The natural behavior of mice includes burrowing, foraging, exploring, gnawing, and nest building. These behaviors are essential to the well-being of the animal, and if prevented from performing these activities mice may engage in non-productive repetitive behaviors1,2. Prevention from responding to their natural inclinations could result in undue stress for the animal, possibly impacting their health and increasing stress-related hormone levels. Due to their importance in maintaining the health and well-being of laboratory rodents, many Institutional Animal Care and Use Committees (IACUCs) require provision of one or more enrichment items unless an exception is approved for scientific reasons, or veterinary staff advise against provision for clinical reasons. However, researchers often have the option to choose enrichment items for cages.
This Taconic Insight will offer a brief review of the importance of enrichment and what forms are most common for laboratory mice and rats.
What is Enrichment?
Laboratory mice and rats that are kept in cages without proper sensory and motor stimulation cannot exhibit species-specific behaviors such as foraging, exploring, hiding, and building. It is for this reason that environmental enrichment products were created — to encourage the inherent behaviors of mice that do not have access to the natural environment.
According to the Guide for the Care and Use of Laboratory Animals, there are key features that contribute to environmental enrichment3,4:
Group housing of compatible animals
Providing animals with sensory and motor stimulation
Promoting species-specific behaviors
Providing animals with a means for control over their environment (e.g., nest-building materials, hiding places)
Novel items (e.g., toys, special food treats)
Opportunity for exercise (e.g., running wheels, climbing structures)
Due to the constraints of cage size, toys and exercise equipment may not be available to mice. However, the addition of cardboard houses, plastic tubes, and/or shredded paper serve as environmental enrichment and allow for natural hiding, burrowing and nesting behavior. Mice are usually housed in groups of compatible individuals after weaning to encourage social interactions and maintain their well-being.
Products Available for Environmental Enrichment
Hiding places in the form of disposable igloos or homes are commonly used forms of cage enrichment. Mice are naturally burrowing creatures, and providing them with areas where they can hide from view is essential to their well-being. This is especially relevant when co-housed with other mice as well as when housed in clear-sided cages. One example product are Shepherd Shacks which provide a hiding place, climbing structure, and can also be chewed and shredded. An important consideration of using huts/igloos is that these can increase aggression in some groups of animals due to resource hoarding, especially in lines prone to aggression.
Cardboard tunnels offer the opportunity for hiding, gnawing and shredding, while plastic tunnels are autoclavable, reusable and mainly used for hiding/sheltering and play.
Toys and Equipment
Depending on the size of the cage, toys such as spinning wheels or swings may be added to stimulate movement and exercise. Some spinning wheels may require cages large enough to accommodate the structure; but once in place, allow for exercise and entertainment for the group of mice.
Bedding and Nesting Material
With regards to bedding, most standard cages contain wood, paper, or corn cob-based bedding. It is important to remember, however, that many conventional types of bedding material do not allow rodents to express nesting behavior when used alone since they cannot be used to build nests. Bedding types such as Alpha-Dri Plus contain mini paper twists which provide nesting material that is beneficial to the mice. The Bed-r'Nest, which comes pre-packaged in 4 grams or 8 grams sizes, is designed to resemble thin strands or grass similar to those found in nature. It is easy to dispense, dust-free, and provides adequate thermoregulation once shredded apart. Compressed cotton nesting squares are another commonly used form of nesting material.
Additionally, wooden chew sticks or nylon bones can be used as a beneficial form of enrichment for laboratory mice. Gnawing is not just natural behavior, but rather an essential process for dulling the animal's teeth. If they are not worn down, rodent teeth will eventually become overgrown. Therefore, items such as chew sticks, in addition to other sources of enrichment present, are essential for maintaining proper health of all laboratory rodents.
1. Bailoo, J. D.; Murphy, E.; Boada-Saña, M.; Varholick, J. A.; Hintze, S.; Baussière, C.; Hahn, K. C.; Göpfert, C.; Palme, R.; Voelkl, B.; et al. Effects of Cage Enrichment on Behavior, Welfare and Outcome Variability in Female Mice. Frontiers in Behavioral Neuroscience. 2018, p 232.
2. Clipperton-Allen, A. E.; Page, D. T. Decreased Aggression and Increased Repetitive Behavior in Pten Haploinsufficient Mice. Genes, Brain Behav. 2015, 14, 145-157.