DIFFERENCES IN BODY SIZE AMONG CHUCKWALLA (SAUROMALUS OBESUS) POPULATIONS

Abstract

I raised juvenile chuckwallas (Sauromalus obesus; Iguanidae) that were collected from six different populations under identical conditions in a laboratory common-garden experiment. This experiment allows partitioning of genetic causes from environmental causes of the differences in adult size found in different populations. Chuckwallas from different home populations showed what appear to be genetic differences in size and patterns of growth. Lizards from populations with small adults grew faster before maturity and slower after maturity than those from populations with larger adults. Size and growth rates were correlated with elevation among sites for chuckwallas both raised in the laboratory and in natural populations. For example, at low-elevation sites, chuckwallas grew comparatively rapidly to reach maturity in a shortened growing season. A high frequency of drought at these hot, dry sites may necessitate storage of energy after maturity as a buffer against the relatively high probability that food will be limited in the following year. Lizards at higher elevation sites, however, are freed from this constraint, allowing them to use energy for growth to reach larger size and therefore to produce larger clutches. Although plant species diversity in the wild showed an increase with increasing elevation, food abundance, measured as the aboveground biomass of annuals, did not. This indicates that the length of time food is available, rather than its abundance, is of primary importance to chuckwallas.