Phenotypic selection on morphology at independence in the Chinstrap penguin Pygoscelis antarctica

Every year, shortly after the emancipation of chicks at our study colony (Deception Island, South Shetlands), hundreds of carcasses of presumably starved Chinstrap penguin Pygoscelis antarctica chicks are washed on the shore. In 1997 we measured the flippers of fresh carcasses and compared their lengths with those of live chicks about to become independent. There was a highly significant difference of 6.5 mm between both distributions, which suggests strong directional phenotypic selection on skeletal size operating through its association with body reserves at independence. Given that heritabilities of flipper length and body weight measured on 36 families are 0.73(± 0.32) and 0.075(± 0.081), and that both characters show a genetic correlation of 0.44(± 0.14), we can expect an evolutionary response to this selection episode. Assuming that the target of selection is weight at emancipation (heavier chicks carry proportionally larger reserves), and that flipper length changes as a consequence of its genetic correlation with weight, we can predict a response of 1.32–2.87 mm or 0.23–0.51 standard deviation units for flipper length. This substantial evolutionary response may be countered by other selective pressures affecting other life stages of these birds. Selection on reserve storage capacity at independence may affect morphological traits also in other species.