• basking;
  • ectotherms;
  • flight distance;
  • optimal escape theory;
  • predation risk;
  • thermoregulation


Optimal escape theory predicts that animals facing a predation threat should react in a manner that balances perceived threats with the costs of escape. Flight distance, the distance at which animals flee in response to an approaching predator, is often used as a quantitative measure of animal risk assessment and is thus useful to examine predictions of optimal escape theory. However, applying this theory to a broader diversity of animals under more complex environmental conditions has been hampered by a paucity of data. In this study, I measure the flight distances of two freshwater turtles, the invasive Trachemys scripta elegans, and the native Emys marmorata, in an urban California water body to examine how flight distance is affected by time of day, initial visibility to the approaching predator, and the presence of conspecifics. I found that E. marmorata was more sensitive to predation risk than T. scripta and would flee earlier (at a greater distance). Time of day and the presence of conspecifics had no effect on E. marmorata's flight distance although visibility and the interaction between time and visibility did. Emys marmorata would flee earlier when visible, and in the afternoon visible turtles would flee earlier than non-visible turtles, but in the morning, visibility did not affect turtle flight distance. In contrast, T. scripta's flight distance was not affected by any measured variables, although this may be due to a lack of statistical power given the low sample size. This study demonstrates that multiple environmental characteristics influence risk assessment in turtles, and implies that interspecific differences in responses to predation threats may lead to increased competition for thermal resources between native and invasive ectotherms. This is a likely mechanism contributing to the negative impacts of T. scripta on native turtle populations, especially in urban environments.