Whole-organism performance and repeatability of locomotion on inclines in spiders

Burst speed, a common metric of locomotor performance, is typically measured on horizontal surfaces. Studies of locomotor performance in spiders have examined vertical and horizontal locomotion, but not the effects of intermediate inclines. We measured the effect of angle of incline, body size, relative leg length (variation not explained by body size) and relative body mass (variation not explained by body size or leg length, often interpreted as ‘condition’) on maximum running speed in two Australian spiders differing in habitat architecture and foraging strategy. Males of a ground-dwelling jumping spider, Jacksonoides queenslandica, and an orb-web spider, Nephila plumipes, were forced to run on raceways inclined at +0°, +30° and +60°. At the population level, maximum running speed decreased on inclined compared to level raceways for both species, but the effects of slope varied substantially between individuals. While speed was not influenced by body size or relative mass in either species, it was influenced by leg length in both. In J. queenslandica, relative leg length did not influence horizontal running speed but was positively related to speed on slopes, principally because relatively short-legged spiders were slowed whereas relatively long-legged spiders were not affected. In N. plumipes relative leg length was related negatively to running speed on horizontal surfaces but positively to running speed on slopes. Unlike J. queenslandica, N. plumipes was little affected by slopes of 30° but slowed markedly on slopes of 60°, irrespective of relative leg length. We suggest that performance advantages on inclines may promote relatively longer legs.