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Annexe IV _____________________________________________ predation, starvation or dehydration (Finkler, 1999, 2001; Steyermark et al., 2001). But, for example, Tucker (2000) showed that the hatchlings of two aquatic turtle species use very different strategies to move overland: to reduce time spent migrating, Chrysemys picta hatchlings increase locomotion speed whereas Trachemys scripta hatchlings use stealth, i.e. slow locomotion speed (see also Kolbe et al., 2002). The same locomotor performance should therefore not been chosen to evaluate the efficiency of overland move for these two species. This underlines the importance of the choice of the locomotor performance measure to obtain reliable conclusions. Second, since a behavioral response is generally complex, different measures of the same behavior are possible, each capturing a particular aspect of that behavior. In our experiment, we studied the composite righting response through two different measures that are independent: the Latency time, during which the turtle stays motionless, and the Time to right, which is the active return of the turtle in a prone position. Surprisingly, the significance and impact of most effects differs widely between the two measures (see summary in Table 3). For example, at constant incubation temperature, the effect of sex on Latency time is highly significant (P = 0.0005) whereas sex does not influence the Time to right (P = 0.9815). A third measure was possible to analyze the righting response: the total duration since turtles were placed on their carapace until they return in a prone position. In fact, this measure represents the sum of Latency time and Time to right. There again, results of significant effects detected were different of the results obtained for the two other measures (analysis not shown). The strong discrepancies between our results for the two measures of the righting response show that the different aspects of a behavior can be influenced by different factors. This may point out the importance of using several measures of a locomotor performance to obtain valid conclusions. 15
Annexe IV _____________________________________________ Several parameters influence a priori the “reaction norm” in locomotor performance performed in laboratory: the environmental conditions during the incubation of the eggs (e.g. temperature and hydric potential), the husbandry environment and finally, the experimental design during the measure. All these environmental conditions may directly affect the expression of hatchling or performance traits, thus these traits are not fixed but are phenotypically plastic (Kingsolver & Huey, 2003). And Arnold (1983) himself said that behavior, ontogeny and environmental variation may complicate the relationship between performance and fitness. Many previous studies have found a significant effect of incubation temperature of the eggs on locomotor performance in sauropsids (Burger, 1989; Shine et al., 1997; Downes & Shine, 1999; Elphick et al., 1999; Brana & Ji, 2000; Freedberg et al., 2001; Webb et al., 2001; Du et al., 2003). However, most of these studies have used extreme temperature regimes to test the effect of temperature. Extreme temperatures generate morphological and physiological constraints that do not exist in nature. Furthermore, most studies also used constant temperature regimes, which are not representative of the fluctuations observed in natural nests (Georges et al., 1994; Valenzuela, 2001). It has been shown that the variance of the incubation temperature, and not only its mean, has a strong impact on various offspring traits, including growth rate, body size or locomotor performances (Shine et al., 1997; Doody, 1999; Demuth, 2001; Shine, 2002). The study of the influence of incubation temperature on performance is particularly important in species with TSD, in which Charnov and Bull (1977) predict a different fitness between males and females incubated in the same thermal environment. In our study, we compared the “reaction norm” of the righting response between juveniles from constant and fluctuating incubation temperatures of the eggs. We found that the significance of most effects (i.e. impact of explanatory variables on the righting response) differs widely between constant and fluctuating temperature treatments of the eggs, even 16
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Annexe IV<br />
_____________________________________________<br />
predation, starvation or <strong>de</strong>hydration (Finkler, 1999, 2001; Steyermark et al., 2001). But, for<br />
example, Tucker (2000) showed that the hatchlings of two aquatic turtle species use very<br />
different strategies to move overland: to reduce time spent migrating, Chrysemys picta<br />
hatchlings increase locomotion speed whereas Trachemys scripta hatchlings use stealth, i.e.<br />
slow locomotion speed (see also Kolbe et al., 2002). The same locomotor performance should<br />
therefore not been chosen to evaluate the efficiency of overland move for these two species.<br />
This un<strong>de</strong>rlines the importance of the choice of the locomotor performance measure to obtain<br />
reliable conclusions.<br />
Second, since a behavioral response is generally complex, different measures of the same<br />
behavior are possible, each capturing a particular aspect of that behavior. In our experiment,<br />
we studied the composite righting response through two different measures that are<br />
in<strong>de</strong>pen<strong>de</strong>nt: the Latency time, during which the turtle stays motionless, and the Time to<br />
right, which is the active return of the turtle in a prone position. Surprisingly, the significance<br />
and impact of most effects differs wi<strong>de</strong>ly between the two measures (see summary in Table<br />
3). For example, at constant incubation temperature, the effect of sex on Latency time is<br />
highly significant (P = 0.0005) whereas sex does not influence the Time to right (P = 0.9815).<br />
A third measure was possible to analyze the righting response: the total duration since turtles<br />
were placed on their carapace until they return in a prone position. In fact, this measure<br />
represents the sum of Latency time and Time to right. There again, results of significant<br />
effects <strong>de</strong>tected were different of the results obtained for the two other measures (analysis not<br />
shown).<br />
The strong discrepancies between our results for the two measures of the righting response<br />
show that the different aspects of a behavior can be influenced by different factors. This may<br />
point out the importance of using several measures of a locomotor performance to obtain<br />
valid conclusions.<br />
15
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