Environmental Enrichment

BU IACUC Revised March 2021, Approved April 2021

Boston University is committed to observing Federal policies and regulations and AAALAC International standards for the humane care and use of animals.1

This policy provides guidelines for environmental enrichment (EE) for particular species.

Humane considerations and current policies require that research animals, whenever possible, must have the opportunity to interact with conspecifics and to benefit from EE as a means to express natural behaviors.

Many laboratory species housed at BU are social animals and are socially housed whenever possible. If they cannot be socially housed, they are housed in the same room as conspecifics with sensory access (visual, auditory, olfactory) with the exception of rodents in IVC caging.

Enrichment devices must fit in the cages without crowding the animals or obscuring assessments for health and welfare, and must be either discarded at cage change or sanitized before reuse. Animal care staff are responsible for the administration of the enrichment program including replacement of devices that are heavily soiled, malfunctioning, or in poor working condition.

Note that many prey species (e.g. rabbits and rodents) are neophobic, so rotation of enrichment items such that unfamiliar objects are provided may be stressful to them while it is desirable in other species such as ferrets, swine, cats, and NHPs to prevent boredom.


  • To define environmental enrichment for species housed at BU, including various ways to “facilitate the expression of species-typical behavior and promoting psychological well-being through physical exercise, manipulative activities or cognitive challenges.”1
  • To outline how exceptions to this policy should be considered by the IACUC or veterinarians.


    • Pair-Housing or Group-Housing
      • Compatible animals will be pair- housed or group-housed if temperament allows. Insufficient space within the primary enclosure is not an acceptable reason for not pair- or group-housing; larger caging should be provided for that purpose.
    • Individual Housing
      • If scientifically justified in an approved IACUC protocol animals may be individually housed. Unless otherwise indicated in the approved protocol, individually housed animals will be provided housing which allows auditory, olfactory and sometimes visual contact with conspecifics.
    • Limiting or Excluding Enrichment
      • If a protocol exception to limit or exclude enrichment is to be included in an IACUC submission, the Principal Investigator (PI) is required to consult with veterinary staff during protocol development to discuss the special circumstances that might limit participation in the enrichment program.
      • The PI will need to include the requirement and scientific justification for single housing in the IACUC protocol, specify the approximate duration of single housing and/or the requested exemptions from any EE strategies included in this policy or associated SOP. If no exemptions are approved within the protocol, then the policy and associated SOP will be followed.
      • Once the protocol is approved, the PI must inform BU ASC of the approved exceptions to this policy.

    Medical or Compatibility Conditions

    • Certain medical or compatibility conditions may require that cats be individually housed or that other elements of the enrichment program be changed. These determinations will be made by a veterinarian and documented in the animal’s record.
    • Veterinary exemptions to this policy do not require IACUC approval.

    Types of Environmental Enrichment

    There are multiple ways to categorize types of environmental enrichment and some overlap. The list below is not exhaustive, but covers the general categories of enrichment provided to animals in BU vivaria.

    • Positive Human Interaction: Frequent and gentle handling and daily routines have a positive, calming effect on some species. Interaction with handlers such as grooming and play may also be provided. Positive reinforcement training is also an example of positive human interaction.
    • Conspecific socialization: Social species are cohoused when study needs, signalment and temperament permit. Minimally, they will be housed with sensory access to conspecifics with the exception of rodent species in IVC caging.
    • Manipulanda: Manipulable objects and toys are provided wherever possible. This can also include nesting material and gnawing blocks for rodents.
    • Edible: Food treats and foraging opportunities are provided to some species as part of or in addition to their balanced daily ration as permitted by the study protocol.
    • Occupational: This type of enrichment allows animals to express species-specific behaviors such as foraging or nesting. Examples include nesting material or gnawing blocks for rodents and forage boards or puzzle feeders for other species.
    • Structural: Structural enrichment encompasses features of the environment that promote species-specific behaviors; such as huts or tunnels for rodents and rabbits, perches for NHPs and cats, and hammocks for ferrets.
    • Cognitive: In addition to puzzles and rotating edible enrichment and manipulanda for larger species, TV and radio may also be provided. Positive human interaction, such as training or handling, and/or socialization with conspecifics may also provide cognitive enrichment.

