Life is about compromises, and energy-hungry processes such as reproduction and immunity compete for an organism's sometimes limited energy supplies. A female tree lizard's energy demands go up when she is reproductive and producing lots of eggs, but if there isn't enough energy to go around, then another process suffers. The availability of energy is probably partially regulated by corticosterone (CORT), which mobilises energy stores and regulates the stress response. Susannah French from Arizona State University and her colleagues tested the idea that CORT regulates the distribution of limited energy between the reproductive and immune systems by manipulating CORT levels in reproductive and non-reproductive female tree lizards under different stresses (p. 2859).
However, as French explains, `one major problem with trying to manipulate hormone levels is measuring the response'. Handling animals is stressful for them, and so is giving them regular injections. `The best thing to do is leave them alone', she says. So when French's colleague Dale DeNardo saw a talk by bioengineer Brent Vernon on an injectable gel implant which releases substances into the body at a controlled rate, French knew that they had arrived at a solution. Collaborating with Vernon, the team developed a lizard implant consisting of a mixture of polymers, plus CORT, which they mixed up and injected into the animals under sedation such that the implant set in the lizards' bodies just after the injection.
For their experiments, the team captured non-reproductive and reproductive female lizards in their Sonoran desert home before transporting them back to the lab in cloth bags. They placed half of both groups on a restricted diet of two crickets a week, while the others could gorge themselves on as many crickets as they could eat. Within each dietary group, half the animals received CORT implants.
Two days after receiving their slow-releasing CORT implants and starting their new diets, the team gave each of the lizards a small circular wound in the top layer of their skin. They were surprised by what they found when they measured the area of the wounds five days later to see how CORT affected wound healing in the different groups. `We expected that all the CORT animals would have a suppressed [immune] response', French says. Instead, the team found that CORT had an effect only when the lizards were energy limited.
They found that the non-reproductive females on a restricted diet had slower wound healing when they received a CORT implant than untreated animals. This suggests that CORT is effective in compromising non-reproductive females'immune response only when they are in energy debt.
Turning their attention to the reproductive females, the team found that all those on restricted and unrestricted diets who had also received a CORT implant had restricted wound healing. So the added stress of reproduction,which uses up a lot of energy, makes a bad energetic situation worse. Having unlimited access to crickets isn't enough to redress the energy deficit. `CORT is involved in mobilising resources, but not directly involved in the immune response', says French, who is working next on tracking down what other factors affect the wound healing response.