The effect of tail morphology on locomotor performance of snakes: A comparison of experimental and correlative methods

A combination of experimental and correlative analyses was used to study the effect of tail length on locomotor performance of garter snakes (Thamnophis sirtalis fitchi). Correlative analysis was used to determine whether naturally occurring morphological variation was associated with differences in performance, whereas experimental modification of morphology was used to test whether this variation was a causative factor for different performance. Burst speed of snakes performing terrestrial lateral undulation was measured for snakes with experimentally altered tail length and for snakes with partial and complete tails from a natural population. For 445 snakes with complete tails, regression analysis revealed a significant quadratic effect of relative tail length on size‐corrected performance, indicating that individuals with intermediate relative tail length had the fastest burst speeds. For 52 snakes that had naturally lost from 0.3% to 80.4% of their tail, there was no significant correlation between size‐corrected performance and the extent of the tail that was missing. Experimental ablation of the distal one third of the tail did not affect performance, whereas ablation of the distal two thirds of the tail caused a significant average decrease in speed of only 4.5%. Contrary to the correlative analysis, the experimental manipulation suggests that there is no detectable mechanical effect for minor variations in relative tail length. We conclude that minor deviations from intermediate relative tail length are not the causative factor for a decrement in performance among snakes, and this decrement was a correlated response to some other unmeasured variable.