Lab Toys

How does cage enrichment affect rodents? 

Oct 1, 2009
Alla Katsnelson
© Chris Collins/CORBIS

In a room full of clinical veterinarians and animal-care technicians, everybody is about to play a game. Gina Savastano, a bubbly-voiced senior supervisor of facility operations at Merck, explains the rules: Each of the 15 or 20 teams will receive a species designation (monkey, mouse, rabbit, dog, pig, rat), a piece of posterboard, a magic marker, and an index card that describes a study using that species. The notes on the index cards describe experimental constraints—for example, as part of a study of the auditory system, the control condition requires complete silence. Teams will then have about 30 minutes to think up and draw some device or toy that improves the cage environment for the animals in a manner appropriate for each species—an activity that generally falls under the heading of “enrichment.” Afterwards, judges attending this 1-day Boston conference held last April on lab-animal enrichment (organized by Savastano) will pick the best ideas.

I join one of the rat groups, faced with an obesity study in which the constraint is “mandatory couch potato.” That means the enrichment can’t involve any activity—such as burrowing, climbing, or running—that would burn extra calories. My four partners—two lab-animal vets and two animal-facility managers from a well-known university one state over—brainstorm species-specific behaviors, but no devices are coming to mind. One jokes about creating something akin to an island paradise where rats can lounge, martini in paw. “That’s it!” says another, and the idea for a rat hammock (suggested trademark name: the Rammock) is born.

Rats don’t especially like to recline; the Rammock in that sense is named less for the benefit of the animals and more for the benefit of their human caretakers. The device, as the team envisioned it, is a suspended platform made from fine chain metal, attached with hooks on each end to the grates of a standard rat cage. The chain metal is flexible, but the weave is fine enough to prevent claws from getting stuck. The whole thing can go through the autoclave machine with the rest of the cage for routine cleaning. Rats could climb or perch on top, and enjoy being suspended above the rest of the cage, or use the space underneath to hide, as they like to do.

In the judging ceremony later that afternoon, the Rammock is awarded honorable mention in the rodent category. First prize, though, goes to a mouse enrichment system called “Stepped Up,” which consists of small pieces of polycarbonate arranged into steps that serve as a hanging and climbing gym within the small cage space.

Would a rat like the Rammock? Would a mouse like a Stepped Up system? Probably—after all, what’s not to like? But whether researchers should be adding such toys into rodent cages is a much different question.

The general reasoning behind enrichment is that it will improve the welfare of animals in captivity. There’s been little systematic study, though, of how specific enrichment practices affect lab animals—arguably less for rodents (the vast majority of animals used in research) than for other species, such as primates and dogs, for which some basic enrichment practices have generally been required by law in most countries. The European Union is debating changes to a directive on laboratory animal care that would mandate specific rodent enrichments; so far there are no such stipulations in the United States. “Mice and rats were left in the dark,” says Michele Cunneen, founder and president of Animal Research Consulting, a Massachusetts-based company that sets up animal research facilities primarily for start-up biotech companies.

That’s starting to change, as empirical data for rodents begin to add up, and the lab-animal community talks more about incorporating enrichment, like at conferences such as the one I attended. But the data, it seems, aren’t coming fast enough. While most facilities nowadays are embracing enrichment practices for rodents, the consensus for what constitutes good practice, or even what constitutes enrichment at all, has remained elusive.

The data, it seems, aren't coming fast enough.

What’s also unclear is how such variability in enrichment practices between labs might affect scientific data. Researchers generally agree that for most types of experiments, basic enrichment practices—adding nesting material to a mouse cage, for example—won’t change the data. Such steps may even improve experimental outcomes, proponents say, by decreasing animals’ stress levels and creating a more natural environment. But some actions—even ones as seemingly innocuous as adding cotton balls, which mice can use in building nests—may have unexpected effects, notes Linda Toth, editor-in-chief of the Journal of the American Association for Laboratory Animal Science . (A paper in the journal’s July issue identified a boost in toxicity-induced gene expression in mice whose cages were enriched with cotton balls, presumably due to dioxin, often present in bleached materials.1 ) And at the very least, she adds, “variation in enrichment practices may increase variation in results across individual animals, groups of animals, or labs.”

Other scientists who study enrichment argue that there are so many differences in animal house set-ups in everything from light to noise level, and so many uncontrolled irregularities—monthly fire alarms, for example, or changes in animal-care staff—that whether a rat gets a cardboard tube to play with or not is a mere drop in the bucket of already extant, and unavoidable, variability. “I don’t think [enrichment] adds any greater variability than any other things that are different at different institutions,” says Joseph Garner, professor of animal behavior and well-being at Purdue University in Indiana. “Variability exists, so let’s embrace it.”

In the 1940s, the famed neuropsychologist Donald Hebb decided to bring home one of his experimental rats, letting it run free in his house and play with his children. The increased variety in the animal’s environment compared to a small bare cage, he found, improved its ability to learn. Psychologists since then have examined the effect of environment on cognitive processes such as learning, fear and addiction. In the 1980s, zoos became the first to change housing conditions in order to improve animal welfare and promote natural behavior. Gradually, as the research community has become more cognizant of animal welfare issues, the concept began to trickle over to lab animals.

