Asymmetry and vestigial structures in extant birds

It is commonly asserted that vestigial structures have a greater degree of fluctuating asymmetry than functional structures. The reason given for this link between vestigial structures and fluctuating asymmetry is relaxed selection pressure, as well as fitness benefits associated with redirection of energy and other resources to functional structures. Perhaps surprisingly, few studies have examined and quantified the amount of fluctuating asymmetry in paired vestigial structures, especially limbs. The present study is an attempt to fill this void and to test the following hypotheses: (1) Skeletal elements of limbs not used for locomotion will be more asymmetric than the skeletal elements of limbs used for locomotion (both within specimens and between groups), and (2) limb asymmetry will increase with time since the limb became vestigial. Four locomotor categories were examined: Tropicbirds (regularly use forelimb, but not hindlimb for locomotion), Flighted rails (regularly use both fore‐and hindlimb for locomotion), Grounded Rails (only use hindlimb for locomotion; lost flight recently), Ratites (only use hindlimb for locomotion; did not lose flight recently). Within each category, attempts were made to obtain a statistically‐significant sample (n ≥ 30) while still controlling for phylogeny and body size. Eight bilateral measures were taken, including maximum lengths and condylar breadths. Dimenstions were converted into percentage directional (%DA) and absolute (%AA) asymmetries. Results reveal significant fluctuating asymmetry in the forelimb elements of flightless taxa; however, tropicbirds do no show significant asymmetry in their hindlimb elements. Despite a small sample size of ratites, results also suggest a greater degree of fluctuating asymmetry in the forelimbs with time since the limb became vestigial. Grant Funding Source : Center for Functional Anatomy and Evolution, Johns Hopkins