Whisking is a behaviour in which the facial whiskers of an animal are repetitively and rapidly swept back and forth. This behaviour occurs particularly during locomotion and exploration. The whisking movements occur in bouts of variable duration, and at rates between 3 and 25 whisks/second. Movements of the whiskers are closely co-ordinated with those of the head and body, allowing the animal to locate interesting stimuli through whisker contact, then investigate them further using both the macrovibrissae and an array of shorter, non-actuated microvibrissae on the chin and lips. Whisking has been reported in a wide range of mammals, including two species of marsupial. Whisking contributes both to exploratory movements, which function to acquire sensory inputs, and to palpation movements, which are used in the discrimination of objects and in the control of spatial navigation.
Types
There are three types of whisking. Exploratory whisking occurs when animals whisk in air without contact or with only light contact. In this case, the stroke on a given whisk approaches 70°, and a total field of up to 160° may be covered as the animal slowly shifts the set point of its whisk. Such whisking is extremely regular and typically occurs with a frequency between 7 and 12 Hz and a mean of 9 Hz. Asymmetric whisking occurs when animals make contact with a large object, such as a wall, while they whisk. It also occurs if they turn their head to the side while whisking. Asymmetric whisking lasts for only one to three whisk cycles. Foveal whisking occurs when animals thrust all their vibrissae forward to palpate an object ahead of them, as occurs when they try to detect a landing on the far side of a gap. In this case the stroke is much reduced, typically to 20°. The frequency of foveal whisking is high, ranging between 15 and 25 Hz, and rats can readily switch between foveal and exploratory whisking.
Facial vibrissae
Many land and marine mammals possess facial vibrissae. For example, rats and hamsters, have an arrangement of cranial vibrissae that includes the supraorbital, genal, and mystacial vibrissae, as well as mandibular vibrissae under the snout. Mystacial vibrissae are generally described as being further divided into two sub-groups: the large macrovibrissae that protrude to the sides, and the small microvibrissae below the nostrils that mostly point downwards. The macrovibrissae are generally large, motile and used for spatial sensing, whereas microvibrissae are small, immotile and used for object identification.
Generally, the supraorbital, genal and macrovibrissae are reported to be motile, whilst the microvibrissae are not. This is reflected in anatomical reports that have identified musculature associated with the macrovibrissae that is absent for the microvibrissae. The effector system generating whisking comprises a set of “extrinsic” muscles controlling movements of the mystacial pad and a group of “intrinsic” muscles, producing vibrissa protraction. The extrinsic muscles in the mystacial pad move many or all of the macrovibrissae together. However, the individual follicles of some groups of facial vibrissae in some species are also motile. A small muscle 'sling' is attached to each macrovibrissa and can move it more-or-less independently of the others. Vibrissa retraction is thought to be a passive process produced by rebound of stretched follicular muscle. Whisking movements are amongst the fastest produced by mammals. In animals that are capable of whisking at high frequencies, the whisking musculature contains a high proportion of type 2B muscle fibers that can support faster contractions than normal skeletal muscles. Sensory innervation of the whiskers is provided by the infraorbital branch of the trigeminal maxillary nerve; motor innervation is attributable to the facial nerve.
Species that whisk
Amongst those species with motile macrovibrissae, some perform whisking. Although whisking is prominent in rodents, there are several rodent genera, such as capybara and gophers, that do not appear to whisk, and others, such as guinea pigs, that display only irregular and relatively short whisking bouts Whisking behaviour has not been observed in carnivores, although some species, such as pinnipeds, have well-developed sinus muscles making the whiskers highly motile.
Control and co-ordination
In rats, whisking movements occur in bouts of variable duration at rates between 3 and 25 whisks/second. The movements have an amplitude over a range from ∼10 to 100°, at an average protraction velocity of ∼1000°/sec, and at a predominant frequency of 5–7 Hz. Three types of whisk are described, single, delayed or double-pumped. Whisking movements relative to the head are described by 3 angles of rotation and translations of the whisker base. Azimuthal rotation moves the whiskers back and forth along the longitudinal axis. This movement co-varies with small changes in elevation. Torsion refers to a rotation of a whisker about its own axis. The torsional angle is correlated with the azimuth, and alters the forward facing surface of the whisker shaft that contacts the opposing surface. Because the vibrissae are curved, torsion also displaces the whisker tips relative to the head. Early studies described co-ordination between vibrissae, nose, head, and sniffing movements. From these studies it was suggested that the animal's whisking behaviour is dependent on the task and that whisking during exploratory behaviors is different from whisking during discriminative behaviours. In all whisking animals in which it has so far been measured, these whisking movements are rapidly controlled in response to behavioural and environmental conditions. Movements of the whiskers are closely co-ordinated with those of the head and body. Sniffing, a high-frequency, highly rhythmic inhalation and exhalation of air through the nose, plays an important role in rodent olfaction. Whisking is thought to be co-ordinated with sniffing and normal respiratory behaviour. It has been shown that breathing and whisking movements are correlated only when the whisking rhythm is less than 5 Hz. Only 13% of whisking movements occur during high-frequency respiration typically associated with sniffing, indicating that high-frequency whisking and sniffing behaviours are not correlated. Whisking and sniffing also constitute the overt expression of an animal's anticipation of a reward.
Development
In rats, whiskers grow to their adult size in the first month of life, although, rats sustaining denervation of the whisker pad grow whiskers that are thinner and smaller than those of normal adults. Whisking begins around post-natal day 11 to 13, prior to the eyes opening, and achieves adult amplitudes and characteristics by the end of the third post-natal week. Prior to the onset of whisking, neonatal rats show behavioural activation in response to whisker stimulation, and tactile learning in a classical conditioning avoidance paradigm, but are not able to orient to the stimulus source. Micro-movements of the vibrissae in the first ten days of life have also been observed.
