Predicting body size: The role of behavior in determining growth rates of juvenile eastern fence lizards, Sceloporus undulatus

Background/Question/Methods

Body size is an important trait that affects an individual’s survival and reproductive success.  Variation in growth rates after birth can lead to differences in body size, and therefore have important fitness consequences. Understanding the factors that affect growth rates, and whether these are fixed at birth or can change through development, can allow us to understand the drivers of this important ecological trait. Increased activity levels have been found to correlate with larger adult body size across many populations and species.  However, it is unclear whether more active animals have higher growth rates and end up larger, or whether larger animals are more active as a consequence of their greater size. Eastern fence lizard (Sceloporus undulatus) siblings hatch at the same size, but grow at very different rates. We used this species to test 1) whether activity levels at birth correlate with within-clutch differences in growth rates, 2) whether individual activity levels remain consistent over time, and 3) the effect of competition on activity levels and growth rates. To do this, we measured the activity of group- and individually-housed hatchlings before and after siblings had diverged in body size at eight weeks of age.

Results/Conclusions

Our results reveal that larger lizards have higher activity rates, and that variation in activity levels exists before differences in body size are evident even within sibling groups. However, behavior at birth did not correlate with  body size at eight weeks. This is likely due to changes in activity of individual lizards across time; the lizards that were most active at birth were not always the most active at eight weeks of age. Competition appeared to drive changes in activity, since individually-housed lizards showed little change in activity over time. These data suggest that an individual’s activity levels at birth do not dictate its future growth rates. Instead, activity level is a flexible trait that changes through development, possibly in response to competition for resources. Future research should investigate the factors that drive changes in activity levels and fitness consequences of activity levels in the field, including survival rates and reproductive success.