HERREL, A.*; JAMES, R.S.; VAN DAMME, R.; University of Antwerp, Belgium; Coventry University, UK; University of Antwerp, Belgium: Muscle physiology constrains behaviour in lizards: physiological basis for the fight vs. flight paradigm.

Previous workers have noted that some agamid lizards show a temperature dependent behavioural shift where a flight response is used at high temperatures. In contrast, at low temperatures, lizards turn towards their aggressor and try to bite (Hertz et al., 1982). Here we examine the physiological basis for this behavioural shift in the lizard Trapelus pallidus. Behavioural experiments confirmed previous observations and indicated that lizards turned around to bite more at lower temperatures. In vivo performance data show that the thermal dependence of bite force is much smaller than that of sprint speed. In vitro physiological data for the m. caudofemoralis (the major hindlimb rtetractor) and the m. adductor mandibulae externus (the largest jaw closer muscle) show that the effect of temperature on contraction speed (eg. twitch time to peak tension, half relaxation time) was much greater than that of temperature on force generation (eg. peak twitch and tetanic force). However, whereas the effect of temperature on force production was most pronounced for the limb muscle, the effect of temperature on contraction speed was greatest for the jaw muscle. Our data suggest that the observed behavioural shift is driven by the direct effect of temperature on muscle contractile performance, which in turn affects whole organism performance traits. Moreover, our data show local physiological adaptation in the jaw and limb muscles to their primary function (force vs. speed). Hertz, P.E., R.B. Huey and E. Nevo (1982) Fight versus flight: body temperature influences defensive responses of lizards. Anim. Behav. 30: 676-679.