Tarantulas adapt their running style after losing legs

Losing a leg or two is not always a big deal for spiders. If one of their eight legs gets stuck in a crevice or if molting goes wrong, the arachnids will simply cut off the limb just beyond the body joints. When they are young, the limb will regrow within a month. 

But what if a spider loses two or more limbs? Do they simply relearn how to live and run on six legs instead of eight? New research with Guatemalan tiger rump tarantula (Davus pentaloris) spiderlings indicates that they do not. Instead, they use a mash-up of different walking styles to compensate and can even move as quickly as they did before losing their limbs. The findings are detailed in a study published June 17 in the Journal of Experimental Biology.

In the study, biologists Tonia Hsieh, Brooke Quinn, and Sarah Xi from Temple University in Philadelphia adhered the front right and rear left legs of some spiderlings to a card. They waited for the young tarantulas to detach their limbs. Once the spiderlings detached, the researchers filmed the animals from above, capturing every detail of their footwork as they scampered away.

Next, the team allowed enough time for the tarantulas to regrow their limbs. They filmed the spiders running around with eight intact limbs, before encouraging them to shed their limbs again and filming their movements. They ended up capturing over 43,000 frames and more than 800 strides. For help analyzing and coding all of this footage, Hsieh, Quinn, and Xi teamed up with physicists Suzanne Amador Kane and Kris Wu from Haverford College in Pennsylvania, and mathematician Michael Ochs from The College of New Jersey. 

The united team found that immediately after losing their limbs, the tarantulas appeared to pick up running just as fast as they had before. The spiders also recovered their ability just as quickly after the second amputation. The major observed differences were splaying their legs wider, twisting their bodies slightly more to one side, and increasingly more weaving.

However, when the team took a closer look at the spiders’ footwork, they saw that the arachnids are remarkably adaptable. According to Hsieh, the spiders with a full set of eight legs alternate between having four feet—the first and third on one side and the second and fourth on the other–that are in contact with the ground at one time. In theory, six-legged tarantulas should have two options. They should either alternate between having four legs and two legs in contact with the ground, similar to limping. Or, they could alternate between using three legs touching the ground more similar to an ant.

In practice, the spiders in this study rarely followed these rules. The tarantulas with all eight legs sometimes left one leg lingering on the ground while the other four descended, or they raised the fourth limb early, leaving only three in contact with the ground. 

The spiders that were down two legs also alternated randomly between limping forward on two and running around like an ant on sets of three legs in order to keep moving fast. The tarantulas seemed to favor their hind legs. These limbs are mainly used for propulsion and keep them on the ground longer when down to six limbs.

According to the team, this indicates that tarantulas don’t just relearn how to run after losing limbs. They alternate between limping and running like ants instead, to just keep running around.  

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