ABSTRACT
Isometric twitch and tetanus parameters, force–velocity curves, maximum shortening velocity (Vmax) and percentage relaxation between stimuli (%R) across a range of stimulus frequencies were determined for a muscle used during call production (the tensor chordarum) and a locomotor muscle (the sartorius) for three species of hylid frogs, Hyla chrysoscelis, H. versicolor and H. cinerea. The call of H. chrysoscelis has a note repetition rate (NRR) approximately twice as fast as the call of H. versicolor (28.3, 42.5 and 56.8 notes s−1 for H. chrysoscelis and 14.8, 21.1 and 27.4 notes s−1 for H. versicolor at 15, 20 and 25°C, respectively). Hyla cinerea calls at a very slow NRR (approximately 3 notes s−1 at 25°C). Hyla versicolor evolved from H. chrysoscelis via autopolyploidy, so the mating call of H. chrysoscelis is presumably the ancestral mating call of H. versicolor.
For the tensor chordarum of H. chrysoscelis, H. versicolor and H. cinerea at 25°C, mean twitch duration (19.2, 30.0 and 52.9 ms, respectively), maximum isometric tension (P0; 55.0, 94.4 and 180.5 kN m−2, respectively), tetanic half-relaxation time (17.2, 28.7 and 60.6 ms, respectively) and Vmax (4.7, 5.2 and 2.1 lengths s−1, respectively) differed significantly (P<0.05) among all three species. The average time of tetanic contraction to half-P0 did not differ significantly between H. chrysoscelis (14.5 ms) and H. versicolor (15.8 ms) but was significantly longer for H. cinerea (52.6 ms).
At 25°C, Vmax differed significantly among the sartorius muscles of H. chrysoscelis, H. versicolor and H. cinerea (5.2, 7.0 and 9.8 lengths s−1, respectively) but mean twitch duration (29.5, 32.2 and 38.7 ms, respectively), P0 (252.2, 240.7 and 285.1 kN m−2, respectively) and tetanic half-relaxation time (56.3, 59.5 and 60.7 ms, respectively) did not differ significantly. The average time of contraction to half-P0 did not differ significantly between H. chrysoscelis (23.7 ms) and H. versicolor (22.9 ms) but was significantly shorter for H. cinerea (15.6 ms).
The only consistent contractile differences found in this study between the calling muscle and locomotor muscle of H. chrysoscelis, H. versicolor and H. cinerea were that the calling muscles generated less tension and their force–velocity relationship was much more linear. These differences may be attributable to ultrastructural differences between calling and locomotor muscles.
The relationship between %R and stimulus frequency clearly shows that the tensor chordarum of H. chrysoscelis is capable of functioning at higher contractile frequencies (i.e. NRRs) than the tensor chordarum of H. versicolor which, in turn, can function at higher contractile frequencies than the tensor chordarum of H. cinerea. However, the calling muscle of H. versicolor appears to be much faster than it needs to be. Possibly, neurological changes regulating NRR have evolved faster than physiological changes to the muscle.
The principal modification to the tensor chordarum of H. versicolor in response to the evolution of a slower NRR is a reduction in deactivation rate.
