Effects of habitat-related visibility on escape decisions of the Spanish Terrapin Mauremys leprosa Pilar López 1 , Iván Marcos 1,2 and José Martín ∗,1 Optimal escape theory and empirical evidence suggest that prey should not flee immediately upon detecting an approaching predator, but should wait until the predator approaches closer than the point at which predation risk is equal to escape costs (Ydenberg and Dill, 1986). Ap- proaching predators do not always pose an im- mediate threat, and several environmental fac- tors may affect the escape decisions of prey, such as microhabitat structure, which affect both the probability of being detected by the predator, as well as the probability of detect- ing the predator, and the relative safety and costs of potential refuges (Lima, 1990; Martín and López, 1995, 2000a, 2000b; Cooper, 1998). Thus, animals should tend to adjust the char- acteristics of their escape responses according to perceived levels of risk (Martín and López, 1995, 2000a; Cooper, 1997). Several aspects of antipredatory behaviour in many animals have been successfully predicted by the optimal escape theory (Ydenberg and Dill, 1986; Lima and Dill, 1990). However, fur- ther tests are needed to ascertain its applicabil- ity to a wider range of prey species, and to more environmental conditions. Many turtles are pre- dominantly aquatic, but come to the water’s banks for basking (Lefevre and Brooks, 1995). Basking increases body temperature (Crawford et al., 1983; Hammond et al., 1988) and, thus, activates metabolism and may increase diges- 1 - Dept. Ecología Evolutiva, Museo Nacional de Cien- cias Naturales, CSIC, José Gutiérrez Abascal 2, 28006 Madrid, Spain Corresponding author; e-mail: Jose.Martin@mncn.csic. es 2 - Present address: Instituto Pirenaico de Ecología, CSIC, Avda Montanana 177, 50080 Zaragoza, Spain tive turnover rates (Parmenter, 1981). However, basking behaviour also exposes turtles to pre- dation risk, because on land they are potential prey of many predators, such as birds or mam- mals (Martín and López, 1990). Thus, during basking bouts many turtle species are very wary, being extremely alert and vigilant, and diving quickly into the water, apparently at the least disturbance. However, because stopping bask- ing could be costly and because the risk posed by an approaching predator is not always the same, optimal escape theory predicts that tur- tles should not dive into water immediately af- ter detecting a predator, but when costs of flee- ing equal benefits. This decision should be in- fluenced by predation risk level, which may depend on many environmental factors. How- ever, to our knowledge, this hypothesis remains untested in turtles. Moreover, in spite of the con- spicuous escape diving of many turtle species in varied freshwater habitats, only anecdotal ob- servations refer to this behaviour (see reviews in Greene, 1988; Ernst and Barbour, 1989). In this paper, we tested factors that determine the escape decisions of Spanish Terrapins, Mau- remys leprosa, in the field under various habitat conditions that may affect their risk perception. We simulated predatory attacks in different wa- ter habitats and specifically examined whether differences in relative conspicuousness of bask- ing turtles to visually oriented predators and in the potential ability of turtles to detect the predator may explain variability in their escape behaviour. Mauremys leprosa is a semiaquatic small turtle (max- imum carapace length of 20 cm, but population mean of adults is around 16 cm) widespread in the South and Cen- tral Iberian Peninsula and Northwestern Africa (Andreu and López-Jurado, 1998; Keller and Busack, 2001). We per- © Koninklijke Brill NV, Leiden, 2005. Amphibia-Reptilia 26 (2005): 557-561 Also available online - www.brill.nl