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Geckos have an extraordinary ability to cling to smooth vertical surfaces thanks to special gripping toe pads. Less well known is that some geckos have another sticky pad underneath the tip of their tails 鈥� and a study shows it is comfortably strong enough to support the weight of the animal on its own.

Adhesive toe pads are the subject of much scientific interest, says at Marquette University in Wisconsin. But the tail pads are considerably less understood, although it is more than 120 years since they were first recognised.

鈥淭hey have been truly mysterious structures for a really long time,鈥� says Griffing.

To compare the tail pads to the toe pads, Griffing and his colleagues studied the crested gecko (Correlophus ciliatus), a tree-dwelling gecko in possession of a tail pad that is native to New Caledonia in the Pacific Ocean.

The team used high-powered microscopes to view the pads鈥� microscopic anatomy, finding that the tail pads, like those on the toes, are covered in thousands of branched, hair-like structures called setae that can cling to surfaces through intermolecular attraction. In laboratory experiments, the researchers discovered that a tail pad can support five times the gecko鈥檚 body weight 鈥� roughly the stickiness of a single gecko toe.

This means the crested gecko鈥檚 tail can act like another limb, says Griffing, which may be useful in the treetop environment where 鈥渉aving a firm grasp on your environment is quite important鈥�.

The researchers also tracked how the tail pad arises during embryonic development. They found that the tail pad grows and changes shape like toe pads do, and it does so at about the same point in the development of the embryo. Griffing thinks that sometime in the gecko鈥檚 evolutionary history, the 鈥渄evelopmental programming鈥� for toe pads was turned on in the tail tip.

鈥淲hat is even more amazing is that this has been repeated in at least five different gecko lineages over evolutionary time,鈥� says Griffing.

at Bielefeld University in Germany says the cling strength of the tail pad is 鈥渦nexpected鈥�, but adds that 鈥渙ur knowledge on the variation in adhesive performance across geckos is still restricted to a relatively small number of species鈥�.

Dozens of gecko species have tail pads, and Griffing is curious to know if these gripping structures follow the same developmental patterns as in crested geckos.

Also, unlike most geckos, crested geckos don鈥檛 regenerate their tails if they are lost. Griffing wants to investigate why that is, especially given the tail鈥檚 usefulness.

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