Egg characteristics in relation to nesting microenvironment in captive Southern Rockhopper Penguins, Eudyptes chrysocome

Background/Question/Methods

Southern Rockhopper Penguins, Eudyptes chrysocome, practice a reversed asynchronous brood strategy in which second eggs are up to 70% larger and hatch a day or more sooner than first eggs (St. Clair 1996). Previous research has shown that first eggs are viable (Poisbleau et. Al. 2008). If first eggs are not “throwaway” eggs there may be some advantage given to smaller first eggs to help them compete with their larger older siblings. I hypothesized that first eggs of rockhopper penguins experience developmental advantages and preferential treatment over second eggs which offset disadvantages associated with reversed hatching asynchrony.

Research was conducted on captive rockhopper penguins during the 2010 and 2011 breeding seasons at The Riverbanks Zoo and Garden in Columbia, SC. Egg characteristics, as well as those of the nesting environment, were measured to better understand this approach to incubation regime. Length and breadth measurements of the eggs were taken, and mass was recorded every day. Candling techniques were used to monitor the development of eggs. The temperature and humidity under each parent in the nest was recorded using data logging hygrometers in dummy eggs.

Results/Conclusions

The mean laying interval was 4.0 ± 0.6 days. First eggs had significantly smaller masses and greater surface-area-to-volume ratios (p<0.05) than second eggs but were equal in density. The interval between laying date and onset of development was equal for first eggs, second eggs, and first eggs that never had a partner. The change in mass over the course of incubation was equal in fertile and infertile eggs, indicating that mass loss patterns are the result of the microhabitat of the nest and not the metabolic processes of the embryo. First eggs that never had a nestmate lost mass more rapidly after day 4 than first eggs with a nestmate (p<0.05), and experienced higher temperatures, which were equal to those of second eggs (p<0.50), reflecting a reallocation of resources by the parents. The presence of the second egg was not necessary to elicit this rise in temperature. Humidity availability in the nest did not vary by day or egg type. Rockhopper penguins appear to adjust their incubation regime in relation to the expected lay date of their second egg.