      Edible enrichment (all species): If a PI has received approval in a protocol that the animals on study will receive edible treats for enrichment, these may be given by the PI or research staff as a form of positive interaction. The administration of these edible treats should be clearly documented in the animal’s records.

      Species-Specific: Mammals


      • Behaviors: Include climbing, perching, self-grooming, mutual grooming, vocalization, exploration of physical and olfactory environment, hunting and capture behaviors, scent-marking, physical marking with claws, and hiding.2
      • Observations of Singly Housed Cats
        • Singly housed cats are presumed to need more environmental enrichment than pair- or group-housed cats and may be provided with extra cage enhancements unless an exception to limit or exclude EE is justified in the IACUC approved protocol for scientific reasons. It is expected that if social housing is limited or excluded, then enhanced enrichment (including direct contact with caregivers) will be provided as a substitute for lost social contact.
        • Singly housed cats will be observed daily and will have their well-being evaluated at least monthly by a veterinarian or adequately trained veterinary staff. The assessment and recommendations for continued single housing will be reviewed and approved by the Attending Veterinarian or designee.
        • Group Activities for Compatible Cats
          • The best way to provide EE for cats is to allow them time out of their cages with access to EE as described below. Regularly scheduled free time (group activities) on the floor of the animal room allows cats to exercise, to manipulate objects, and to socialize.
          • Cats should not be released for group activities and to interact with other cats during the quarantine period, if applicable. Novelty of enrichment through rotation or replacement of items should be a consideration.
        • Examples of EE
          • Scratching poles or boards: These should be provided to allow scratching and stretching.
          • Perches: These should be provided in the animal room and may consist of free-standing high perch posts, shelves installed on the walls, or other devices.
          • Toys: These should be replaced when worn or broken.
          • Brushing the cat is usually a pleasurable activity for the animal and should be done regularly to accustom the cat to the handler. This is also a good time to perform a routine physical examination.


      • Behaviors: Include climbing, jumping, hiding, huddling with and grooming conspecifics, gnawing, and dust bathing.2
      • Examples of EE
        • Enrichment that allows for opportunities for foraging, hiding, exploration, gnawing, and exercise such as running and climbing are optimal.
        • Hay and PVC tunnels are frequently provided.
        • Chinchillas should be given the opportunity for dust baths at least three times weekly. This should be logged in the animal’s record.


      • Behaviors: Include tunneling, digging, rough-and-tumble play with conspecifics, climbing, exploration of environment, chewing/biting, sleeping, interaction with balls/manipulanda.2
      • Examples of EE
        • Ferret balls, Nyla bones or other chew toys, food treats, PVC pipes for tunneling.

      Guinea Pigs

      • Behaviors: Include foraging, exploration, hiding, huddling, burrowing, gnawing, and vocalizing. 2
      • Examples of EE
        • Enrichment which allows for opportunities for foraging, hiding, exploration, and gnawing are optimal.
        • Nesting materials: Guinea pigs don’t build nests but may burrow and hide in hay, straw, shredded paper, and similar substrates.7
        • Toys: Suitable options include ferret roll-about balls filled with foraging material, balls with bells inside, and bedding bags.
        • Sources of cover: PVC pipes, hide boxes, red guinea pig huts.8
        • Foraging material: Hay, corn husks.

      Naked Mole Rats

      • Behaviors: tunneling, colony housing, gnawing, nesting.
      • Examples of EE
        • Housing with tunnels and multiple dedicated activity chambers.
        • Colony housing for demonstration of complex social behaviors.
        • Fresh produce, nesting material or bedding, gnawing items.

