The Dalila effect: C57BL6 mice barber whiskers by plucking.

Group-housed laboratory mice are frequently found with their whiskers and facial hair removed. It has been proposed that dominant mice are responsible for barbering the hair of the recipient (the Dalila effect), and early studies suggest that the hair is removed by nibbling. In the present study, pairs of C57BL6 mice, composed of a barber and recipient, were separated to allow hair to regrow. The animals were then placed together in an observation box and their social behavior was videorecorded. The videorecording was subjected to frame-by-frame analysis. Barbering was found to occur during acts of mutual grooming. During grooming, one member of a mouse pair removed the vibrissae of the conspecific and did so by grasping individual whiskers with the incisors and plucking them out. Although plucking appeared 'painful', recipients were passive in accepting barbering, and even pursued conspecifics for further grooming. Other measures indicated that barbers were heavier than recipients and brain weights were not different. Although cortical barrel fields appeared normal to cytochrome oxidization and zinc staining, Golgi analysis of layer three, barrel-field basilar dendrites indicated changes in cell morphology. The results are discussed in relation to the hypothesis that barbering is an expression of social dominance, the origins of the barbering behavior, and the consequences of barbering on brain function.