The Ontogeny of Carapace Morphology in Gopher Tortoises (Gopherus polyphemus)
Juvenile gopher tortoises (Gopherus polyphemus) have a domed carapace whereas adults have a noticeably flattened carapace. Given their domed appearance and small size, we hypothesized that the domed carapace of juveniles decreases the righting response time in the event that the individual is flipped onto its back (e.g. by a predator). This study quantified the ontogeny of carapace morphology in Gopherus polyphemus. Seventy five juvenile to adult gopher tortoises were photographed in lateral and dorsal views. Using morphometric software (tpsDig), 26 landmarks were digitized from each photograph. This approach allowed us to quantify the important region(s) of shape change over a broad range of sizes including hatchlings, juveniles, and adult tortoises. Furthermore, we linked changes in morphology with behavior by quantifying the righting response across this same sample. Large adult gopher tortoises can not right themselves and can die if left on their back. Given the initial observation of the domed carapace of juveniles, we hypothesized that juveniles would be able to right themselves faster than adult tortoises with their dorsoventrally flattened carapaces. To test this, we flipped tortoises onto their backs and measured the time it took them to right themselves. Trials were conducted in an arena containing loose, very sandy soil that is identical to the sandhill environs that gopher tortoises inhabit. Regardless of size or shape, only 6.7% of tortoises were able to right themselves. We conclude that the change in carapace morphology is not related to the righting response. Instead, carapace shape may relate to allometric patterns of growth, and/or the pronounced increase in burrowing activity individuals exhibit as they mature.
Society for Integrative and Comparative Biology Annual Meeting (SICB)
San Antonio, TX
Waddell, Janice C., Lance D. McBrayer, David C. Rostal.
"The Ontogeny of Carapace Morphology in Gopher Tortoises (Gopherus polyphemus)."
Biology Faculty Presentations.