How a tokay gecko sticks to glass without any glue

The tokay gecko is named after its own shouting. Males call a loud, two-part “to-kay” again and again, and the Smithsonian’s National Zoo says the call carries several meters. An animal this loud should be easy to find. Yet it is a nocturnal ambush hunter that lives on trees, cliffs and walls, which means it is usually stuck to something vertical.

How it sticks is the real puzzle. Science News reported in 2000 that scientists had been studying how geckos cling to smooth surfaces for at least 75 years. The tokay (Gekko gecko) can hang from glass, and in a 2005 experiment a dirty toe regained enough grip after a few steps to carry the animal’s weight. There is no glue, and it is not suction.

This article follows the grip from the toe down to the molecules: the hairs, the forces, the trick of letting go, and the explanations that scientists ruled out. Where the evidence is a calculation rather than a proof, we say so.

Meet the tokay gecko

The tokay lives in the tropical forests and rocky areas of southern and Southeast Asia. The Animal Diversity Web, a University of Michigan database, gives its native range as northeast India through the Indo-Australian Archipelago, and The Reptile Database lists many countries and islands, from Bangladesh to Indonesia. It has also been introduced to Florida and Hawaii, among other places, and the National Zoo says it is considered a pest in some of them. It is kept in captivity and traded as a pet. This article is not a care guide.

The two color forms matter more than they look. A 2012 chromosome study, as described by The Reptile Database, suggested that red-spotted and black-spotted tokays might be different species. The same page says that an earlier name, Gekko reevesii, has been revived and that a 2023 study found tangled family lines among these animals. So “the tokay gecko” may turn out to be more than one species.

What scientists ruled out

Before anyone found the answer, they crossed off wrong ones. The 2000 news release from UC Berkeley, which accompanied a Nature study by Kellar Autumn, Robert Full and colleagues, lists the explanations that did not hold up.

• Suction. The calculated force is far too weak, and geckos also stick in a vacuum.
• Glue. Geckos have no glue glands, and their feet leave no residue.
• Velcro-style interlocking. Also on the release’s list of ideas that did not fit.
• Friction alone. Friction cannot explain walking on a ceiling.
• Static electricity. Other researchers had already ruled it out.

That leaves a quieter answer. It is physics, not chemistry or plumbing.

Hairs on hairs, and a force between molecules

A tokay’s toe pad is covered in setae, tiny hairs about one-tenth the width of a human hair, according to the 2000 release. Each seta ends in hundreds of flat tips called spatulae. Science News says the sole carries roughly a billion spatulae, and the release also speaks of a billion working together. The 2005 self-cleaning paper describes each spatula as about 200 nanometers wide, and a nanometer is a billionth of a meter.

A tip that small can get very close to a surface. When two surfaces are that close, molecules attract each other through a weak pull called the van der Waals force. It comes from the way electrons shift around inside atoms, and it exists between any two materials. One pair of molecules barely notices it. A billion tips add it up. The 2000 release says a single seta could lift an ant, and a million of them over an area the size of a dime could lift a child of about 45 pounds. It also says the combined force of all the toe hairs is about 10 times the force needed to pull a live gecko off a wall, and that geckos use only a fraction of their hairs at once.

Was this proven in 2000? Not quite. Science News reported that the van der Waals idea was a calculated explanation, and that the team could not rule out water on the surface as a helper. The 2002 PNAS paper by Autumn and colleagues went after that gap. Water film, called capillary adhesion, predicts a weak grip on water-repelling surfaces. Van der Waals forces predict a strong grip on any surface the hairs can sit close to. The tokay toes stuck about equally well to water-repelling and water-loving surfaces, and so did a single isolated seta. One summary of the paper reports about 40 millionths of a newton for a single seta on both surface types, within roughly 2% of each other. We could not check that number against a second source. The authors rejected capillary adhesion as the deciding force and reported what they called the first direct experimental evidence for van der Waals adhesion.

Sticking is easy; letting go is the trick

A sticky foot that cannot unstick is a problem. The release explains that merely touching a hair to a surface is not enough, because it slips. A push in, plus a small pull toward the body, makes it grip. To release, the gecko peels. Both the release and Science News report that the hairs hold until they are levered up to about 30 degrees from the surface. Pulling straight away does nothing, and peeling does the work.

The sources give different numbers for the benefit of the sideways tug. The release says a push-and-pull gives about 600 times more sticking power than friction alone, and Science News says the tug makes each seta grip about ten times better than pressing alone. They compare different things, so treat both as rough.

Then there is the dirt problem. Geckos do not groom their feet, yet they stay sticky for months between sheds of skin, according to the 2005 paper by Wendy Hansen and Kellar Autumn. In their tests, toes coated with tiny spheres lost much of their grip, and steps on clean glass brought back enough for a single toe to hold the animal’s weight again. The authors attribute this to an imbalance of attraction: a dust particle is pulled harder by the glass than by the tiny tips, so each step leaves more particles behind than it keeps. The method used glass spheres as stand-ins for dirt, so real dust may behave differently.

What this looks like in practice

Here is how the tests translate into everyday situations.

That last row is the gecko’s rare defeat. The authors attribute it to the material’s weak polarizability, which is a measure of how easily its electrons shift, rather than to its slipperiness.

Common mistakes and myths about gecko feet

Myth: the feet are sticky like tape. Tape sticks because of glue, and gecko feet have none. The grip depends on angle and contact, which is why the foot can release on cue.

Wrong guess: it is suction. Suction cups need a seal, and geckos hold on in a vacuum.

Wrong guess: it is glue. The 2000 release reports no glue glands and no residue.

Too narrow: only the toe matters. The grip comes from the hierarchy: toes, pads, hairs and tips. The 2005 paper notes that the self-cleaning belongs to the setae as a structure, not to a habit of the animal.

Mix-up: every lizard climbs this way. Lizards solve the problem differently. The Panther chameleon grips branches with fused toes, the Common flying dragon glides from tree to tree on rib-supported wings, and the Common basilisk runs across water. The tokay’s hairs are one answer, not the only one.

Conservation status

The tokay gecko is listed as Least Concern on the IUCN Red List of Threatened Species. That is the category as distributed by GBIF, the Global Biodiversity Information Facility, and we checked it on November 9, 2026. Least Concern means the species was evaluated and does not meet the criteria for a threatened or near-threatened category. The Animal Diversity Web still lists it as Not Evaluated, a difference that shows why the date matters.

Least Concern does not mean no problems. The Smithsonian’s National Zoo names over-harvesting for traditional medicine and the pet trade as the main threat, and reports national concerns: China’s Red Data Book lists the species as endangered, and the same page reports a 50% decline in Nepal since 2009, a figure we could not confirm elsewhere. The Revelator reports that CITES, the treaty that controls wildlife trade, agreed to place the tokay in Appendix II, with the listing taking effect in late 2019 (the Revelator says “this December”). Appendix II means exports need a permit. We have no confirmed global population trend, because none of our sources states one.

Categories are reviewed and can change, and the copy GBIF distributes can trail the IUCN website by some months. GBIF’s copy does not give the year of the assessment, so we do not state one.