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

Every year, shortly after the emancipation of chicks at our study colony (D eception Island, South Shetlands), hundreds of carcasses of presumably star ved Chinstrap penguin Pygoscelis antarctica chicks are washed on the shore. In 1997 we measured the nippers of fresh carcasses and compared their leng ths with those of live chicks about to become independent. There was a high ly significant difference of 6.5 mm between both distributions, which sugge sts strong directional phenotypic selection on skeletal size operating thro ugh its association with body reserves at independence. Given that heritabi lities of nipper 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 corre lation of 0.44(+/- 0.14), we can expect an evolutionary response to this se lection episode. Assuming that the target of selection is weight at emancip ation (heavier chicks carry proportionally larger reserves), and that nippe r length changes as a consequence of its genetic correlation with weight, w e can predict a response of 1.32-2.87 mm or 0.23-0.51 standard deviation un its for nipper length. This substantial evolutionary response may be counte red by other selective pressures affecting other life stages of these birds . Selection on reserve storage capacity at independence may affect morpholo gical traits also in other species.