      Nonhuman Primates

      • Behaviors: With some variation among nonhuman primate species but include foraging, grooming, perching, climbing, brachiation, exploration of physical environment, vocal and visual interaction with conspecifics, and exploration of manipulanda and devices that present cognitive challenges.2
      • Refer to BUASC Environmental Enrichment Plan for Nonhuman Primates for more detail.
      • Examples of EE
        • Cognitive, Social:
          • Daily interactions with animal husbandry staff, veterinary care staff, and other familiar and knowledgeable persons such as research staff. Properly trained staff are encouraged to socialize with NHPs in non-contact activities, in a manner consistent with personnel safety precautions.
          • Television, radio, and/or nature sounds are provided for animals.
        • Edible, Structural, Manipulanda:
          • Stainless steel perch
          • Mirrors
          • Foraging behavior opportunities include daily provision of varied fruits, vegetables, seeds, and nuts.
          • Hard plastic toys, metal triangles, task-oriented feeding puzzles, Kong toys, soft toys, destructible items such as glove boxes, etc. For some studies, the type of manipulanda provided may require review and discussion with investigative staff to avoid interference with research activities.
          • Swings may be provided in some cages.


      • Behaviors: Most commonly include lie alert, doze, groom, sleep, eat, locomotion, nest building, and gnawing. Rabbits are typically timid and nonaggressive but will display defensive behaviors when frightened, including charging at the front of the cage, thumping, and biting.
      • Social housing: although rabbits are a social species, co-housing adult rabbits may result in repeated fighting and injury, especially males not previously known to each other.
      • Examples of EE
        • Food and Treats
          • Food enrichment and treats should be scheduled into the regularly provided feeding regimen and given daily or several times a week. Treats providing roughage should be given unless contraindicated by the experimental protocol.
          • Because rabbits are herbivores and have a large cecum, the regular feeding of items such as grass hay (cubes or loose), carrots, leafy greens, and other vegetables is beneficial to their digestion. In addition, since rabbit incisors as well as molars grow continuously, chewing allows teeth to wear down.
        • Suitable enrichment devices include large plastic balls which may be pushed around the cage, bells and bar bells, and chew sticks which may be either edible or made of wood. Hay can be provided for nest building and foraging.
        • Novelty of enrichment through rotation or replacement of items should be a consideration; however, changing animals’ environment too frequently may be stressful.1
        • Gentle handling and supervised exercise outside of the primary enclosure in a pen or designated safe area may be beneficial for the animal.

      Small Laboratory Rodents (mice, rats, hamsters, gerbils)

      • Behaviors: Degree of expression varies among rodent species but include nest-building, burrowing, gnawing/chewing, rearing, grooming, locomotion, climbing, exploratory behavior, and thigmotaxis.2
      • For specific guidance on co-housing male mice, please refer to Male Mouse Housing Policy.
      • Examples of EE
        • Wooden chew sticks, Nyla bones, nesting material, Plexiglas tubes, cardboard huts or tubes.


      • Behaviors: Include rooting, vocalizing, exploration, locomotor behavior, foraging, huddling, nose touching, resting, body scratching.2,9
      • Examples of EE
        • Supervised exercise: Letting individual animals outside their runs may also be implemented as a form of enrichment.
        • Rooting materials: Hay, straw, shavings, and similar substances.
        • Toys: Replace when worn or broken. Suitable options include balls, chains, hoses.2
        • Grooming/body scratching material: Includes brushes.
        • Foraging material: Includes palatable food crumbles or pieces strewn in hay, straw, or shavings.
        • Bedding: Includes sanitizable mats for resting and sleeping.
        • Interaction with caregivers.

                            Species-Specific: Other Vertebrates

                            Amphibians and Reptiles

                            • Behavior: Natural behaviors will vary considerably among different species of amphibians and reptiles, each according to its evolution and niche in its native habitat. Opportunities to exercise these behaviors should be provided in tank or housing environments, including the presence or absence of conspecifics and enrichment devices and structures within the enclosure.
                              • Amphibians: Although not all amphibians are social animals, many will benefit from cohabitation. Additionally, swimming, hiding, exploring, foraging, climbing, and burrowing are common behaviors of many amphibious species.
                              • Reptiles: Behavior varies considerably among reptilian species, with some species living solitary and some in groups. Additionally, exploring, climbing, hiding, foraging, basking, and burrowing are common behaviors of many reptiles.
                            • Examples of EE
                              • Enrichment will vary by species, but, in particular, enrichment that allows for opportunities for enhanced foraging, hiding, and exploration are optimal.
                              • For reptiles, basking areas with appropriate UV spectrum lighting are required both for maintenance of good health and exhibiting species-specific behavior.


