In many species, there is extreme variation in adult body size, with some individuals growing much larger than others.  Body size can have important implications for social dominance and therefore access to food, mates, and shelter. The goal of this research was to investigate the behavioral and physiological causes of natural body size variation in eastern fence lizards (Sceloporus undulatus). Specifically, we examined traits that might influence the amount of food an individual will obtain, such as bold behavior and willingness to eat in a stressful situation.  These traits can directly impact growth rates and body size.

Results/Conclusions

We found in behavior trials that larger individuals were not bolder and were just as likely to eat available food as their smaller counterparts, indicating that boldness is unlikely to be a driver for adult body size variation.  However, these larger lizards were more active during the trials.  Increased activity could enhance an individual’s chances of encountering food, leading to increased growth.  Activity rates may be driven by egg yolk steroids, such as testosterone.  Differences in maternal allocation of these steroids to eggs could result in behavioral and ultimately body size variation among siblings.  Both small and large body sizes may be favored under different environmental conditions; therefore, differential allocation may be a strategy for females to ensure some of their offspring will thrive within varying, unpredictable environments.