Growth in pygoscelid penguin chicks

Abstract

A study of sympatrically breeding Adelie (Pygoscelis adeliae), Chinstrap (P. antarctica), and Gentoo (P. papua) penguins was undertaken on King George Island, South Shetland Islands to compare their rates and patterns of growth, and to relate these findings to pygoscelid breeding ecology. In contrast to other large oceanic birds, the logistic equation best described the biomass growth of pygoscelid penguins, as it did their culmen and flipper growth. Growth constants were 0146, 0127 and 0113; asymptotic weights were 3940 g, 4025 g and 5715 g for Adelie, Chinstrap and Gentoo penguins, respectively. Growth of feet and flippers was more rapid than culmen growth.Among the pygoscelids, the Adelie penguin has the most southern breeding range, the Gentoo the most northern; while the Chinstrap penguin's range is approximately intermediate to its congeners. In this and other studies, pygoscelid penguins show both an inverse correlation between their growth constants and asymptotic weights, and between their asymptotic weights and latitudinal distributions. Mean adult weights, however, are not correlated with latitudinal distributions as adult Adelie penguins weigh more than adult Chinstrap penguins. It is suggested that lower asymptotic weights are advantageous in the shorter breeding seasons found at higher latitudes.The similarity of growth rates of Adelie and Chinstrap penguins to day 37, and the decrease of the rate at which Chinstraps grow thereafter, does not support the hypothesis that Chinstrap penguin growth rates are retarded due to food competition with the Adelie penguin in areas of sympatry. The preponderance of krill (Euphausia spp.) in the diet of all these pygoscelid penguins suggests that differences in growth or survival rates resulted from factors other than dietary differences. Due to lack of available data, other such variables affecting survival and growth rates (e.g. predation, foraging strategies) could not be assessed.Recently pygoscelid penguin populations have expanded, probably in response to increased food resources. Adelie and Chinstrap populations have increased more rapidly than Gentoo populations. It is suggested that the lower rate of increase of Gentoo populations reflects basic differences in ecological strategies among pygoscelids, as evidenced by their growth patterns.