[caption id="attachment_179" align="alignleft" width="275" caption="Male Schizocosa sp. CC license photo courtesy of Marshall Hedin on Flickr (click for full)."][/caption]
When our news editor asked on twitter what sort of animal "this six-legged spider thing" is? Her question was answered in less than one minute by entomologists. It was indeed an unfortunate spider who has lost two of his legs. While Robin may be fraught with concern or sympathy for the poor little bugger, she need fear not because spider commonly self-amputate a leg here and there. Of course the process of regeneration may leave the spider a bit more vulnerable out in the wilds of nature. Thankfully, her little friend will likely be much safer in the confines of the Scientific American offices.
Spiders may amputate their legs as a defense strategy, but it’s not clear what trade-offs exist. For instance, if a spider amputates its leg and undergoes regeneration, is future reproduction impacted, is it more susceptible to predation, is it less mobile or less of a competitor?
In a 2007 study by Wrinn & Uetz, the hypothesis that leg regeneration puts spiders at a physiological and developmental disadvantage was put to the test. The authors examined the frequency of self-amputation in the field and the relationship to size, mass and physiological condition in the wolf spider (Schizocosa ocreata, similar to picture above). Additionally, they did laboratory experiments to test the hypothesis that leg regeneration specifically impairs foraging, decreases growth or affects development time.
The field data they collected indicated that leg loss impaired foraging ability, evidenced by decreases in mass, size and physiological condition as one would expect. The laboratory experiments also suggest additional trade-offs. Though not significant, spiders regenerating legs took an average of 3.7 days longer to molt, which they must do regularly in order to grow. One interesting observation was that
“Although spiders appear to show costs of regeneration, the differences in molt interval, size, and mass between intact and regenerating spiders were only true for the first molt after autonomy. During the second molt after autonomy, regenerating spiders were able to compensate for previous costs by either shortening their molt interval or increasing their growth.”
It appears these spiders are bounce back pretty readily, minimizing the costs associated with leg loss and regeneration to only one molt. Another trade-off was between development time and mass. Regeneration resulted in either longer time to molt or lower mass, but not both. Regeneration is a fascinating phenomena. The selection of improved regenerative capabilities comes at a cost, but clearly for the individual the costs do not outweigh the benefits of continuing to live and reproduce. Our news editor needn't fear for this poor spider's handicap. He should be able to head back out into the spider work force in no time!
Wrinn, K., & Uetz, G. (2007). Impacts of leg loss and regeneration on body condition, growth, and development time in the wolf spider Schizocosa ocreata. Canadian Journal of Zoology, 85 (7), 823-831 DOI: 10.1139/Z07-063