The Dalila effect: C57BL6 mice barber whiskers by plucking

Group-housed laboratory mice are frequently found with their whiskers and f acial hair removed. It has been proposed that dominant mice are responsible for barbering the hair of the recipient (the Dalila effect), and early stu dies suggest that the hair is removed by nibbling. In the present study, pa irs of C57BL6 mice, composed of a barber and recipient, were separated to a llow hair to regrow. The animals were then placed together in an observatio n box and their social behavior was videorecorded. The videorecording was s ubjected to frame-by-frame analysis. Barbering was found to occur during ac ts 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 'painf ul', recipients were passive in accepting barbering, and even pursued consp ecifics for further grooming. Other measures indicated that barbers were he avier than recipients and brain weights were not different. Although cortic al barrel fields appeared normal to cytochrome oxidization and zinc stainin g, Golgi analysis of layer three, barrel-field basilar dendrites indicated changes in cell morphology. The results are discussed in relation to the hy pothesis that barbering is an expression of social dominance, the origins o f the barbering behavior, and the consequences of barbering on brain functi on. (C) 2000 Elsevier Science B.V. All rights reserved.