Mystery revealed: How geckos stick to panes of glass

Geckos amaze with their ability to run up walls with ease.

The fact that the mostly nocturnal climbers can even find a hold on smooth panes of glass is thanks to the adhesive strips on their toes.

These climbing aids, which are regularly renewed through moulting, consist of bristles that each fan out into a large number of filigree little hairs.

Each of these hairs ends in a spatula-shaped adhesive disk that nestles tightly against the surface.

Presumably so narrow that van der Waals forces occur.

These are forces of attraction between neighboring molecules that only act over extremely small distances.

Since traction improves with increasing humidity, it could not be ruled out until recently that the capillary forces of water play a role.

Mette H. Rasmussen, Katinka Rønnow Holler and Tobias Weidner from the University of Aarhus have now discovered that water-repellent substances in the form of lipids are involved instead.

The Tokee

, native to Southeast Asia, served as a research object .

Originally found in the branches of forests, this grey-blue gecko, decorated with red spots, also likes to frolic in gardens and buildings today as a cultural successor.

Having escaped from the terrarium, the adaptable reptile has even found a new one here and there, far away from its ancestral home, for example in Florida and on the Hawaiian island of Oahu.

Together with scientists from Oregon State University in Corvallis, from the National Institute of Standards and Technology in Gaithersburg (Maryland) and from the University of Kiel, the Danish team studied the surface of the adhesive lamellae at the molecular level.

With a variant of absorption spectroscopy, the chemical composition could be analyzed more precisely.

Considerably more lipids were found on the densely packed adhesive disks of the adhesive lamellae than on other skin samples.

Clean and dry feet on the go

However, the shell formed by these fatty, water-insoluble substances is only a few millionths of a millimeter thick.

The spectroscopy, which only records a layer of five millionths of a millimeter, also recorded signals from proteins that lie under the lipid envelope.

As Rasmussen and his colleagues report in the "Biology Letters", the measured spectra also allow conclusions to be drawn about the spatial arrangement of the lipids: aligned almost parallel, these molecules are perpendicular to the keratin structure of the spatulate adhesive disks.

Since geckos also leave lipids in their footprints, such substances have to be replenished on an ongoing basis.

How this happens has not yet been clarified, nor has the function of the delicate enveloping layer.

By reducing the evaporation of water, lipids could, for example, prevent the wafer-thin adhesive disks from drying out and thereby losing their suppleness.

Incidentally, when the humidity is high, even completely dried-up bristles from the adhesive lamellae of geckos become somewhat softer and somewhat adhesive again.

However, the lipid layer may also directly ensure better traction on water-repellent material, because both surfaces interact perfectly in the close-up molecular range.

In their traditional habitat, geckos often walk around on suitable ground: leaves and twigs usually have a thin layer of wax with which they stop the evaporation of water.

In addition, a water-repellent surface not only helps to protect the adhesive strips from dirt.

It could also allow the animals to climb through rain-soaked branches with largely dry feet.

The functionality of gecko toes is far from completely unraveled.