In some fields of research, death is only the beginning. For one team based at the University of California, Riverside, the goal was to answer a macabre mystery: Do gecko toes still cling fast to surfaces if the animals die mid-climb?
The question traces back to a decade-old discovery, when scientists famously found that geckos can skirt up walls and across ceilings thanks to millions of tiny hair-like structures on their toes. The bristles utilize van der Waals force—a weak attraction between molecules that gives the lizards their surprising abilities. In life, each of a gecko’s four feet has a clinging strength of up to 20 times the animal’s body weight. The finding spawned a plethora of designs for technologies that mimic gecko grip, including gecko-force tape and hand pads that let humans climb up walls.
But while we know why geckos stick to walls, we don’t understand as much about how they actually do it. For instance, no one was sure whether geckos actively control their grip—like a cat flexing its claws to climb a tree—or whether their toes automatically cling to whatever surface they come into contact with, like a cockroach’s sticky appendages.
Now the question has been answered. Geckos are indeed undaunted by death, the UC Riverside researchers report today in the journal Royal Society: Biology Letters. Even after life leaves their body, their grip remains just as strong as it was when they were living, breathing organisms.
Arriving at this finding required the researchers to create a novel gecko-pulling machine. The device attaches to a gecko's foot and applies a controlled, steadily increasing pulling force. The contraption provided “unprecedented standardization” for measuring gecko clings, the researchers write. Two living tokay geckos were first tested for their adhesive strength. When placed near an acrylic plate, the animals voluntarily attached their feet to that surface. The team dislodged them in a series of six trials, measuring the force it took to pull them off each time. The toes hyperextended just before the animals lost their grip, the team noticed.
The next step, unfortunately, was euthanization. Seven minutes after death, the trials resumed, and the animals’ grips were tested for the next 30 minutes. To the researchers' surprise, the dead geckos’ cling was just as strong as it was in life, with the strongest sustaining up to 19.9 times the animals’ mean body weight. The position, axis and angle of the toes—including the hyperextension element—matched, too, even though the surface area the dead animals’ toes connected with was smaller. Sometimes a toe or two would pop loose before the device applied its full pulling power, but that didn't affect the gecko's total adhesive strength. For a dead lizard, the results were quite impressive.
Since the results show that life force is not required for a gecko to generate its incredible grip, it seems that the hyperextended toes could be key for helping scientists improve designs for their gecko-like adhesives.