The development of diving capacity in juvenile emperor

penguins (Aptenodytes forsteri) should be critical to their survival and success, especially during their first trip to sea in which they travel beyond the Antarctic Circle (Kooyman et al., 1996). However, little is known about the diving ability of juveniles or about the development of the physiological factors underlying that ability. In a study of penguin myoglobin (Mb), Weber et al. (1974) reported extremely low to almost undetectable Mb concentrations in one young Adelie (Pygoscelis adeliae) and one young gentoo (Pygoscelis papua) chick, and a Mb concentration in a fledged Adelie chick that was 43 % of the adult value. In a study of another diving bird, the pigeon guillemot (Cepphus columba), Haggblom et al. (1988) reported a similar developmental pattern for Mb, and also found that blood hemoglobin (Hb) concentration and pectoral muscle lactate dehydrogenase (LDH) activity had reached adult levels by the time of fledging. These studies suggest that Mb, but not Hb or muscle enzyme activity, may be a critical determinant in the development of diving capacity by fledged chicks. This is especially pertinent to emperor penguins, in which the muscle O2 store constitutes 47 % of the estimated total O2 store of adults (Kooyman and Ponganis, 1998). Therefore, in this study, we report diving characteristics of juvenile emperor penguins from the Cape Washington colony (74°32′S; 165°22′E) and developmental changes in body mass (Mb), Hb concentration, Mb content and muscle enzyme activities. Muscle LDH activity was examined as an indicator of anaerobic capacity and to determine whether the developmental changes observed in LDH activity in pigeon guillemots (Haggblom et al., 1988) also occurred in emperor penguins. Citrate synthase (CS) activity was examined as an index of aerobic capacity.