Chelonian Antipredator Strategies: Preliminary and Comparative Data from Tanzanian Pelusios

– Turtle carapaces clearly function as antipredator adaptations, but their varied structure has rarely been matched to differential predation pressure in nature. In addition, the role of antipredator behaviors, and how they interact with morphological defenses in turtles, is poorly understood. Here we show that a small turtle, Pelusios subniger, is found predominately in pools and watercourses where the Nile crocodile, Crocodylus niloticus, a species that imposes intense predation pressure on them, is absent. Our ecological data are consistent with the interpretation that P. subniger can share the same ecological landscape with C. niloticus through behavioral avoidance. Aquatic turtles and crocodilians have a long and complex history of ecological coexistence. For example, many species of crocodilians routinely prey on turtles (Pope 1949; Cott 1961; Platt et al. 2006; Heithaus et al. 2008), and turtles can constitute the dominant prey remains in American alligator (Alligator mississippiensis) stomachs (Delany and Abercrombie 1986; Janes and Gutzke 2002). Crocodilians also exhibit positive allometry of bite force throughout development (Erickson et al. 2003), and this may make them particularly potent predators of many turtle species. Even so, some turtle species appear to benefit from the presence of crocodilians; a comprehensive analysis in Florida found that 26.6% of alligator nests were used by a variety of turtle species as nesting sites and suggested that female alligators may provide incidental protection to conesting turtles (Enge et al. 2000). These varied, sometimes opposing ecological relationships imply that predator avoidance mechanisms in turtles may be complex, species specific, and dependent on local conditions. Although it is generally recognized that the chelonian shell has evolved as a protective antipredator defense mechanism, morphometric analyses have shown that different shell geometries confer more or less protection from large predators that crush their prey, including crocodilians. A recent Finite Element Analysis demonstrated this point among 3 species of North American emydine turtles that varied in shell shape. Relatively higher-domed and thicker-shelled Glyptemys muhlenbergii were more resistant to crushing than more dorsoventrally flattened Emys (Actinemys) marmorata, which in turn were more crush resistant than Clemmys guttata, the least domed and thinnest shelled of the 3 species (Stayton 2009). Similar reasoning, but with less quantitative support, has been applied to other turtles with respect to specific predators. For example, the relatively highdomed thick shell of Pseudemys concinna in the coastal southeastern United States (below the ‘‘Fall Line’’) has been postulated to have evolved as a response to American alligator predation (Aresco and Dobie 2000). Alternatively, turtles with low-domed, easily crushed thinner shells in the same areas may instead utilize behavioral antipredator strategies, including hiding in refuges, crypsis, or increased speed and agility (Greene 1988). Although we know that this pertains in other taxa (Caro 2005), to our knowledge this trade-off between 302 CHELONIAN CONSERVATION AND BIOLOGY, Volume 9, Number 2 – 2010