A combination of experimental and correlative analyses was used to study the effect of tail length on locomotor performance of garter snakes (Thamnophis sirtaZis 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.
relative and experimental approaches applied to a single system.
The relationship between morphology and locomotor performance is a particularly interesting and fruitful area of investigation (Alexander and Goldspink, '77; Garland, '85). The importance of axial morphology and function to locomotion has been discussed for lower vertebrates in general by Gray ('68), and among squamate reptiles one of the most conspicuous sources of morphological variation is in the axial skeleton. This variation is expected to have a significant impact on locomotor capacity. Among different species of lizards, for example, numbers of body vertebrae may range from 16 to 110, and within snakes these may vary from 120 to 320 (Hoffstetter and Gasc, '69). Considerable interspecific variation also exists in the relative length of the tail which may range from about 10% to 75% of total length