COMPARATIVE CONTRACTILE DYNAMICS OF CALLING AND LOCOMOTOR MUSCLES IN 3 HYLID FROGS

Isometric twitch and tetanus parameters, force-velocity curves, maximu m shortening velocity (V-max) and percentage relaxation between stimul i (%R) across a range of stimulus frequencies were determined for a mu scle used during call production (the tenser chordarum) and a locomoto r muscle (the sartorius) for three species of hyIid frogs, Hyla chryso scelis, H. versicolor and H. cinerea. The call of H. chrysoscelis has a note repetition rate (NRR) approximately twice as fast as the call o f H. versicolor (28.3, 42.5 and 56.8 notes s(-1) for H. chrysoscelis a nd 14.8, 21.1 and 27.4 notes s(-1) for H. versicolor at 15, 20 and 25 degrees C, respectively). Hyla cinerea calls at a very slow NRR (appro ximately 3 notes s(-1) at 25 degrees C). Hyla versicolor evolved from H. chrysoscelis via autopolyploidy, so the mating call of H. chrysosce lis is presumably the ancestral mating call of H. versicolor. For the tenser chordarum of H. chrysoscelis, H. versicolor and H. cinerea at 2 5 degrees C, mean twitch duration (19.2, 30.0 and 52.9 ms, respectivel y), maximum isometric tension (Po; 55.0, 94.4 and 180.5 kNm(-2), respe ctively), tetanic half-relaxation time (17.2, 28.7 and 60.6ms, respect ively) and V-max (4.7, 5.2 and 2.1 lengths s(-1), respectively) differ ed significantly (P<0.05) among all three species. The average time of tetanic contraction to half-Po did not differ significantly between H . chrysoscelis (14.5 ms) and H. versicolor (15.8 ms) but was significa ntly longer for H. cinerea (52.6 ms). At 25 degrees C, V-max differed significantly among the sartorius muscles of H. chrysoscelis, H. versi color and H. cinerea (5.2, 7.0 and 9.8 lengths s(-1), respectively) bu t mean twitch duration (29.5, 32.2 and 38.7 ms, respectively), Po (252 .2, 240.7 and 285.1 kN m(-2), respectively) and tetanic half-relaxatio n time (56.3, 59.5 and 60.7ms, respectively) did not differ significan tly. The average time of contraction to half-Po did not differ signifi cantly between H. chrysoscelis (23.7ms) and H. versicolor (22.9 ms) bu t was significantly shorter for H. cinerea (15.6 ms). The only consist ent contractile differences found in this study between the calling mu scle and locomotor muscle of H. chrysoscelis, H. versicolor and H. cin erea were that the calling muscles generated less tension and their fo rce-velocity relationship was much more linear. These differences may be attributable to ultrastructural differences between calling and loc omotor muscles. The relationship between %R and stimulus frequency cle arly shows that the tenser chordarum of H. chrysoscelis is capable of functioning at higher contractile frequencies (i.e. NRRs) than the ten ser chordarum of H. versicolor which, in turn, can function at higher contractile frequencies than the tenser chordarum of H. cinerea. Howev er, the calling muscle of H. versicolor appears to be much faster than it needs to be. Possibly, neurological changes regulating NRR have ev olved faster than physiological changes to the muscle. The principal m odification to the tenser chordarum of H. versicolor in response to th e evolution of a slower NRR is a reduction in deactivation rate.