Photograph by Guy Edwardes
Tree frogs often find their way to perches high in the cloud forest canopy, but one night in Ecuador’s Sumaco Napo-Galeras National Park, England-based photographer Guy Edwardes spotted this Shreve’s Sarayacu tree frog (Dendropsophus sarayacuensis) relaxing on a leaf within easy reach of his lens. The species has a yellow-and-brown patterned back, bulging copper-colored eyes, and a flame-orange belly and inner legs. The diminutive, rain-inspired singer with a distinctly herbal scent is emblematic of the vast amphibian biodiversity of the Neotropics. Dozens of tree frog species, including the Shreve’s Sarayacu, make their homes in the Amazon Basin alone. And just like the frog that Edwardes met at eye level, they are exceptional climbers, as are most of the world’s other tree and torrent frogs—able to cling to vertical and overhanging surfaces that are smooth or rough or even wet, sometimes dangling by a single toe.
These superpowers are the product of convergent evolution across multiple families of frogs that adapted to moving through the vertical world of plants and trees (or waterfalls, in the case of torrent frogs). The frogs’ long toes end in wide pads covered in pentagonal, hexagonal, heptagonal, or octagonal columnar skin cells separated by channels and covered with bristly features called nanopillars. Glands secrete mucus-like fluid into the channels, covering each toe pad and forming a liquid bond with the surface that the frog grips. The pads’ unique, pliable softness and complex surface also help the toes mold to every fold and bump they traverse, and create friction to firm the frogs’ footing. Researchers believe a groove around each pad may help shunt excess fluid that could interfere with adhesion. Adopting a sprawled posture on a precipitous surface also helps frogs stay put.
Because tree frog toe pads are sticky, they have evolved a self-cleaning mechanism to ensure that crud doesn’t accumulate and interfere with their adhesion. Researchers using a species found in Australia have shown that tree frog toes shed debris as the animal moves—possibly because friction rubbed the debris off or the constant secretion of toe mucus flushed it away—leaving it behind in slimy footprints.
The superpowers in tree frog feet may have human applications for soft-tissue engineering, reversible adhesives, and other technological innovations. One research team used structural complexity similar to that of tree frog toes to improve the pad on a safety razor. The design channels away water and simultaneously grips skin, stretching it ahead of the blade for a closer shave. Another has developed an adhesive patch inspired by tree frog toes and octopus suction cups that can adhere to human skin even under flowing water. Such inventions are just a sampling of the biomimetic technologies inspired by nature—a reminder that our fellow beings have much to offer, right down to the genius of their sticky little toes.
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