Patterns of rectrix rachis modification in pintails and the evolution of sexually selected traits

Abstract

In order to transmit aerodynamic forces to the rest of the body, tail feathers need to be stiff to resist lift forces with minimum deformation. Because delta-wing theory predicts that such feathers do not produce lift forces beyond the point of the maximum continuum width of the tail, species with pintails should not require stiff central rectrices distal to that point. We tested this prediction by comparing the relative thickness of the central rectrix rachis in taxa with pintails and triangular tails. Fourteen pairs of closely related species or species groups belonging to the families Phaethontidae, Phalacrocoracidae, Anatidae, Stercorariidae, Psittacidae, Trochilidae, Alcedinidae, Momotidae, Meropidae, Bucerotidae, Tyrannidae, Pipridae and Nectariniidae were compared. Twelve of the phylogenetically independent comparisons showed that the taxa with triangular tails have higher relative rachis thickness (RRT) than their pintailed relatives just behind the point of the maximum continuum width of the tail. In contrast, two taxa with pintails showed proportionately higher RRT than their triangular-tailed relatives. Triangular tails showed an approximately linear relationship between RRT and relative rachis length, which contrasts with a proportionately greater increase in RRT from distal to proximal parts of the feather in 12 pintailed taxa. These results show that in most of the pintailed taxa studied the distal part of the central rectrix rachis has not been selected to resist lift forces and may be adaptively reduced to attenuate the costs of a hypertrophied ornament. However, the presence of distally reinforced rachices in Eumomota superciliosa and Colonia colonus suggests that a different explanation may be required to account for the design of pintail structure in other taxa. © 2005 The Linnean Society of London.