Study of the carapace shape and growth in two Galápagos tortoise lineages

Abstract

Galápagos tortoises possess two main shell forms, domed and saddleback, that correlate with the biogeographic history of this species group. However, the lack of description of morphological shell variation within and among populations has prevented the understanding of the contribution of evolutionary forces and the potential role of ontogeny in shaping morphological shell differences. Here, we analyze two lineages of Galápagos tortoises inhabiting Santa Cruz Island by applying geometric morphometrics in combination with a photogrammetry 3D reconstruction method on a set of tortoises of different ages (from juvenile to adult). The aim of this study is to describe morphological features on the carapace that could be used for taxonomic recognition by taking into account confounding factors, such as the morphological changes occurring during growth. Our results indicate that despite the shared similarities of growth patterns and of morphological changes observed during growth, the two lineages and the different sexes can be distinguished on the basis of distinct carapace features. Lineages differ by the shape of the vertebral (especially concerning their width) and pleural scutes, with one lineage having a more compressed carapace shape, whereas the other possesses a carapace that is more elongated and expanded toward the sides as well as an higher positioning of the first vertebral scute. Furthermore, females have a more elongated and wider carapace shape than males. Finally, carapace shape changes with growth, with vertebral scutes becoming narrower and pleural scutes becoming larger during late ontogeny.

Geometric morphometrics in herpetology: modern tools for enhancing the study of morphological variation in amphibians and reptiles

“…Most studies investigating allometric relationships, both in an ontogenetic and static context of allometry (i.e. GoULD, 1966), indicate that large part of the shape variation observed is attributable to sizeshape allometry (see MonTEIro & ABE, 1997;BIrcH, 1999;BonnAn, 2007;SMITH & coLLyEr, 2008;pIrAS et al, 2010;cHIArI & cLAUDE, 2011;IvAnovIć et al, 2011 for illustrative examples from different groups). caution is advised to avoid conceptual misunderstandings: while GM methods remove size variation through GpA or similar superimposition procedures, the allometric dependence of shape on size still remains in the data and can be investigated (MonTEIro, 1999).…”

“…regarding other reptile groups, size variation also seems to be a main determinant of shape variation in turtles, including ontogenetic, static and evolutionary allometric effects (cLAUDE et al, 2003(cLAUDE et al, , 2004DEpEckEr et al, 2006;AnGIELczyk, 2007;MyErS et al, 2007;nISHIzAwA et al, 2010;AnGIELczyk et al, 2011;cHIArI & cLAUDE, 2011). Different characters show varying degrees of variation in allometric trajectories.…”

“…In turtles, some studies have reported significant SD in shell shape, which is the only trait that has been examined. However, there is a marked discordance among authors regarding the comparison between traditional and geometric morphometrics for quantifying shape in turtles; while some authors use GM as a powerful method for capturing patterns invisible to traditional approaches, such as SD in hatchlings (vALEnzUELA et al, 2004;LUBIAnA & FErrEIrA JúnIor, 2009), others report a weaker SD in geometric shape as compared to that observed using linear measurements (cHIArI & cLAUDE, 2011). Such a disagreement may be due to differences in the degree and direction of shape SD in different taxa, a possibility supported by the limited data available (vALEnzUELA et al, 2004;cEBALLoS & vALEnzUELA, 2011).…”

Assessing 3d Photogrammetry Techniques in Craniometrics

“…New morphometric methodological approaches are effective in capturing reliable information about the shape of an organism and result in powerful statistical procedures for testing differences in shape (Rohlf and Marcus 1993). There is increasing evidence that the improvements of 3D reconstruction methodologies will aid in 3D morphometrics studies (Zollikofer and Ponce de León 2002;Claude et al 2003;Sholts et al 2010;Chiari and * Corresponding author Claude 2011;McLean et al 2012;Ifflaender et al 2013). The actual bio 1 logical materials (specimens) are not linear as used to be visualized in traditional methods, and the modern techniques such as 3D modelling could be more effective in enabling a researcher to visualize differences in shape (Rohlf and Marcus 1993).…”

“…PMSc® is an accurate method for 3D modelling and measurement recording (Walford 2008). In addition, no decision on which variables should be measured is required in advance and therefore, one can evaluate the usefulness of alternative suites of variables without handling the original specimens again (Rohlf and Marcus 1993).The technique of 3D photogrammetry by PMSc® in morphometrics was successfully used on tortoise carapace (Chiari and Claude 2011) and on horse hoof deformation (Jordan et al 2001). The study investigated the use of 3D Photogrammetry in craniometrics.…”