River cooters (Pseudemys concinna) are a novel and compelling system in which to study adaptations and trade-offs in biology teaching labs. This species of turtle lives in the southeastern United States in both fast-moving (lotic) waters lacking predators, and slow-flowing (lentic) waters inhabited by alligators. We used a high-resolution CT scan to create realistic 3D-printed models of P. concinna shells with a range of shell-arching ratios (height/length) published in the literature. Students begin by making qualitative observations of two sets (i.e., populations) of shells. They then make measurements and use Excel to calculate ratios, create box-plots, and run a t-test. After determining whether the observed difference between populations is significant, they read a short journal article that proposes mechanisms that might explain why such differences in shell ratio exist; namely, shell strength and hydrodynamic efficiency. Students then test these predictions. Shell strength is measured using a soda can crusher to determine how much force a shell can support prior to sudden structural failure. Hydrodynamic efficiency is measured by recording the time it takes shells to drop through a column of water (a proxy for hydrodynamic drag). This data is collected by student pairs and then combined for analysis as a class data set. At the end of the lab, students are asked to draw conclusions about the trade-off between taller (high-ratio) shells adapted to lentic waters with predators and shorter (low-ratio) shells found in lotic waters without predators.
In this lab students collect data, create box-plots, run multiple t-tests, read literature, test predictions, and draw conclusions about a natural system. The use of CT scans to produce high quality 3D printed models makes this lab accessible to any institution with a 3D printer.
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