Ear tags are inexpensive and easy to perform, and as a result are commonly used for the identification of individual mice and rats. Tags can be custom-made with a series of numbers and/or letters. Assigning specific codes to a mouse/rat strain, or to a Principle Investigator (PI), aides in recordkeeping and database management, especially in large breeding colonies where there are multiple strains and PIs. In addition, there are a variety of tags commercially available. Metal tags have characters etched on them. Although this etching is clear, when working with a mouse it is often necessary to pick it up in order to read the code on the tags due to the small size of the tags. Other options are plastic tags with either numbers and/or letters on brightly colored backgrounds, or plastic tags with a barcode on the front surface. Both of these options allow the tags to be read without having to handle the animals. Another advantage to plastic tags is their compatibility with MRI imaging.

The positioning of the tag on the ear is critical, as improper positioning can have several unfortunate consequences. Tags must be placed such that they do not cause a bend in the pinna, interfere with the animal's mobility, or catch on any part of the caging. If the tag is placed too close to the edge of the ear it can easily be torn from the pinna tissue. If positioned too close to the head, the skin of the neck area can catch in the tag, which will create irritation, pain, and restricted movement. This can then lead to self-trauma from the animal scratching at the tag, ripping it from the ear, or getting a foot caught in the tag. Training for proper placement and technique is essential.¹

While tags provide individual identifiers for mice or rats, they are not infallible. Tags can be lost due to fighting, over-grooming, dermatitis in susceptible strains, infection, or self-trauma.² Adult rats in particular are less tolerant of ear tags and often remove them, inflicting self-trauma. The more aggressive breeding behavior in rats can also lead to an ear tag being torn from the pinna. If mice or rats are paired or group-housed, the loss of an ear tag on more than one animal can be troublesome. To circumvent this problem, a secondary form of identification-such as a punch code used on the ear opposite of the tag, or a tail or toe tattoo-should be utilized. A facility should standardize both the choice of ear to be tagged and the code to be used on the other ear or on the toes. All codes should be indicated on the cage card.

The use of ear punch codes allows for a quick and simple method of identification, but the technique has limitations. Commonly used codes have an upper limit for numbers available for use, usually a maximum of 100 to 399. To read the ear punch codes, most animals must be grasped at the scruff. The punches can double as tissue samples for PCR genotyping, which allows for animal identification and genotype sampling to be accomplished simultaneously. This results in less stress to the animals from repeat handling.³

There are three styles of ear punches: the scissor punch, the thumb punch, and the lever punch. Although all are equally effective, there are advantages and disadvantages to each. For example, as rats age, the skin of their ears thickens. It is best to perform the ear punches on young rats as they are weaned. For adult rats that require identification, the use of inhalant anesthesia should be considered. Ergonomically, the level punch is easier to use than a thumb punch, especially for the thicker ears of older rats. However, both of these punches require a pinching motion of the fingers. The scissor punch has the ergonomic advantage of not requiring a pinching motion, thus avoiding carpal tunnel fatigue and injury. In contrast, the thumb punch is advantageous for collecting genotyping samples, as the tips with the tissue material will fit into an Eppendorf tube. The style of punch to use should be determined by the individuals performing the task and the age of the rats.

For rats, although metal tags are most often used, studies have shown that there are consequences to using them. Inflammation, neoplastic changes, and even chronic pain have been observed in animals that have worn metal ear tags for several months.^3,4,5,6 The tags should be promptly removed from the ear if there is an adverse reaction, and the subsequent injury should be examined and treated, if necessary, by the veterinary staff.

Proper care and maintenance of punches will extend their usage. All types of punches will dull if autoclaved, so it is recommended to clean them with a disinfectant and rinse them with alcohol. Then, they must be thoroughly dried to prevent rusting. If the punches are used for PCR, they should be soaked for 3 minutes in a solution that removes residual DNA or RNA. Before use they are rinsed in alcohol and wiped dry.

When choosing the appropriate identification method, many factors must be deliberated. Each technique has advantages and disadvantages that must be considered in relation to the experimental needs. While the costs must be weighed along with other factors, the ease of the technique and the level of discomfort to the animals should be the primary considerations.^3,4

Ear punching/notching is commonly used in mice and rats because there is little cost, and the procedure is easily done. There are a limited number of codes that can be used, with most codes having an upper limit of 100 to 399. To read the ear punch codes, most animals must be grasped at the scruff. However, the punches can double as tissue samples for PCR genotyping, which allows for identification and genotype sampling to be accomplished simultaneously, thus resulting in less stress to the animals from repeat handling. ⁵