Introduction to biotribology: Animal locomotion

Abstract Limbs of vertebrate animals have synovial joints. Many arthropod joints and the hinges of bivalve molluscs owe their mobility to components that bend, rather than to surfaces that slide over each other. No lubrication systems have been described for the few sliding surfaces in invertebrate joints. Peak forces on animal joints tend to be roughly proportional to (body mass)2/3. The fin spines of some fishes have modified joints at their bases, which enable the fish to lock the spine in the erect position. The ventral surfaces of crawling animals, including earthworms, snakes, and snails, slide over the ground. The conditions required for forward progress are discussed. Particularly, interesting tribological questions arise in the case of snails, which crawl on a mucus trail. For running animals, fast turns and other manoeuvres involving acceleration may be limited by friction with the ground. The feet of insects and geckos adhere to solid surfaces, enabling these animals to climb vertical walls and even to walk on ceilings. Van der Waals adhesion seems to be responsible in the case of geckos and capillary adhesion in many insects. The trap of pitcher plants captures insects by making the adhesive mechanism ineffective.