Maternal effects are maternal traits that influence offspring phenotype via non-genetic pathways. In many cases maternal effects can have profound influences on offspring, often rivaling the effects of genetic contributions to offspring fitness. In birds, an important maternal effect is incubation behavior. Although incubating parents must leave the nest to forage, maintaining constancy of the incubation environment is vital to the hatching success of the eggs. Changes in the incubation environment can also influence offspring phenotype. Although this phenomenon is poorly-understood in birds, in reptiles we know that the incubation environment can influence sex ratios, offspring size, body shape, growth rate, locomotor performance, behavior, and survival.
Recent research on wood ducks (Aix sponsa) demonstrates that subtle changes (~1 degree C) in incubation conditions can influence hatchling characteristics. Hepp et al. (2006) incubated wood duck eggs at three temperatures that encompassed the range of naturally-incubated wood duck nests. They found that ducklings incubated at low temperatures took longer to hatch and were smaller and had lower protein content than ducklings incubated at higher temperatures. In the Hopkins lab, we have explored the possible mechanisms underlying the observed effects of incubation temperature on hatchling mass and protein content. In our lab, Sarah DuRant and colleagues measured energy expenditure of embryos incubated at various temperatures and found that embryos incubated at lower temperatures expended more energy throughout incubation than those incubated at higher temperatures.
In addition, DuRant and colleagues explored other effects of incubation temperature on hatchling phenotype to provide further insight into the effects of incubation environment on offspring and maternal fitness. Specifically, she found that ducklings incubated at lower temperatures had lower immunocompetence (i.e., bacterial killing ability of the blood, cell-mediated immunity, antibody production in response to exposure to a novel antigen, and wound healing), lower thermoregulatory ability, slower growth, higher baseline and stress-induced corticosterone levels, lower thyroid hormone levels, and slower locomotor ability (with Brittney Hopkins Coe) than ducklings from warmer temperatures. Following that, our collaborators, Gary Hepp (Auburn University) and Robert Kennamer (University of Georgia) found that lower incubation temperature led to lower duckling survival and recruitment in the wild.
Currently, our lab is investigating how clutch size may affect incubation in natural populations. Clutch size varies considerably in natural wood duck populations because hens participate in conspecific brood parasitism, wherein hens lay some of their eggs in other hens’ nests. Because incubation behavior is costly for parents, incubation behavior and/or incubation temperature may be affected by clutch size. If clutch size influences optimal incubation, then incubation may be a mechanism by which optimal clutch size in constrained, or an overlooked cost of brood parasitism. In the summer of 2014, the Hopkins lab created model duck eggs with temperature loggers inside them and used these to monitor incubation temperatures throughout the wood duck breeding season at Department of Energy’s Savannah River Site in South Carolina. This temperature data is being analyzed to determine the effects of clutch size on thermal dynamics of the nest as well as behavior of the incubating hens.
Additionally, our lab is continuing to investigate how incubation temperature may affect duckling phenotype by studying duckling behaviors that are crucial for early survival. For her dissertation, Sydney Hope is studying how incubation temperature affects duckling exploratory and boldness behavior, nest exodus performance, and competition for heat and food.
In conjunction with the data collected by Hepp, our research will provide unprecedented insight into the links between the incubation environment, offspring and maternal fitness, and the tradeoffs between parental care and self-maintenance. Additionally, our research will provide insight into constraints on the evolution of clutch size and the costs of brood parasitism. In light of the fact that human disturbances such as climate change and habitat degradation can influence the delicate balance between parental care and self-maintenance, our work will elucidate how these relationships are modulated by parents in a changing world.