Horizontal and vertical optimal running speeds of cellar spiders are impacted by leg autotomy, sex, and body mass

Abstract Animal locomotion occurs at different speeds in nature, but much of the current literature focuses on investigating maximal speeds. Voluntary speeds are likely a better approximation of optimal speeds that are more economical and more widely used in nature, which should be more widely considered. Autotomy, voluntary self‐amputation, is a common strategy beneficial for escaping life‐threatening situations. Spiders frequently autotomize legs in response to predatory encounters. It has been shown that despite this benefit, autotomy causes a reduction in maximal speeds, which makes it more difficult for spiders to evade predators, locate food, and search for mates. However, maximal speeds are rarely used in nature because of their energetic cost and negative impacts on maneuverability and accuracy of leg placement. Studies are lacking on how leg autotomy influences voluntary (optimal) speeds which spiders use to mitigate the costs of maximal speeds. This study examined the effects of leg autotomy, incline (horizontal and vertical), and body mass on optimal running speeds of cellar spiders, Pholcus phalangioides Fuesslin, 1775. Voluntary speeds were assessed twice for all spiders on both inclines. The first run consisted of non‐autotomized spiders. Then, half of the spiders were induced to autotomize one front leg, while the other half were run again intact. Overall, while autotomy did not result in significantly different speeds, vertical speeds were slower than horizontal speeds and males were faster than females. Running speeds decreased with increasing size for both autotomized and control spiders. We found the speeds of control spiders slightly increased between the first and second set of trials, whereas autotomized spiders showed a slight decrease. These results suggest that, regardless of incline, cellar spiders are able to compensate for leg autotomy with only a small hindrance on optimal performance. Future work should seek to directly compare forced and voluntary movements to further examine how spiders compensate for leg autotomy.