Travis W. Rusch and Michael J. Angilletta Jr.

Animals spend much of their time keeping themselves at the right temperature (i.e. thermoregulating) to maximize performances such as digestion, growth, and locomotion. For most species, the primary means of thermoregulation is shuttling between sun and shade. However, because temperature varies throughout space and time, organisms regularly compete for access to suitable microclimates, especially when these resources are rare. Such competition is costly, as competing individuals lose opportunities to thermoregulate, risk injury from fighting, and may experience hormonal changes. Often, features such as body size determines the outcome of competition, with larger individuals gaining greater access to resources. Thus, the costs of competition are often greater for smaller individuals.

In this study, we used heat lamps to investigate the behavioural and physiological costs of competition for mountain spiny lizards by observing the behaviors of solitary or paired males in thermal gradients. When paired, large males outcompeted small males for access to a heat source. Surprisingly, this behavior caused large males to thermoregulate less accurately and less precisely. This result likely occurred because larger males increased their use of the heat lamps in the presence of a small male. In fact, some of the large males allowed their temperatures to approach the lethal limit before leaving the heat source. Conversely, small males experienced lower body temperatures when paired with a large male, because males prevented them from regularly accessing the heat source. Competition also caused a stress response in the form of elevated corticosterone in both large and small males, though this effect was much larger in small males. Lastly, competition caused changes in testosterone in both large and small males, but in opposite directions. Large males tended to increase testosterone while small males decreased testosterone after being paired. Therefore, both large and small males incurred costs of competition for a limited heat source, including reduced thermoregulatory performance and greater physiological stress. These results highlight that thermoregulatory behavior is influenced by both abiotic and biotic factors, such as available thermal resources and competitors.

Image caption: Male mountain spiny lizard (Sceloporus jarrovi) basking under heat lamp in laboratory thermal arena.Photo Credit: Travis W. Rusch.
Read the article in full here.