`Stress has serious consequences.' These are words you might expect to hear from a psychiatrist, family physician or even your mom, but what about a zoologist? Absolutely. The study of how animals respond to and cope with stressful social situations or environments has blossomed in the last 10–20 years. While many of us may be more familiar with work of this nature in primates and other mammals, it turns out that reptiles, particularly lizards, are an important model for understanding the effects of stress on animal behavior and physiology. Stress commonly leads to elevated levels of the hormone corticosterone in the blood, and in lizards this can have detrimental effects on a variety of behaviors that have consequences for reproductive success. It is less clear what effects corticosterone has on whole-organism physiology, but recent work by Donald Miles at Ohio University,Ryan Calsbeek at Dartmouth College and Barry Sinervo at the University of California, Santa Cruz, sheds more light on how elevated stress hormones impact traits like locomotor performance and metabolic rate.
Miles and colleagues traveled to the coastal mountains of California and collected male blue-throated side-blotched lizards (Uta stansburiana), known from previous work to get stressed by interactions with both more dominant and sneakier conspecifics. Once back in the laboratory, the team tested the lizards' endurance, by twice measuring how long they maintained a 0.5 km h–1 pace on a motorized treadmill. They then implanted either a corticosterone treatment or a saline sham into the lizards to find out how this affected their locomotor performance; the corticosterone implant is known to elevate hormone levels approximately 2- to 5-fold for several months. Testing the lizards' endurance again every 2–7 days for 1 month following treatment, the team found that in sham animals endurance remained steady before and for a month after treatment at about 200–250 s. In contrast, males with hormone implants lasted longer on the treadmill from approximately 2 weeks after the implantation surgery, peaking at day 29 where they ran for around 350 s.
To find out what impact the hormone implants might be having on metabolic rates, the team measured resting and maximum metabolic rates for 3–4 weeks in a second group of lizards with identical sham or hormone implants. They measured the resting rates at 15 min intervals during a 6 h period in a dark chamber, noting in which 15 min interval the lowest rate occurred. The single highest value obtained from each individual during treadmill exercise to exhaustion at 1 km h–1 gave the maximum rate. Metabolic tests revealed that males implanted with corticosterone had significantly lower resting rates than animals with saline shams and although maximum rates were not significantly different, hormone-implanted animals also exhibited values about 15% lower, on average, than the sham lizards.
Thus, stress hormones appear to help lizards save metabolic energy and also improve their locomotor endurance. While the authors can only speculate as to what the mechanism underpinning these effects might be, it is clear that increased circulating corticosterone is not necessarily all bad news. Perhaps I should look on the bright side the next time I'm stressing out; better yet,maybe I should go for a run – a long run.
