Key difference between female and male turtle shells may aid survival.
One way to tell a male turtle from a female turtle is a difference in the shape of their shells. While this pattern of sexual dimorphism is common among turtle species, it could have other effects on turtle biology. The differing shell designs may exhibit different strengths in standing up to the attack of a predator, and therefore influence the survival rate of individuals of one sex compared with the other.
Christine Vega and C. Tristan Stayton, from the Department of Biology at Bucknell University in Lewisburg, PA have investigated the mechanical implications of shell shape differences between two species of male and female turtles. The painted turtle (Chrysemys picta) is an aquatic species, and the wood turtle (Glyptemys insculpta) is a terrestrial species.
Female turtles tend to have larger, domed shells, which may indicate the ability to have large clutches of eggs. Males, on the other hand, typically have flatter shells and larger openings for their limbs, allowing them greater movement. This improves their success in courtship and mating.
To test the strength between the varying shells, researchers created digital models for finite element analysis. This computer model uses simple geometric shapes to create a complex structure—the turtle shell in this case. It then calculates the response of those elemental shapes to create the response of the shell as a whole. Twelve load conditions, representing the bite of a predator at different locations, were applied.
What Was Discovered
The females of both species fared better than the males, although male and female wood turtles were more evenly matched. Female shells showed less stress than male shells. The concave shape of the plastron, or underside, of the male wood turtle’s shell in particular exhibited significantly increased stress, indicating it would be more easily punctured by a predator.
These differences between sexes may stem from selection based on other factors besides predation. Currently, there are no data showing that females are more likely to encounter predators, for instance, and that therefore their shells have become better able to withstand such attacks. However, these variations might have an impact on turtles’ lives and, according to the researchers, may have important implications for turtle conservation efforts.
Christine Vega and C. Tristan Stayton (2011) Dimorphism in Shell Shape and Strength in Two Species of Emydid Turtle. Herpetologica: December 2011, Vol. 67, No. 4, pp. 397-405. doi: http://dx.doi.org/10.1655/HERPETOLOGICA-D-10-00037.1