Leaf biomechanics as a potential tool to predict feeding preferences of the geometric tortoise Psammobates geometricus

Abstract

Food and non food plant characteristics were studied for the geometric tortoise,  Psammobates geometricus, to test the hypothesis that feeding preferences in  tortoises may be influenced by the mechanical properties of plant leaves. Twelve focal feeding observations made between 8 October and 31 October 2002 were synthesized with literature reports to develop species lists of food and non food grasses and geophytes in renosterveld habitat. Failure load and tensile strength were determined in the field for leaves of food and non food plants that were spatially available to the tortoises and present in relative abundance during the course of the study. Leaves of geophyte food plants were fleshy and succulent with a low failure load and tensile strength. Leaves of food plant grasses had significantly higher failure loads and tensile strengths compared to leaves of food plant geophytes. In non food plants there was no signifi cant difference in failure load between grasses and geophytes, but tensile strength was signifi cantly higher in the grasses. Non food plant grasses and geophytes overall had significantly higher failure load and tensile strength values than did food plants. Two non food plants that overlapped in mechanical strength to the food plants either have toxic/unpalatable chemicals in high amounts or are recent invasive species to the renosterveld. Thus, these studies support the hypothesis that leaf biomechanical properties may be an important factor in determining the diet of the geometric tortoise and perhaps tortoises in general.

Key words: biomechanics, failure load, herbivore, Psammobates geometricus, tensile strength.