(a) SH performance in the golden-collared manakin, whereby SH percent relaxation declines across stimulation trains that are greater than 80 Hz. This is just above the species maximum observed roll-snap speed of 68 Hz. Solid lines represent represent significant regression slopes (p<0.05, β <0), with corresponding shaded areas denoting 95% confidence bands. Non-significant slopes (p>0.05) are indicated by dotted lines. (b) SH performance in the white-collared manakin. Note that all slopes are non-significant (p>0.05), as denoted by the dotted lines; however, the y-intercept appears to progressively decrease as the stimulation frequency increases, suggesting that the muscle fuses at the onset of stimulation and stays that way across the entire stimulation train. (c) Regression lines of the slope of the lines in (a) and (b) plotted as a function of stimulation train frequency. Note that SH performance—as measured by the SH’s ability to resist ‘rapid fatigue’ during high frequency stimulations—declines in golden-collared manakins (F1,6=27.7, p=0.004, R2 = 0.82), but not in white-collared manakins (F1,7=0.15, p=0.711). Solid lines associated with each species represent the mean change in percent relaxation at a given frequency,±95% confidence bands (shaded areas). The red line indicates the stimulation frequency at which the 95% CI of the SH performance line in golden-collared manakins intersects with the slope = 0 point. In theory, this represents that maximum twitch speed that the muscle can attain without incurring an endurance cost, and it notably corresponds to the species’ average roll-snap speed (denoted by horizontal box and whisker plot, where the vertical line is at the mean, shaded box indicates ±1 SEM, and whiskers extend to the species range).