Animal locomotion on the surface layer of water is the study of animal locomotion in the case of small animals that live on the surface layer of water, relying on surface tension to stay afloat.
There are two types of animal locomotion on water, determined by the ratio of the animal's weight to the water's surface tension: those whose weight is supported by the surface tension at rest, and can therefore easily remain on the water's surface without much exertion, and those whose weight is not supported by the water's surface tension at rest, and must therefore exert additional motion in a direction parallel to the water's surface in order to remain above it. A creature such as the basilisk lizard, often dubbed the 'Jesus lizard', has a weight which is larger than the surface tension can support, and is widely known for running across the surface of water. Another example, the western grebe, performs a mating ritual that includes running across the surface of water.[1]
Surface living animals such as the water strider typically have hydrophobic feet covered in small hairs that prevent the feet from breaking the surface and becoming wet. Another insect known to walk on the water surface is the ant species Polyrhachis sokolova. The pygmy gecko (Coleodactylus amazonicus), due to its small size and hydrophobic skin, is also able to walk on the water surface.[2]
According to biophysicist David L. Hu, there are at least 342 species of water striders.[3] As striders increase in size, their legs become proportionately longer, with Gigantometra gigas having a length of over 20 cm requiring a surface tension force of about 40 millinewtons.
Water striders generate thrust by shedding vortices in the water: a series of U-shaped vortex filaments is created during the power stroke. The two free ends of the "U" are attached to the water surface. These vortices transfer enough (backward) momentum to the water to propel the animal forwards (note that some momentum is transferred by capillary waves; see Denny's paradox for a more detailed discussion.)
- ^ "Aechmophorus occidentalis western grebe". Animal Diversity Web. University of Michigan Museum of Biology. Retrieved 2014-01-02.
- ^ Walker, Matt (13 October 2009). "Lizards filmed 'walking on water'". BBC News.
- ^ Hu, David L.; Chan, Brian; Bush, John W. M. (2003). "The hydrodynamics of water strider locomotion". Nature. 424 (August). Nature Publishing Group: 663–6. Bibcode:2003Natur.424..663H. doi:10.1038/nature01793. PMID 12904790. S2CID 4362791.