Is the Pelvis Sexually Dimorphic in Turtles?

Abstract

Variation in the pelvis is intrinsically linked to life history evolution. This is perhaps best exemplified by sexually dimorphic pelvic variation in bipedal primates. Yet, whether this trend is applicable to other taxa is unclear. Using turtle anatomy as a model, I tested the hypothesis that the pelvis is also sexually dimorphic in egg-laying tetrapods. I sampled a natural turtle population with female-biased sexual size dimorphism (i.e., larger females). I show that the area of the egg canal (pelvic aperture) is greater in females. Morphological differences between sexes were predicted by body size, such that skeletal shape deformation of the female ilium increased proportionally with pelvic aperture area. These results suggest that sexual pelvic dimorphism might be indirectly maintained by selection for large female size, consistent with the pelvic constraint hypothesis in reptiles. However, subsampling of similarly sized individuals revealed that pelvic aperture area and shape may vary in disproportion to body size. Comparisons of pelvic ontogenetic trajectories across multiple lineages are needed to clarify the occurrence of sexual pelvic dimorphism in turtles and other egg-laying tetrapods. My findings provide impetus to further explore how sex-specific functional demands influence the architecture of the pelvic girdle. Anat Rec, 2018. © 2018 Wiley Periodicals, Inc.

Disentangling the correlated evolution of body size, life history, and ontogeny in miniaturized chelydroid turtles

“… Note : Data on embryonic S. odoratus were from this study and Smith Paredes et al (2020); embryonic C. serpentina data were mostly from Cordero et al (2018). Post‐hatching growth data were from Iverson (1991) and Iverson et al (1997).…”

“…Offspring survival in miniaturized species of Chelydroidea is perhaps not as dependent on body size because juveniles are highly adept at hiding in shallow‐water habitats or can remain within terrestrial shelters until environmental conditions are favorable (Cordero & Swarth, 2010; Ernst & Lovich, 2009). These species, therefore, invest in clutch sizes that may exceed five small eggs that nonetheless test the spatial limits imposed by the pelvic canal (Cordero, 2018). On the contrary, miniaturized tortoises that lay a single massive egg might be able to transiently weaken articulations of the pelvic girdle to permit the passage of eggs (Hofmeyr et al, 2005).…”

Shell histology of the Triassic turtle, Proterochersis porebensis Szczygielski & Sulej, 2016, provides novel insights about shell ankylosis

“…Sexual dimorphism expressed as size differences between the males and females is common in turtles (e.g. Berry & Shine 1980;Pritchard 2008;Leuteritz & Gantz 2013;Cordero 2018). It was also discussed previously for Proterochersis spp.…”

Flexible embryonic shell allies large offspring size and anti-predatory protection in viviparous snails

“…While our study on obstetric selection gives a new perspective in study of shelled molluscs, researchers have well explored the link between life history evolution, calcification and skeleton morphology in mammals and other vertebrates 1 , 6 , 27 , 28 . Those studies have mainly focused on the sexually dimorphic pelvis 4 , 29 and differences in egg-shell properties (brittle-shelled vs. pliable-shelled eggs) 30 , 31 .…”

“…Trade-offs which resemble obstetric selection may function in any animal with a skeleton which limits the space available for embryo development or hinders their delivery through a narrow, relatively inflexible passage. For example, in tetrapods, a pelvic constraint model convincingly explains a functional upper size limit for progeny both in live-bearing and egg-laying species 4 – 6 . Another group of animals most likely experiencing some kind of obstetric selection are shelled gastropods.…”