Identification Methods for Rodents
Revised April 22, 2005
There are several IACUC approved methods used to identify rodents. Each method has both advantages and disadvantages. In long-term studies it is important to choose a method that is permanent and easily read. If you would like to inquire about equipment used in methods discussed here, please email the OACU Training/Compliance Team. You may send questions to the general IACUC email account at iacuc@med.unc.edu.
Long Term Methods
Ear Notching
This method is frequently used in both mice and rats. There are several tools that may be purchased to achieve this. Most resemble a hole puncher and are very cheap. There are previously created maps that serve as a numbering system, or the researcher may create a map.
Advantages
- Ear notching can be done quickly causing very little pain or distress.
- The instruments are not costly and can be obtained easily.
Disadvantages
- This method cannot be applied until the ears are fully developed. This may be too late for those that use young rodents.
- This may not work with fractious strains. Rips or tears caused by fighting may leave the pattern indiscernible.
- Tools used to notch ears dull easily so must be replaced frequently.
Ear Tagging
Ear tags can be purchased with numbers and/or letters. Correct placement of the tag makes them fairly easy to read.
Advantages
- Ear tags are inexpensive and are fairly easy to apply.
- This method does not require the use of anesthesia.
- Tagging can be done quickly and does not seem to cause pain and only minor distress.
Disadvantages
- Tags can fall out if not applied properly. They can also be lost if ears are ripped or torn in strains that fight.
- Tags are relatively heavy for weanlings and may cause young mice to tilt their head even when the proper sized tag is applied. (Different sized tags are available for different species.)
- Some strains are prone to scratching the tagged area which can lead to infection, hematomas, and granulomas. In a very limited number of cases, a member of the DLAM veterinary team has seen ear tags stimulate tumor growth.
Microchipping
Microchips (electronic transponders) are safe and reliable.
Advantages
- Microchips may be applied without the use of anesthesia. Applying microchips seems to cause little or no pain.
- Even though chips may migrate to a different area, they are not lost so prove to be a reliable method.
- Animals can be identified without handling and removing them from the cage.
- Some microchips are designed to provide other information, such as core body temperature and heart rate.
Disadvantages
- The equipment used to read the chips is fairly expensive.
- Microchips cost five to ten dollars each. (Despite what manufacturers say, the chips can be reused. In order to reuse the chips they must be sterilized by ethylene oxide. The hospital will do this as a service for fee.)
- If not implanted properly there is a slight risk of infection.
- In a very limited number of cases, a member of the DLAM veterinary team has seen microchips stimulate tumor growth.
Microtattooing
This method seems to be growing in popularity. It is both permanent and fairly easy to apply.
Advantages
- This method works in all strains, even the more fractious.
- The cost is very reasonable after the initial expense.
- Tattoos can be applied to rodents of any age.
- The markings are easily read, especially when applied to the tail of light-colored rodents.
- When the tattoo is placed in the proper area, it is not necessary to handle the animal to read the tattoo.
- Tattooing causes only minor pain and distress and does not require the use of anesthesia.
Disadvantages
- The identifying marks may be a little difficult to read in young
pigmented mice. This improves as the mice age.
- The initial cost is rather expensive.
- There is a small chance of inducing infection if the tattoo is not applied correctly.
Toe Clipping
"Toe-clipping, as a method of identification of small rodents, should be used only when no other individual identification method is feasible and should be performed only on altricial neonates."1 The IACUC allows toe clips on a maximum of four toes and on no more than two per foot. Do not cut the hallux ("dew-claw" or "little toe") as this may decrease the rodent's grasping ability.
Advantages
- Toe clipping can be done at a very early age, postnatal day one. The ideal time is between postnatal day five and seven when the toe is large enough to work with, yet the bones are not calcified. (Toe clipping may not be performed after postnatal day ten.)
- The tissue can be used for genotyping.
- No anesthesia is needed. This seems to cause little or no pain when performed early enough. The young react to being removed from their mother, but do not react to the clipping of the toe.
Disadvantages
- The young show signs of distress when removed from their mother and siblings.
- This may cause a small amount of pain.
- The general public views this as a form of brutal mutilation.
- There is a small possibility of infection.
- A reduction in the number of toes may reduce the ability to grasp objects.
Short-term Methods
Hair Clipping
Trim patterns in fur. Keep a record or picture to identify rodents.
Advantages
Disadvantages
- This procedure is very temporary. The hair will grow back within ten days and the procedure must be repeated.
Permanent Markers and Fur Dyes
It is easy to apply marks or dyes to different body parts.
Advantages
- This method is non-invasive. It causes only minor stress due to restraint.
Disadvantages
- This method can be time-consuming because it must be redone soon.
- If you are working with nursing pups, the mothers will groom the neonates excessively and the markings may disappear overnight. This could result in a loss of identity.
1 Guide for the Care and Use of Laboratory Animals, National Research Council, 1996. National Academy of Sciences. 46 pp.