Evolution of the humeral plexus in penguins

Thomas, Daniel Blair.

Hot-blooded pen gums (Sphenisciformes) forage in cold oceans, and face the constant risk of hypothermia. The energetic costs incurred through foraging are partially mitigated by a complex set of heat-conserving upper-wing arteries and veins known as the humeral plexus. The humeral plexus provides a counter-current heat exchange system that redirects heat back towards the body core. Such a system is hypothesised to be a major contributor towards the evolutionary fitness of marine homeotherms. Determining the antiquity of the humeral plexus within the relatively ancient penguin lineage was the central aim ofthis thesis. Dissections of Eudyptes sclateri, Eudyptula minor and Megadyptes antipodes wings confirmed that the humeral plexus occurs in all living penguin species, whereas the absence of a humeral plexus in Puffinus griseus and other outgroup taxa suggested that the soft tissue structure evolved after the split between Sphenisciformes and Procellariiformes. The humeral arterial sulcus, an osteological correlate to the humeral plexus, was identified in 15 species from all six modem penguin genera. Two fossil penguin species, Palaeeudyptes klekowskii and Tonniornis mesetaensis from Seymour Island, Antarctica, are the oldest penguins studied with humeral arterial sulci and indicate that humeral plexi have been part of the penguin lineage since at least the Late Eocene. Additional evidence for humeral plexi in fossil penguins was sought from the paleophysiological proxy of oxygen isotopes incorporated into bone while the animal lived. Two new methods were developed as part of this work: 1) a nitric acid method for extracting phosphate from bleached bone; 2) a Raman-spectroscopic method of assessing diagenetic alteration. Wing temperature profiles calculated from phosphate oxygen isotopes were calibrated using modem species. Megadyptes antipodes exhibited a strong cooling profile along the. length of the wing, whereas Puffinus griseus exhibited thermal constancy between the middle humerus and proximal phalanx of the second digit. Isotopes from Palaeeudyptes gunnari demonstrated a cooling pattern similar to M antipodes, providing further evidence that the humeral plexus had evolved by the Late Eocene. The humeral plexus must, therefore, have evolved within the penguin lineage during the warmest times of the Cenozoic, or Late Cretaceous, indicating that heat retention conferred a significant energetic advantage to stem-penguins foraging in waters that were cooler than the core body temperature of ~38.5°C.

169 leaves : ill. (some col.) ; 30 cm.

2009Thomas