Effects of leg loss on male crab spiders Misumena vatia

Although shedding a leg may save an individual's life, it may lower its subsequent fitness by decreasing its ability to obtain food, increasing its vulnerability to predators, or lowering its success in reproduction. We studied the effects of leg loss on common locomotory tasks and activity levels in adult male crab spiders Misumena vatia. Over three field seasons approximately 30% of adult males lost one or more legs, usually among their four large anterior legs. Although M. vatia do not build webs, they use their front legs extensively to traverse lines, their most important means of movement. Field-captured males missing one or more legs moved 38% more slowly across lines and 28% more slowly on surfaces than intact males. Since males must hunt to find unmated females, this reduced mobility suggests a significant fitness cost for males that have lost legs. However, a set of males with experimentally autotomized legs moved as rapidly on lines as intact males. Since individuals that lost a limb in the field weighed significantly less than intact individuals of the same size (carapace width), adjusting for leg mass, the slow rates of movement on lines by the field-caught individuals missing a leg probably resulted from poor body condition, rather than a loss of dexterity. Most studies have explained potential loss in fitness from losing a leg as a direct consequence of leg loss, seldom invoking indirect effects. The potential for leg loss to reduce fitness via impacts on body condition warrants further attention, for it could play an important role in cost of autonomy.