Evolutionary Morphology of Flightlessness in the Auckland Islands Teal

The morphological bases of flightlessness in the Auckland Islands Teal (Anas [a.] aucklandica) are described through mensural and qualitative comparisons with its flighted relatives--Grey Teal (A. gibberifrons), Chestnut Teal (A. castanea), and Brown Teal (A. chlorotis)--using 448 study skins, 107 skeletons, dissections of three anatomical specimens, and supplementary data on body mass and wing areas of other dabbling ducks. Limited mensural data also were collected from two skin specimens and 38 skeletal elements of the flightless Campbell Island Teal (A. [a.] nesiotis). Anas aucklandica is the smallest of the four species, has the smallest relative wing length in the subtribe Anateae, and has tail lengths showing higher variance and sexual dimorphism than those of its closest flighted relatives. Numbers of primary remiges show a modal reduction of one in A. aucklandica, but counts varied from seven to 10 per wing and 21% of the specimens were laterally asymmetric in counts; a loss of several secondary remiges also is indicated. Anas aucklandica is characterized by a 50% reduction in the mean length of primary remiges and increased variation of the lengths of primary remiges; these changes together result in the highest wing loadings in Anas (2.2 g cm-2). Canonical analysis of external measurements confirmed that "relative wing size" of A. aucklandica is substantially less than that of its flighted relatives, a proportionality which also characterizes juveniles of all four species; A. chlorotis was intermediate in relative wing size. Sexual dimorphism is comparatively great in A. aucklandica, despite its smaller overall size. Osteological measurements confirmed a roughly 50-mm reduction in skeletal wing length of A. aucklandica, and revealed that the humerus is disproportionately long and the ulna and carpometacarpus disproportionately short in the species; a similar pattern of disproportionately long proximal segments and disproportionately short mid-elements occurs in the leg. Multivariate comparisons of five sternal measurements indicate that A. aucklandica, and to a lesser degree A. chlorotis, show juvenile characters of shallow carinae and caudal emargination. Canonical analysis of 23 skeletal measurements revealed that the juvenile skeletal proportions of A. aucklandica and the intermediate proportions of A. chlorotis, relative to A. gibberifrons and A. castanea, define a complex axis contrasting dimensions of the pectoral girdle, antebrachium, and manus with those of the skull, humerus, leg elements, and other measurements of the manus and sternum. Dissections indicate that A. aucklandica is virtually identical (qualitatively) to A. castanea and A. gibberifrons in its pectoral musculature, but that most pectoral muscles are disproportionately small in the flightless form. The unique morphometrics and presumed genetic isolation of A. aucklandica justify its recognition as specifically distinct from A. chlorotis; the taxonomy of the group has had significant implications for evolutionary inferences in the past. Anas aucklandica is the only anatid in which loss of flight was coincident with a derived decrease in body size. Small body size, underdevelopment of the pectoral appendage, and lessened sexual dichromatism of A. aucklandica, and to a lesser extent A. chlorotis, are interpreted as paedomorphic conditions, adaptive as developmentally economic changes related to year-round residency in a predator-depauperate insular environment, and are most likely the result of progenesis. Correlated changes in life histories, particularly increased egg size and decreased clutch size, characterize both A. chlorotis and A. aucklandica and distinguish the paedomorphosis of A. aucklandica as an exception to the general association between K-selection and neoteny.