Walk with your tail, not the legs

During tetrapod evolution, the transition from living in water to living on land was a big deal. Although, walking on land is not as easy due to physical stressors like gravity. Scientists hypothesize that a more robust vertebral column helped to facilitate early forays onto land and played a large role in generating propulsion (forward movement). Unfortunately, early tetrapods existed millions of years ago so we’ll never know for sure.

This small detail did not stop Drs. Christine Vega and Miriam Ashley-Ross from searching for an answer. In a recent study, the tag-team from Wake Forest University identified the effect of lateral flexibility (or rather, the lack thereof) on limb use in terrestrial walking.

A tiger salamander (Ambystoma tigrinum). Photo by Carla Isabel Ribeiro on Wikimedia Commons
Figure 1. Tiger salamanders (Ambystoma tigrinum). Photo by Carla Isabel Ribeiro.

Salamanders are a group of slender amphibians with long-tails, and are often used to model early tetrapod locomotion (Figure 1). On land, they primarily use their limbs to generate propulsion but exhibit a varying amount of side to side bending (undulation) in the trunk region of their body. By contrast, turtles and tortoises only use their limbs to get around on land with no lateral undulation because of their restrictive shells. Because turtles and salamanders are so different, it’s difficult to compare them directly and isolate the role of lateral flexibility.

Figure 2. Tiger salamanders with no shell (left), a flexible tygon shell (middle), and a rigid PVC shell (right). Photos provided by Dr. Christine Vega.

To make salamanders more like turtles, Vega and Ashley-Ross (2021) creatively put shells on tiger salamanders (Ambystoma tigrinum) to restrict flexibility in their trunk (Figure 2). The authors filmed the animals walking with two different shells, one flexible and one rigid, as well as without a shell. They predicted a “shelled” salamander would either compensate by changing the way they use their limbs (presumably to be more like a turtle) or walk slower. Expectedly, the rigid shell restricted the animals’ lateral flexibility (Figure 3). Unexpectedly, the authors found that the salamanders did not change the way they moved their limbs and experienced only a slight reduction in walking speed. Instead, the animals compensated by moving and bending their tail more!

Figure 3. Modified Figure 2 from Vega and Ashley-Ross (2021) showing differences in average trunk curvature (left) and tail curvature (right) between the unrestricted, Tygon (flexible shell), and the PVC (rigid shell) walking trials. Trunk flexibility decreased with the stiff shell, while tail movement increased with both the flexible and stiff shells.

This surprising result suggests that tail undulation may help salamanders traverse challenging environments. In non-salamander systems, tail movement can serve many functions such as providing stability, generating propulsion, or helping with responses to unexpected disturbances. So why don’t turtles use their tails instead of changing how they walk? Likely because most turtles have highly reduced tails, but the few species that do have long tails provide exciting avenues for future research (Figure 4).

Figure 4. The common snapping turtle (Chelydra serpentina), a turtle species with a long tail. Photo from the Wikimedia Commons.

What can salamander tail use tell us about early tetrapods that lived millions of years ago? That bit is still unclear, but the tail should not be taken lightly. Vega and Ashley-Ross conclude by proposing that future studies should investigate the effects of restricted tail flexibility because a loss of tail movement might be the catalyst that promotes changes in limb use.

Jonathan Huie is a PhD student at George Washington University studying salamander limb biomechanics and the water to land transition. His past work includes fish feeding morphology and anole ecomorphology. Find out more about his work at www.jonathanhuie.com, follow him on twitter @jmhuiee, or email him at jonathanmhuie@gmail.com.

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