A new study suggests that a tree-dwelling salamander may be able to control its grip on trees' bark by pumping blood in and out of the tips of its toes. This rather crafty strategy could one day be replicated in human technology such as grippy prosthetic hands and reusable adhesives.
Native to the Pacific Northwest region of North America, the wandering salamander (Aneides vagrans) makes its home high in the crowns of giant coast redwood trees.
Not unlike a flying squirrel, it's capable of leaping and gliding from tree to tree. And needless to say, it's important that the salamander sticks its landings – if it were to lose grip and fall all the way to the forest floor, the creature would be in for a long climb back up.
Scientists had already noticed that blood-filled cavities (aka sinuses) were easily visible within the square-ended translucent tips of the salamander's toes. It was theorized that these might be utilized for oxygenating the blood via the amphibian's gas-permeable skin, perhaps even functioning like gills.
Now, however, it appears that the blood sinuses may instead be utilized to adjust the pressure exerted by the toes upon the bark, and to increase or decrease their contact area.
Washington State University postdoctoral researcher Christian Brown first started wondering about the toe-tips when he was serving as a "salamander expert" for a nature documentary. When viewing close-ups of wandering salamanders climbing up surfaces, he and camera assistant William Goldenberg noticed that blood rushed into the tips of the creatures' toes right before they took each step.
This observation led to a study conducted by Brown, Goldenberg and colleagues, involving three live A. vagrans that were collected from the wilds of northern California. As the animals made their way across a horizontally- or vertically-oriented clear acrylic sheet, a camera shooting through the other side of that sheet captured extreme close-up video of their toes in action.
It was found that the blood sinus in each toe-tip is almost entirely divided into two parallel chambers via a membrane running down the middle. Blood can still flow in either direction from chamber to chamber, however. It can also be completely drained from each sinus, or pumped in to completely fill each sinus.
After scrutinizing the footage, the scientists noted that not only was the toe-tip blood distribution asymmetric (from toe to toe and even within each sinus), it also varied depending on whether the salamanders were standing, stepping, hanging or climbing.
So, how could this arrangement help the creatures keep a grip?
For starters, it's possible that by selectively swelling one sinus – or even one side of one sinus – a toe-tip could be temporarily wedged into a groove in the bark when climbing.
On the other hand, when a sinus is drained of blood, the toe-tip softens and thus conforms better to the contours of the bark beneath it. Its contact area with that bark also increases, allowing the vertically-oriented salamander to effortlessly hang in place.
That said, when the animal is lifting its foot off the bark to step forward/upward, it needs to release its grip. This may be achieved by pumping the sinus full of blood, making the toe-tip harder, rounder and stiffer – thus reducing its contact area with the bark.
The scientists now plan on conducting further research to get a better understanding of what's really going on, with an eye towards employing a similar mechanism in human technology. An unrelated adhesive tape inspired by the sticking power of gecko feet is already on the market.
"This pilot study did not capture footfall nor did it examine blood activity during grip-adjustment," Brown tells us. "We know that the blood can be controlled on a side-by-side basis because we observed blood concentrations changing in one sinus at a time as well as both at the same time, meaning [the salamanders] are capable of the fine-scale control between sides, but we still don’t completely understand when one side might drain or fill vs the other."
A paper on the study, which also involved scientists from Gonzaga University, was recently published in the Journal of Morphology.
Source: Washington State University