The mechanical design of woodpecker tails as an adaptation to scansorial habits

POLEDRI, BRENDA L; LIJTMAER, DARÍO A.; TUBARO, PABLO L.

Congreso; Evolution 2021; 2021

Society for the Study of Evolution (SSE)

Woodpeckers (Picidae) typically move vertically on tree trunks using the tail as a support for body weight. This unusual means of support (scansorial) correlates with several putative morphological adaptations including an increase in the flexural stiffness of rectrix rachices. However, a rigorous test of the adaptive value of these characters is still lacking. Here we present the results of a comparative morphological study focused in woodpeckers, including other scansorial and non-scansorial bird groups. Rachis diameter was positively associated with body mass as expected if tail structure was a simple allometric product of body size. Scansorial birds had larger rachis diameters for a given body mass than cursorial birds, and an increased proportion of cortex. However, woodpeckers differed from woodcreepers in several critical aspects: 1) they had significatively shorter tails; 2) they evidenced a faster decrease of the dorso-ventral diameter of rachices towards the tip, and 3) they had a larger proportion of cortex and a smaller proportion of medullary tissue along the distal parts of their rachices. These results indicate that woodpecker tail feathers are not designed to resist Euler bucking (i.e. the tendency to collapse by arching along their length when the feather is loaded axially by body weight) as in woodcreepers, but to allow a ?controlled? distal buckling that increase tail adherence to the trunk surface as well as the body weight that the proximal part of the tail can support without significant deformation.