Maternal effects in Wood Ducks: incubation temperature influences incubation period and neonate phenotype

G. R. Hepp¹, R. A. Kennamer², and M. H. Johnson³

¹School of Forestry and Wildlife Sciences, Auburn University, Auburn, AL 36849
²Savannah River Ecology Laboratory, Drawer E, Aiken, SC 29801
³Department of Biostatistics AE-3035, Medical College of Georgia, Augusta, GA 30912-4900

Summary:
1. Maternal effects often explain a significant amount of variation in offspring phenotype, and can be important in the evolution of life histories. Incubation of eggs is an important maternal effect, and optimal growth and development of avian embryos takes place within a narrow range of incubation temperatures, but the effect of incubation microclimate on neonate phenotype remains relatively unexplored in birds.
2. In this study of Wood Ducks (Aix sponsa Linnaeus) we examined effects of incubation temperature on the length of incubation period and neonate quality. We monitored nest temperatures and incubation periods of naturally incubated Wood Duck nests and found a strong inverse relationship between incubation period and average nest temperature.
3. Next, we collected three unincubated eggs from each of 48 nests, and randomly assigned eggs from each nest to one of three incubation temperatures (34·6, 36·0 and 37·4 °C). Experimental incubation temperatures overlapped average nest temperatures of naturally incubated Wood Duck nests.
4. Hatching success varied with incubation temperature and was lowest for eggs incubated at the lowest temperature. Incubation period of experimental eggs decreased with increasing temperature but was not affected by fresh egg mass.
5. Wood Duck embryos catabolized an estimated 34–38% of egg lipids and 25–33% of egg protein during incubation. Percentage change of lipids increased with decreasing incubation temperature, but not significantly. Embryos incubated at lower temperatures used a greater percentage of protein than embryos incubated at higher temperatures.
6. In analyses using fresh egg mass as the covariate, we found that wet and dry mass of ducklings increased with increasing incubation temperature. Decreases in lipid content of Wood Duck neonates with decreasing incubation temperature were not significant, but eggs incubated at low temperatures produced ducklings that had reduced protein mass and that were structurally larger than ducklings from eggs incubated at high temperatures.
7. Our study illustrates the importance of incubation temperature on the development of Wood Duck embryos. Decisions made by incubating parents that influence egg temperature can modify incubation period and offspring phenotype. Investigations of incubation as a reproductive cost should consider how parental decisions influence both parents and offspring.

Key words: Aix sponsa, Anatidae, parental care, reproductive success