the caudal skeleton is the pygostyle, represented by a single, co-ossified unit consisting of the fused caudal-most vertebrae, ranging from three to seven in number [18], [23]. This serves as an attachment site for caudal musculature, tail feathers, and as an anchor for the tail fanning mechanism itself [16], [18].

Bottom Line: The evolution of this function for the tail contributed to the diversification of birds by allowing them to utilize a wider range of flight behaviors and thus exploit a greater range of ecological niches.This study explores whether differences in flight behavior are also associated with variation in caudal vertebra and pygostyle morphology.Thus, distinct locomotor behaviors influence not only feather attributes but also the underlying caudal skeleton, reinforcing the importance of the entire caudal locomotor module in avian ecological diversification.

View Article: PubMed Central - PubMed

Affiliation: Department of Biological Sciences, Ohio University, Athens, Ohio, United States of America ; Ohio Center for Ecology and Evolutionary Studies, Ohio University, Athens, Ohio, United States of America.

ABSTRACT
Birds exhibit a specialized tail that serves as an integral part of the flight apparatus, supplementing the role of the wings in facilitating high performance aerial locomotion. The evolution of this function for the tail contributed to the diversification of birds by allowing them to utilize a wider range of flight behaviors and thus exploit a greater range of ecological niches. The shape of the wings and the tail feathers influence the aerodynamic properties of a bird. Accordingly, taxa that habitually utilize different flight behaviors are characterized by different flight apparatus morphologies. This study explores whether differences in flight behavior are also associated with variation in caudal vertebra and pygostyle morphology. Details of the tail skeleton were characterized in 51 Aequornithes and Charadriiformes species. Free caudal vertebral morphology was measured using linear metrics. Variation in pygostyle morphology was characterized using Elliptical Fourier Analysis, a geometric morphometric method for the analysis of outline shapes. Each taxon was categorized based on flight style (flap, flap-glide, dynamic soar, etc.) and foraging style (aerial, terrestrial, plunge dive, etc.). Phylogenetic MANOVAs and Flexible Discriminant Analyses were used to test whether caudal skeletal morphology can be used to predict flight behavior. Foraging style groups differ significantly in pygostyle shape, and pygostyle shape predicts foraging style with less than 4% misclassification error. Four distinct lineages of underwater foraging birds exhibit an elongate, straight pygostyle, whereas aerial and terrestrial birds are characterized by a short, dorsally deflected pygostyle. Convergent evolution of a common pygostyle phenotype in diving birds suggests that this morphology is related to the mechanical demands of using the tail as a rudder during underwater foraging. Thus, distinct locomotor behaviors influence not only feather attributes but also the underlying caudal skeleton, reinforcing the importance of the entire caudal locomotor module in avian ecological diversification.

Show MeSH