Because many factors influence growth, this latter source of shape variation could be very important, particularly for ectotherms. Nevertheless, few studies have tested whether variation in growth rate per se really can induce size-independent shape variation in vertebrates. However, this latter result might be just another example of the training effect on trophic structures. So whether environmentally induced growth rate variation per se is a likely source of metric shape variation for head and limb traits in vertebrates remains an open question. Here we test whether temperature-induced variation in tadpole growth rate can cause variation in relative head width or leg length at metamorphosis.
Temperature is one of the most important variables controlling growth in ectotherms.
Approximately 50 eggs from each of two recently laid egg masses of the Cascades frog, Rana cascadae, were collected from a roadside pond on route 20 near Santiam Pass, Oregon. These two temperatures were chosen in order to generate a large variance in growth rate. They are well within the range of temperatures experienced by the species in the field. We used the average of two measurements on each trait. One larva died in the cold room shortly after the experiment was set up, but the remaining 23 developed normally and appeared healthy.