Individual growth of the Flathead Knobscaled Lizard, Xenosaurus platyceps, from tropical and temperate populations

Growth rates of ectotherms are frequently affected by environmental conditions, such as temperature, precipitation, and prey availability. However, local adaptation to environmental conditions could influence geographic variation in growth rates. We studied growth rates of the Flathead Knob-scaled Lizard, Xenosaurus platyceps, from a tropical population and a temperate population in Tamaulipas, Mexico. We used a field study and a common garden experiment to examine the extent and potential causes of variation in individual growth rates between these two populations from contrasting environments. Field-determined growth rates in the tropical population were greater than those in the temperate population. In a laboratory common garden experiment that manipulated food availability and temperature for neonates from each population, neonates from the tropical population grew faster than those from the temperate population. Neonates kept at higher temperatures grew faster than those kept at lower temperature. In addition, growth rates of neonates from temperate and tropical populations did not differ at low temperature, but neonates from the tropical population grew faster at the higher temperature than did the neonates from the temperate population. Food treatment did not affect individual growth rates in these lizards, although there was a trend for neonates grown with higher food availability to grow faster. Our results suggest that not only does the warm environment of the tropical population contribute to the observed differences in field growth rates, but there is a genetic difference in the ability of the tropical population to take advantage of the warmer temperatures to increase growth rates.