The effect of leg autotomy on terrestrial and aquatic locomotion in the wolf spider Pardosa valens (Araneae: Lycosidae)

Abstract

Many species use autotomy, the self-amputation of an appendage, as a last-gasp method to escape a predator. Although this behavior can have immediate survival benefits, it can also negatively affect future survival or reproduction. The wolf spider Pardosa valens Barnes 1959 occurs along small mountain streams in southeastern Arizona, where it moves both on cobble along the stream and on top of the water's surface. Autotomy of legs is common in this species, and we hypothesized that such leg loss could lead to decreased sprint speed in both terrestrial and aquatic locomotion. We examined burst speed in the laboratory on artificial terrestrial and aquatic racetracks during 2005 (both males and females) and 2006 (females only). In 2005 terrestrial trials, intact spiders were faster than autotomized spiders, but there was no effect of sex on speed. In contrast, 2005 aquatic trials revealed that females ran faster than males, but that autotomy had a negative impact on the speed of females only. Additionally, female spiders generally ran faster on the terrestrial track later in the day than earlier in the day, suggesting that environmental variables such as temperature may have some influence on spider locomotion. Males were less likely to run on water than were females, and ran shorter distances when they did run. Results for females during 2006 also showed a decline in speed with autotomy, and an increase during later trials, although the results were weaker than during 2005, with only the aquatic trials showing a significant difference. These results suggest that leg autotomy in this spider does have a cost, but that the magnitude of this cost depends on aspects of the spider (e.g., sex) and habitat (e.g., substrate and environmental conditions).