Albino Rodent Tail Tattooing: Two Years of Experimental Data and Observations

By Norman B. Guilloud, DVM, and A. Neill Johnson, DVM, Ph.D.

PRESENT METHODS FOR IDENTIFYING rats and mice are not always satisfactory. Ear notch- ing and ear tagging, though widely used by researchers, have inherent problems. Applying and reading ear notches is time consuming. When one person applies ear notches others may find them difficult to read. Using ear notches to represent a five or six -digit number is also very difficult. The notching of ears is not, in our opinion, the most humane procedure.

Researchers can apply ear tags rapidly, read them more accurately than ear notches, and can stamp them with letters and numbers. By using both letters and numbers, a researcher may identify very large populations of animals without repeating the same identification number. Two major problems with ear tags are tag loss, and inflammation, bleeding, and necrosis of the area adjacent to the tag. If the tags become imbedded in the tissue of the ear, they become difficult to read. During two year studies, rats have been reported to develop neoplastic lesions at the ear tag site. Recently, there have been reports of short-term experiments on tail tattooing in rats3.4. Long-term tail-tattoo studies in rats1, and both rats and mice5, showed that tattooed numbers and letters were legible for two years. This paper addresses additional efficacy data and toxicology information concerning tail tattoos in rats and compare these with data concerning ear tags. In addition, we summarize five years of practical observations on the use of tail tattoos in albino rats and mice.

We used an electrical tattoo machine with 12-volt regulated power supply and foot control switch (AIMS, Inc., 221 Second Avenue, Piscataway, NJ 08854). We distributed weanling Sprague Dawley albino rats from Charles River Laboratories into four treatment groups (20 males and 20 females per group) as follows:

Group 1. Untreated controls; 
Group 2. Tattooed with AIMS™ black #242; 
Group 3. Controls tattooed with physiological saline; 
Group 4. Tagged with Monel ear tags. 

We manually restrained the rats and tattooed each animal in Group 2 on the tail with one letter and four numbers. We did not use anesthesia during these procedures. We tattooed Group 3 with saline to exactly duplicate the handling and restraint used in Group 2. We used standard procedure to ear-tag members of Group 4. We individually housed the rats in wire-floored cages. The data we collected during the two-year study included mortality, body weight, loss of identifying factors, speed of the identification process, hematology, gross pathology and histopathology.

Results: Figure 1
We observed no differences between the groups in the parameters of mortality, body weights, or hematology (red cell, white cell and differential counts). We conducted hematology tests during the first four weeks following identification. Since we noted no differences, we discontinued these tests.

Our visual comparisons of Group 2 and the control groups (1 and 3) showed that Group 2 suffered no adverse offects. Of the Group 4 rats, though, 15% exhibited ear neerosis and 35% required ear tag replacement during the two years. Tattoos in Group 2 were legible throughout the study, though some tattoo's faded. At the end of the study, the tattoos on one rat were difficult to read. Most tattoos were legible and easy to read (See Fig.I).

Twenty-three months into the experiment, an experienced animal handler was able to read the tattoos in Group 2 at a Speed of 360 per hour while ear tags in Group 4 were readable at a rate of 206 per hour.

Table 1 Histopathological examination of the adrenal cortex,kidney's, liver, pancreas, pituitary, skin and mammary gland showed no differences among treatment groups. We noted differences, however, in the histology of the tail and in the lymph nodes proximal to the tail. Group 2 rats showed some fibrous tissue infiltration in the tall area surrounding the tattoos. A few animals in Group 2 had small amounts of black pigment in the popliteal and sublumbar lymph nodes adjacent to the tall (See Table I). The pigment panicles in the lymph nodes did not mask the visibility of the cells in these nodes. These nodes showed no differences in lymphoid hyperplasia, sinusoidal histiocyte proliferation, plasma cell proliferation, sinusoidal dilation, lymphatic dilalion, and hemorrhage when compared with 27 rats in control Groups 1 and 3, and 31 ear-tagged rats in Group 4.

Tattoos and ear tags are safe to use for rat identification. We found no differences in mortality, body weight and hematology between the treatment groups. The mild tail fibrosis in Group 2 in the area of the tattoos is similar to reported reactions in human tattoos. With human tattoos, mild fibrosis of the papillary dermis is an almost constant feature. The presence of a small amount of pigment panicles in the popliteal and sublumbar lymph nodes did not cause any abnormalities in these nodes. Since we saw that our rats' tail tattoos had no carcinogenic or other adverse effects, we conclude that the pigment #242 is safe for use. Permanence, easier and faster readability and the humane aspects of tattooing make this procedure more effective than ear tags.

Our laboratory has been using tail tattoos with AIMS™ black #242 in rats and mice for the past five years. In addition to the advantages just mentioned, we found that animal technicians were bitten less oftcn when reading tattoos than when reading ear tags. The animals experienced little or no trauma from the tattooing process.

The tattooed taiIs can also be saved at necropsy with other tissue specimens as a positive source of identification since the tattoos are readable in formalin for an indefinite period.

In long-term experiments, some animals' tattoos may require touch-ups. Others may need retattooing if the tattoo starts to fade. Fading is caused by improper tattooing techniques. Technicians require good vision and proper training in order to obtain permanent, easily-readable tattoos.


  1. Guilloud, N.B. Johnson, A.N. Barbour, LL,and Duncanson, A. Comparison of Ear Tags and Tail Tattoos for Rat Identification.Lab Anim. Sci. ; 34 (5):499, 1984
  2. Waalkes, M.P., Sabine, R., Kasprzak, K.S. and Issaq, H.J. Inflammatory, Proliferative, and Neoplastic Lesions at the Site of Metallic Identification Ear Tags in Wistar [Crl:(W1)BR] Rats. Cancer Res.47: 2445-2450, 1987
  3. Donohue, N.M. and Greenbert, N.J. Procedural Determinants of Tail Tattoo Stability in Sprague Dawley Rats.Lab. Anim. Sci.39(5):494, 1989
  4. Farwell, L.M. An laternative for the Arabic Numbering System Used in Tattooing Laboratory Animals. Lab Anim. Sci. 39(5):495, 1989
  5. Yacowitz, H., and White, E. Improved Tattooing Equipment and Proceduers for Laboratory Rat and Mouse Identification. Abstract P-3 33rd Annual Meeting Amer. Assoc. Lab. Anim. Sci., Oct, 1982
  6. Goldstein, A.P. VII. Histologic Reactions in Tattoos. J. Dermtol. Surg. Oncol.; 5(11):896-900, 1979

Norman Guilloud Is Director. Laboratory Animal Medicine and A. Neill Johnson is Assistanr Director of pathology, Department of Drug Safery Evaluation at the R.W. Johnson Pharmaceutical Research Institute. Re-print requests to Guilloud at the R.W. Johnson Pharmaceutical Research Institute, P.O. Box 300, Raritan, N.J. 08869


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