INTRASPECIFIC VARIATION AND EVOLUTIONARY REDUCTION OF TENDON OSSIFICATION IN DENDROCINCLA WOODCREEPERS

We compared levels of intraspecific variation in hindlimb tendon ossif ication and other hindlimb myological traits in the Tawny-winged Woodc reeper (Dendrocincla anabatina) with our previous data on the Barred W oodcreeper (Dendrocolaptes certhia) and the Ivory-billed Woodcreeper ( Xiphorhynchus flavigaster). In D. anabatina, a member of a clade chara cterized by reduced tendon ossification, variation in tendon ossificat ion was markedly greater than in other aspects of the hindlimb muscles , as were occurrences of bilateral variation and the number of individ uals exhibiting atypical conditions. Variation in tendon ossification in D. anabatina is 5-10 times more frequent than in the other species, which have not experienced reduction in ossification. Tendon ossifica tion in D. anabatina, and possibly in other Dendrocincla species, prov ides a rare example of extensive intraspecific variation in the hindli mb myology of passerines. The variant conditions in D. anabatina evolv ed within the Dendrocincla clade, either since D. anabatina and its si ster group split, or as ancestral polymorphisms retained through speci ation events. In either case, the available evidence does not support a role for directional selection in the process of evolutionary reduct ion in tendon ossification in Dendrocincla.