Arboreal locomotion small branches, as they may wrap too far around and prick the animal's own paw. Adhesion is an alternative to claws, which works best on smooth surfaces. Wet adhesion is common in tree frogs and arboreal salamanders, and functions either by suction or by capillary adhesion. Dry adhesion is best typified by the specialized toes of geckos, which use van der Waals forces to adhere to many substrates, even glass. Frictional gripping is used by primates, relying upon hairless fingertips. Squeezing the branch between the fingertips generates a frictional force that holds the animal's hand to the branch. However, this
Prostitution among animals observed. A study at Yale–New Haven Hospital trains capuchin monkeys to use silver discs as money in order to study their economic behavior. The discs could be exchanged by the monkeys for various treats. During one chaotic incident, a researcher observed what appeared to be a monkey exchanging a disc for sex. The monkey that was paid for sex immediately traded the silver disc for a grape. The researcher took steps to prevent any possibility of coins being traded for sex after his suspicions were aroused. Prostitution among animals A few studies have suggested that prostitution exists among different species