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, 4 (11), e7999

Predatory Functional Morphology in Raptors: Interdigital Variation in Talon Size Is Related to Prey Restraint and Immobilisation Technique

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Predatory Functional Morphology in Raptors: Interdigital Variation in Talon Size Is Related to Prey Restraint and Immobilisation Technique

Denver W Fowler et al. PLoS One.

Abstract

Despite the ubiquity of raptors in terrestrial ecosystems, many aspects of their predatory behaviour remain poorly understood. Surprisingly little is known about the morphology of raptor talons and how they are employed during feeding behaviour. Talon size variation among digits can be used to distinguish families of raptors and is related to different techniques of prey restraint and immobilisation. The hypertrophied talons on digits (D) I and II in Accipitridae have evolved primarily to restrain large struggling prey while they are immobilised by dismemberment. Falconidae have only modest talons on each digit and only slightly enlarged D-I and II. For immobilisation, Falconini rely more strongly on strike impact and breaking the necks of their prey, having evolved a 'tooth' on the beak to aid in doing so. Pandionidae have enlarged, highly recurved talons on each digit, an adaptation for piscivory, convergently seen to a lesser extent in fishing eagles. Strigiformes bear enlarged talons with comparatively low curvature on each digit, part of a suite of adaptations to increase constriction efficiency by maximising grip strength, indicative of specialisation on small prey. Restraint and immobilisation strategy change as prey increase in size. Small prey are restrained by containment within the foot and immobilised by constriction and beak attacks. Large prey are restrained by pinning under the bodyweight of the raptor, maintaining grip with the talons, and immobilised by dismemberment (Accipitridae), or severing the spinal cord (Falconini). Within all raptors, physical attributes of the feet trade off against each other to attain great strength, but it is the variable means by which this is achieved that distinguishes them ecologically. Our findings show that interdigital talon morphology varies consistently among raptor families, and that this is directly correlative with variation in their typical prey capture and restraint strategy.

Conflict of interest statement

Competing Interests: The authors have declared that no competing interests exist.

Figures

Figure 1
Figure 1. Feet of representative raptors.
Note the digit length and relative enlargement and curvature of claws within each foot: Accipitridae bear hypertrophied talons on D-I and II; Falconidae have only modest talons on each digit and only slightly enlarged D-I and II; Strigiformes bear large talons with comparatively low curvature on each digit; Pandionidae have enlarged, highly recurved talons on each digit. (A) Accipitridae: goshawk, Accipiter gentilis, MOR OST-1276; (B), Accipitridae: red-tailed hawk, Buteo jamaicensis MOR OST-1275; (C) Falconidae: peregrine falcon, Falco peregrinus, MOR OST-1265; (D) Strigiformes: great grey owl, Strix nebulosa, MOR OST-1284; (E) Pandionidae: osprey, Pandion haliaetus, MOR OST-1268.
Figure 2
Figure 2. Size and curvature measurements taken from each claw, using methodology of Pike and Maitland (2004).
(A) Outer curvature measurements. ALo, arc length from claw base to tip; Ao, straight line (chord) distance from claw base to tip; Hmo, height of claw at midpoint; Oo, angle of curvature. (B) Inner curvature measurements. ALi, arc length from claw base to tip; Ai, straight line (chord) distance from claw base to tip; Hmi, height of claw at midpoint; Hp, height of claw at base; Oi, angle of curvature.
Figure 3
Figure 3. Correspondence analysis of relative claw and toe sizes on each digit amongst taxa.
Raptor taxa group tightly into discrete family clusters. Axis 1 accounts for 58.1% of the variation within the data set, and Axis 2 accounts for 25.5%. Axis 1 is controlled by the sizes of all claws relative to toe 3. Axis 2 is mainly driven by the sizes of claws 1 and 2 relative to claw 4 and toe 3, with relative toe sizes also influencing taxa distribution. Measurement ratios used: claw-I/claw-IV, claw-II/claw-IV, claw-III/claw-IV, claw-I/toe-III, claw-II/toe-III, claw-III/toe-III, claw-IV/toe-III, toe-I/toe-IV, toe-II/toe-IV, toe-III/toe-IV. These ratios are displayed because they best explained the variation within the data set using the fewest number of axes.
Figure 4
Figure 4. Bivariate plots of claw morphology illustrating family level differentiation.
(A) Raptors show variation in the relative size of D-I and II claws among families; Accipitridae have hypertrophied D-I and II claws; Falconidae have only slightly enlarged claws on D-I and II; Strigiformes and Pandionidae have claws that are all similar in size. (B) Falconids can be most easily differentiated from non-raptors by the greater inner curvature of D-II claw, and that the D-II/D-III claw arc length ratio is greater than or very close to 1.
Figure 5
Figure 5. Phylogenetic diagram plotting occurrence of morphologic traits.
Numbered traits in parentheses are present only in selected taxa within the clade (see main text). 1. D-II talon as large or larger than D-III; 2. short tarsometatarsus; 3. hypertrophied D-I and D-II talons; 4. elongate toes; 5. highly recurved talons on all digits; 6. subequally large talons on each digit. General arrangement of families after Hackett et al . Nomenclature and arrangement of Falconidae and Accipitridae after Griffiths et al , respectively (see main text for exceptions).

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