Tattooing Various Combinations of Ears, Tail, and Toes to Identify Mice Reliably and Permanently

Uniquely identifying research animals is a widespread and essential procedure. Potential disadvantages of commonly used identification methods such as toe clipping, ear punching, and ear tagging include tissue loss and adverse effects in physiologic homeostasis and animal behavior. In addition, the labels produced by using these methods can become unreadable, potentially leading to misidentification. In this study, we proposed a combined approach involving ear, tail, and toe tattooing that can be used to permanently identify mice regardless of their age. Four groups (neonatal and adult C57BL/6J [black] and CD1 [white] mice) were used. Single- or 2-color tattooing (ear, tail, or toe or combinations thereof) was performed to identify a defined or unlimited number of mice, respectively. Tail tattooing using both green and red pastes was suitable for identifying white-haired neonatal mice as early as postnatal day 1, whereas toe tattooing with green paste was an effective alternative approach for labeling black-haired mouse pups. In comparison, single-color (green) or 2-color (green and red) ear tattooing identified both white and black adult mice older than 3 wk. Ear tattooing can be adapted to labeling an unlimited number of adult mice by adding the cage number. We conclude that tattooing various combinations of the ears, tail, and toes provides an easy and permanent approach for identifying mice of all ages with minimal disturbance to the animals.