Assessing the relative influence of island climatic and geographical factors on body size in an endangered iguana species of the French Caribbean

Body size is an important organismal trait on which many physiological and behavioral factors depend, and can be used to study how animals adapt to insular environments. In this regard, reptiles on islands exhibit remarkable size extremes, ranging from giants to some of the smallest vertebrate species in the world. In addition, the dependence of ectotherms on external temperature makes them particularly sensitive to climatic conditions. We tested the hypotheses that adult body size in Lesser Antillean iguanas (Iguana delicatissima) would increase with island area and isolation, as well as with increasing annual rainfall and ambient temperature. We used a database of 6878 individuals collected on seven islands in the French West Indies from 2009 to 2021. We measured individual size by the snout-vent length. GLMM analyses showed that iguana body size increased with island size, and that iguanas on the islands located closer to the mainland were larger compared to those on islands located the furthest from the mainland. Regarding climatic conditions, we found that annual rainfall and ambient temperature had no significant effect on iguana body size. These findings indicate that geographical conditions have a greater influence on iguana size compared to climatic conditions. The lack of influence of climatic conditions may be related to the fact that iguanas are critically endangered, meaning that their populations consist of limited numbers of individuals far from the carrying capacity of their habitat. If ecological resources are not currently a limiting factor for Lesser Antillean iguanas, then conservation efforts could successfully promote the growth of their populations.