THERMAL SENSITIVITY OF SWIMMING PERFORMANCE AND MUSCLE CONTRACTION IN NORTHERN AND SOUTHERN POPULATIONS OF TREE FROGS

The effects of temperature on sprint swimming ability and muscle contractile properties were examined in northern and southern populations of the holarctic tree frog, Hyla crucifer Wied-Neuwied, acclimated to 20-23°C. Maximal swim-ming velocities of 29 (southern) and 32 (northern) cm s- ' and stroke frequencies of 4-1 (southern) and 5.5 (northern) strokess- ' were attained at 30°C, and maximal stroke lengths (i.e. distance moved per stroke) of 8.0 (southern) and 7.4 (northern) cm at 20°C. The thermal dependence of swimming velocity decreases with increasing temperature (e.g. Qlo = 4.0 from 6 to 10°C, 1.2 from 20 to 30°C), as reported for locomotion in other ectothermic vertebrates. Over a temperature range of 1-5-30°C, velocity increases by a factor of 5.5, frequency by a factor of 4, and length by a factor of 1-7. Thus, increased velocity at higher temperatures can be attributed mostly to increased stroke frequency; increments in stroke length contribute less. Muscle contractile properties have similar thermal dependencies to those of other vertebrates: rate processes [including twitch time-to-peak tension (TPT), twitch half-relaxation time (RTIl2), maximal rate of tetanic tensio