The relation of size to climbing, line-crossing and running performances of male crab spiders

Abstract

Mobile predators employ a variety of locomotor tasks in hunting, avoiding predators and searching for mates. However, few studies have evaluated the performances of a population’s members in all of their major locomotor tasks, even though all of these performances may affect both an individual’s fitness and how members of a population exploit resources. Male crab spiders Misumena vatia, diminutive members of an extremely sexually dimorphic species, regularly run, climb and cross silken lines. For the most part, individuals moved at similar speeds relative to each other in the three tasks: the 15th fastest runner was approximately 15th fastest in the other two moves as well. Thus, these spiders provided little evidence that any individual excelled at one task or another, relative to other members of its population, thereby exhibiting a marked behavioral syndrome. However, large individuals moved faster than small ones in these tasks, especially when climbing. Although the gravity and bridging hypotheses predict that small individuals climb and cross silken lines faster than larger ones, at ca. 4.15 mg (extremes of 2.5–8.0 mg), male Misumena, and most terrestrial invertebrates, fall well below weight constraints currently predicted by either hypothesis. Data on sexual cannibalism, overwintering, and lifespan suggest possible countering advantages of small size, and hence balancing selection, which may contribute to Misumena’s striking sexual size dimorphism and explain the males’ wide size range.