Head-Cocking and Visual Exploration in Marmosets (Saguinus Fuscicollis)

Abstract

S. fuscicollis, like most primates that lack ocular dominance columns in the visual striate cortex, often cocks its head from side to side as it orients toward an external stimulus. The present paper attempts to describe this response more accurately and to examine its immediate causes, its ontogenesis and its probable functions. A variety of observations and special tests were conducted, using a total of 26 subjects. Head-cocking occurs equally often in the clockwise and counter-clockwise directions, with no obvious sequential regularities here. The angle of head rotation is ordinarily between 30 and 90 degrees but can be as much as 180 degrees. Head-cocks are nearly always associated with visual fixation of a stimulus object and are usually accompanied by characteristic bodily postures. The response is evoked principally by novel visual stimuli rather than familiar ones or odors or sounds. It habituates quite rapidly with continued or repeated exposure to a given object, following the same general trend as approach reactions except that its peak frequencies occur sooner. It most often precedes physical contact with the object and is made from within a short distance of the object. The percentage of head-cocks that are made while the animal orients in the direction of the novel object (rather than in any other direction) also declines steadily across time. The response is characteristic principally of very young animals. It is first seen at about nine days of age; its peak frequencies are estimated to occur between 20 and 40 days of age (at which the time they may easily exceed 100 per hour); and most responses are made while the infant is riding dorsally on another animal, rather than clinging to its ventrum or locomoting independently. After five weeks of age the frequency of head-cocking declines according to a straight-line log-log function; older adults might make less than one head-cock per hour. All evidence suggests that for this species the functions of head-cocking are principally if not solely visual. The more precise nature of the visual information that might be picked up, and the probable physiological and phyletic variables that underlie head-cocking, are discussed.