Geographic Variation in Morphology and Allozymes of South American Imperial Shags

Abstract

To evaluate the hypothesis that Pleistocene glaciations in southern South America effected differentiation in the Imperial Shag (Phalacrocorax atriceps), I analyzed geographic variation in plumage, skeletal size and proportions, and allozymes in populations from seven South American localities. Alar bar size, crest length, and the size of the white dorsal patch vary geographically among juveniles but not adults, and the alar bar and white dorsal patch are both present all year in adults. Geographic variation in osteology is marked, with males showing more interpopulational differentiation than females. Coastal populations follow Bergmann's rule, while freshwater Fuegian lake shags are smaller than coastal populations even though the geographically nearest coastal population is the largest one in body size. Shags from the lowest latitude, in central Chile, have relatively long culmens, conforming to Allen's rule. All populations except that from Chubut Province are very similar in allozyme frequencies, and the very low genetic distances between populations are noncongruent with the greater degree of morphological differentiation, particularly between lake and coastal populations. The genetically differentiated Chubut population was probably somewhat isolated on the Patagonian Massif, a circumstance unrelated to Pleistocene glaciations. Chilean shags were probably not long reduced during glaciations to a small isolate, because they can form colonies on unglaciated headlands; they are only slightly differentiated from Atlantic populations. The treatment of the Falkland Islands population as a distinct subspecies is supported by plumage characters. The freshwater Fuegian shags must have differentiated since glacial recession. The populations from the southern Atlantic coast and coastal Tierra del Fuego, which were probably contiguous throughout the Pleistocene, differ from each other only clinally in size. Unlike the cases of other Fuego-Patagonian species studied, Pleistocene vicariance events appear to have had little influence on the evolutionary history of the Imperial Shag. This is probably related to life-history traits of this species such as coloniality, vagility, longevity, trophic generalism, and cold tolerance.