Being stressed out is something everybody can relate to. Driving in heavy traffic, taking an exam, interviewing for a job, or dealing with family issues could all produce a similar stress response. Humans aren’t the only ones to experience stress. Zebras and lizards may not do math problems, but they may flee from or respond to a dangerous predator. These types of short-duration stressors are called “acute” and often involve only a single event. In humans, that might involve driving in heavy traffic or doing some difficult mental math. In the ecological world, acute stressors can include fighting with a competitor for food or territory or an encounter with a predator. Stressors that are persistent or long-lasting are called “chronic.” In humans, this could include getting over a recent break-up, being unemployed, caring for sick relatives, or worrying about money problems. In other animals, chronic stress could include exposure to a long winter storm or drought or continued difficulty finding food.
Although the kinds of stressors are quite different, the way an animal’s (or human’s) body responds is actually quite similar in most vertebrates. This stress response includes a suite of physiological changes that happen inside the body to help deal with the stressor. Part of what helps encourage these responses is the production of “stress hormones,” like cortisol in humans and fish or corticosterone in reptiles, amphibians, and birds. This suite of changes is often thought of in terms of energy: stress hormones help redistribute energy toward things that are immediately important, like escaping from a predator. This energy is taken away from functions like reproduction or growth, which are not as important in the short-term.
The stress response is very important for dealing with stressors, but what happens when the stressor doesn’t stop or is frequently repeated for long time (e.g. is chronic)? Under conditions of chronic stress, growth, reproduction, and immune function are often suppressed. Perhaps frequently taking energy away from these processes is not the best idea if it means an animal won’t be able to become big enough to mate or successfully produce offspring.
If you’ve ever been in the Southern US, I’m sure you can relate to the stressful experience of being bitten by a fire ant! Fire ants can bite, sting, and even kill a lizard, so it makes sense that they have elevated levels of stress hormones (corticosterone) after a fire ant attack. Because they are frequently attacked, these lizards experience chronic stress, which in theory should lead to immune suppression. This got Tracy and I thinking: although they have some neat ways of dealing with fire ants, lizards from fire ant invaded sites may frequently get bitten, which breaks the skin, or stung (envenomated), which activates the immune system. Thus, it seems that the immune system is important to lizards at invaded (high-stress) sites and that suppressing the immune system under chronic stress would be a bad idea for these lizards. Perhaps populations from invaded high-stress sites have adapted so that they do not suppress their immune function under chronic stress.
Last year (summer 2012), we designed a project to determine if this were the case. I simulated the elevation of stress hormones that occurs from a fire ant attack by directly applying corticosterone (CORT) to the backs of lizards. (Corticosterone is dissolved in sesame oil, which is easily absorbed through the lizard’s skin.) We applied CORT every day for 23 days to lizards from invaded high-stress and uninvaded low-stress sites. Some lizards received just oil each day, to act as a control and base of comparison. At the end of treatments, we measured immune function to see if lizards from invaded and uninvaded sites responded differently to this chronic stress.
We expected all lizards that received CORT treatments would have reduced immune function compared to lizards that received just oil (control lizards). Additionally, we expected that this reduction would be less in lizards from invaded high-stress sites versus those from uninvaded low-strew sites. The graph below demonstrates this expectation. Taller bars indicate “better” immune function.
We used two different measures of immune function, both of which roughly measure the ability of lizard blood to deal with foreign particles, in this case E. coli (not a virulent strain!) and sheep red blood cells. Surprisingly, lizards that received CORT treatments actually had higher immune function compared to lizards that received just oil. Additionally, lizards from both invaded high-stress and uninvaded low-stress sites responded the same way to treatments.
There are may reasons why me may have seen these results. Here are a few of our many ideas:
- Our lizards were fed a healthy diet of tasty crickets–likely more than they would eat in the wild. Perhaps for immune suppression to occur under chronic stress, lizards need to be food (and thus energy) limited. (With a healthy supply of energy, there may have been no need to divert energy from other systems like the immune system to deal with the stressor.)
- Perhaps some parts of the immune system are more important than others. Some components that we did not measure in this study may be “okay” to compromise under stress in this context, while the components we did measure may not.
- Perhaps CORT levels were able to return to “normal” between treatments, which makes this method more of a “repeated acute” stressor than a “chronic” stressor. We suspect that frequency and duration of a stressor are important predictors for consequences of stress, and I performed a study this past August to investigate this idea. I’ll be sure to share the results of that experiment in a future update!
These results were presented at the 2013 SICB annual meeting.
UPDATE: These results are published in General and Comparative Endocrinology: read it here!