                            • Behavior: Natural behaviors will vary considerably among different species of birds, each according to its evolution and niche in its natural habitat. Opportunities to exercise these behaviors should be provided in cage environment, including presence of conspecifics, enrichment devices or structures within the cage, and manipulable toys or objects.
                              • Interaction with conspecifics, exploring, perching, foraging, scratching, pecking, singing, nest-building, hiding, preening, dust- or water-bathing, interacting with manipulanda, and flying.2
                              • Although not all birds are social animals, many will benefit from cohabitation.
                            • Examples of EE
                              • Perches: These should be provided in the housing enclosure.
                              • Nest boxes or sites: These should be provided in the housing enclosure or adjacent to it in a manner accessible by the birds.
                              • Toys.
                              • Sources of cover for terrestrial birds.
                              • Partial to fully solid flooring with bedding to encourage scratching and foraging.
                              • Foraging opportunities: Provisions of food items among substrate on caging floor, bowl/trough, or in a grain block.


                                • Behavior: Natural behaviors will vary considerably among different species of fish, each according to its evolution and niche in its native habitat. Opportunities to exercise these behaviors should be provided in the tank environment, including the presence or absence of conspecifics and enrichment devices or structures within the tank.
                                  • Many fish are shoaling or schooling species, swimming in groups of individuals. As such, these species should be group- or pair-housed, unless otherwise approved by the IACUC.
                                  • In the wild, many fish are found in areas with submerged or overhanging vegetation and prefer to spawn in sites associated with aquatic vegetation. This environment also provides refuge and satisfies their preference for structure.
                                  • Aggression will typically increase with smaller population densities. However, overcrowding must also be avoided. Thus, established population densities of fish tanks must be closely adhered to according to each species’ need.
                                • Examples of EE
                                  • If species-appropriate, the provision of artificial plants or other structures that imitate natural habitats should be strongly considered, particularly in tanks with low numbers of fish. Such enrichment devices provide the animal a choice within its environment and refuge from real or perceived threats.
                                  • Environmental enrichment options for fish include: Barriers, hides, shelters, aquatic plants (live or plastic), plastic piping, and aquarium substrate (e.g., sand, silt, marbles, gravel, etc.)2
                                  • If species-appropriate, live feed provides EE.


                                1. The Guide for the Care and Use of Laboratory Animals. 2011. NRC ILAR. P. 52-55. Environmental Enrichment.
                                2. Laboratory Animal Medicine. 2015. Fox et al. Academic Press.
                                3. ILAR Journal 2005 Volume 46(2) Enrichment Strategies for Laboratory Animals. http://ilarjournal.oxfordjournals.org/content/46/2.toc
                                4. Thomas L. Wolfe. Introduction. Environmental Enrichment. http://ilarjournal.oxfordjournals.org/content/46/2/79.full
                                5. Enrichment Strategies for Laboratory Animals from the Viewpoint of Clinical Veterinary Behavioral Medicine: Emphasis on Cats and Dogs Karen L. Overall and Donna Dyer. http://ilarjournal.oxfordjournals.org/content/46/2/202.full
                                6. P. 205. Table 1. Enrichment recommendations for cats.  http://ilarjournal.oxfordjournals.org/content/46/2/202/T1.expansion.html
                                7. The Laboratory Rabbit, Guinea Pig, Hamster, and Other Rodents. 2012. Suckow et al. Academic Press. P. 598-599.
                                8. The Laboratory Rabbit, Guinea Pig, Hamster, and Other Rodents. 2012. Suckow et al. Academic Press. P. 607-608.
                                9. Swine in the Laboratory, 3rd edition. 2016. Swindle MM and Smith AC. CRC Press. P. 10-12.

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