Many parents will probably agree that raising a brood is a strain, but a female eider duck has a tougher job than most. A female duck sits tight when incubating her clutch without eating a morsel, losing weight in the process. Scientists have shown that acquired immunity - when the immune system makes antibodies in response to invading pathogens - decreases in female ducks during the fast. Researchers suspect that the stress hormone corticosterone,which causes the body to break down proteins for energy during the final stages of the fast and which also affects immune responses, might be responsible. To investigate, Sophie Bourgeon and Thierry Raclot at CNRS,Strasbourg, France, implanted corticosterone pellets under the skin of incubating ducks to find out how extra corticosterone affects ducks' immunity(p. 4957). The implanted ducks lost 35% more weight than ducks with no implants, and the levels of immune system proteins called immunoglobulins, which include antibodies, decreased by twice as much. These results suggest that corticosterone plays an important role in affecting female ducks' immunity while they are incubating their eggs.

Bourgeon, S. and Raclot, T. (
). Corticosterone selectively decreases humoral immunity in female eiders during incubation.
J. Exp. Biol.