Relationships between female quality, egg mass and eggshell blue-green colouration in southern rockhopper penguins : a test of the sexual signalling hypothesis
Faculty of Sciences. Biology
Faculty of Social Sciences. Instructional and Educational Sciences
Polar biology. - New York
, p. 1805-1811
University of Antwerp
Eggshell blue-green colouration (BGC) is caused by the pigment biliverdin which has antioxidant capacities. Eggshell BGC has therefore been interpreted as being costly for the female and therefore a signal of female quality (sexual signalling hypothesis). Southern rockhopper penguins Eudyptes chrysocome exhibit both a reversed hatching asynchrony and a brood reduction strategy. First-laid (A-)eggs are smaller and hatch on average 1 day after second-laid (B-)eggs, with B-eggs usually producing the only surviving chick. According to the sexual signalling hypothesis, we predicted a positive relationship between BGC and both female body mass and egg mass, and consequently within clutches a stronger BGC in B-eggs than A-eggs. Furthermore, we expected a negative relationship between BGC and clutch initiation date. Contrasting these expectations, we found no effect of female body mass or egg mass on BGC, and BGC in A-eggs increased with clutch initiation date, while there was no effect in B-eggs. Within clutches, A-eggshells were more intensely blue-green coloured than B-eggshells. Concluding, our results appear to contradict the sexual signalling hypothesis. We, however, did not measure pigment concentrations and solely relied on BGC from eggshell photospectrometry, assuming that biliverdin concentrations were positively correlated with BGC. We therefore caution that before precluding the sexual signalling hypothesis, future studies that include measurements of eggshell pigment concentrations in addition to BGC are necessary. Altogether, a better understanding of the biological meaning of biliverdin, its biochemical synthesis and antioxidative function in the avian body is urgently needed.