Vibrissal behavior and function

Tactile hair, or vibrissae, are a mammalian characteristic found on many mammals (Ahl, 1986). Vibrissae differ from ordinary (pelagic) hair by being longer and thicker, having large follicles containing blood-filled sinus tissues, and by having an identifiable representation in the somatosensory cortex. Vibrissae are found on various parts of the body, but those most frequently studied are the facial or mystacial vibrissae, also called whiskers. Long facial whiskers, or macrovibrissae, are found in many mammalian species, projecting outwards and forwards from the snout of the animal to form a tactile sensory array that surrounds the head. For example, in rats, the macrovibrissae form a two-dimensional grid of five rows on each side of the snout, each row containing between five and nine whiskers ranging between ~15 and ~50 mm in length (see Figure 1 for illustration). The macrovibrissae of many rodents and some other species can move back and forth at high-speed thus explaining the term "vibrissa" which derives from the Latin "vibrio" meaning to vibrate.

The study of any behavior involves identifying the circumstances under which it arises, and then characterizing its nature, as precisely as possible, in all of the relevant contexts. However, in addition to describing what animals do, behavioral science also seeks to understand the function of behavior, both 'proximally' in terms of its immediate consequences for the animal, and 'ultimately', in terms of its adaptive significance and contribution to the evolutionary fitness of the species. In the context of the vibrissal system, we are only just beginning to piece together descriptions of how, and in what contexts, animals use their whiskers. Even less is known about the function of vibrissae, beyond the obvious intuition that whiskers are 'for touch' just as the eyes are 'for sight'. We would like to understand much more about the specific contribution of the vibrissae to the life of the animal, both in order to explain the emergence, through natural selection, of this important mammalian sense, and also to be able to frame better functional hypotheses for physiologists investigating its biological and neural substrates. Here, we provide a brief comparative and ethological overview of vibrissal behavior and function.