Roughly two decades ago, rodents were uniformly housed in a shoebox-sized cage containing bedding, food and water, with one animal per cage. Despite early concerns that tweaking this “standard” environment might wreak havoc on experimental models, today this tweaking has become close to routine. Chris Sherwin, an animal behavior and welfare scientist at the University of Bristol in the UK, estimates that 70–80% of universities in his country provide some level of enrichment to rodents. In Denmark, the law requires rodents to be provided with nesting material, a shelter and a gnaw stick, and the Netherlands requires that enrichment specialists oversee practices at different institutions.

A typical mouse cage used at Novo Nordisk after the company redesigned its animal facilities. The larger cages contain paper nesting material, a gnaw stick, and a hiding place; weekly food treats added to the bedding encourage foraging.
Courtesy of Novo Nordisk

The United States is in the process of revising its own guidelines for lab-animal welfare, put out by the National Academy of Sciences, and there has been talk the guidelines may include a requirement for enrichment in the new version, due out next year. Most institutions already do something. “If you don’t at least have some kind of nesting material for rodents, you’re probably not providing the state of the art,” notes Cunneen. All of the facilities she sets up for her clients get enrichment by default—group housing, shredded paper for nesting material, and often a small cardboard hut, a piece of PVC pipe, or toilet paper roll for the mouse to hide. (There’s always pushback from clients initially. “You’ve got to start with the ‘Oh my god it’s going to ruin my model’ discussion,” she says.)

But what should be the “default” enrichment? Both mice and rats are social creatures, for example, and many researchers say the most important improvement to a rodent’s cage involves simply housing animals in groups rather than singly. Nest building seems to be an equally crucial need for mice. In line with this, most animal facilities group-house mice and provide nesting material, unless there is a good reason not to. But these changes are just a start, scientists say. So mice like to build nests—what material should they receive? They like to hide and gnaw—what chew toys are most satisfying? “If you’re going to design a good housing system, you need to be able to view the world in the way the animal is viewing it,” says Garner. “We just haven’t done that for mice.”

Variation in enrichment practices may increase variation in results across individual animals, groups of animals, or labs. —Linda Toth

In a study published earlier this year, Garner and his colleagues set up interconnected cages maintained at different temperatures to assess what temperature animals chose to be in.2 Most animal facilities are kept at 20–24?C, but animals who weren’t active gravitated towards the space that was about 10?warmer. The lower temperature is most likely a stressor, Garner says, noting that at 20?C, the animals show changes in immune function.

Other groups are looking at the physiological effects of enrichments. Mice in the wild routinely combine their litters, raising their pups communally—something that caged life generally prevents. Kathleen Heiderstadt, a veterinarian at Penn State University’s Animal Resource Program, is starting to look at whether mice raised in separate litters show behavioral and neurobiological changes in adulthood, versus animals raised communally. (Earlier work showed changes both in behavior and in physiological markers of neuroplasticity in mice raised in separate litters versus in combined litters.3 )

How, then, to give animals what they physiologically seem to need? Routinely setting animal house temperatures higher won’t work, since needs vary widely depending on the time and the mouse’s activity; lactating animals, for example, like a nice chill, at 15?C. One obvious way to let rodents customize their temperature, Garner says, is to provide nesting material. But mice in the lab rarely build nests as elaborate as what they build in the wild, and his group decided to see if one commonly supplied nesting material, consisting of compressed cotton squares, was to blame. In a study published last year, the group found that mice given cotton squares build the least naturalistic nests, relative to when they receive strips of paper.4 The cotton squares are easy to package, thus are excellent from a human engineering perspective, he notes, “but nobody had ever asked whether it worked” for the animals. In fact, mice don’t seem to be able to shred the material. “The stuff requires a stroke of genius from the mice before they can chew it up,” he says.

In fact, research often shows commercially available enrichment products are not actually of use to the animal. Another study published in 2005 by Vera Baumans at the University of Utrecht in the Netherlands and her colleagues compared two commonly used huts (which rodents use as hiding places inside the cage), one made from cardboard and the other from plastic.5 In a preference test, the mice spent much less time in the cage containing the plastic hut than the cage with the cardboard hut. Commercially available enrichment products as a rule simply aren’t well tested, Baumans says. Some wouldn’t benefit the species they’re geared towards at all. “When I see toys for mice, [such as] little balls, I always ask, what is the idea? In the wild, mice don’t play with balls!”

Indeed, enrichment doesn’t always have the intended effects—adding some types of enrichment, such as nesting material, can boost aggression in some strains of male mice,6 but not in others.7 And though some studies show that enrichment decreases rodents’ stress,8 other studies suggest some measures of stress increase in enriched environments.9 The effects of different types of enrichment are often strain-specific and gender-specific, and are even sensitive to the statistical method used in any given study, notes Baumans. It’s just very difficult to predict what effect an enrichment practice will have, says Toth, who’s also a veterinarian and associate dean of research and faculty affairs at Southern Illinois University. “In some cases it should not matter, but in others it could be crucial.”

Type of enrichment Finding Strain Reference
Several cardboard, paper, or plastic objects, changed every 2 days In a Huntington's Disease transgenic mouse model, enriched mice showed delayed onset of motor disorders and no loss of cerebral volume. Transgenic HD mice on a CBA background strain Nature 404: 721-22, 2000
Larger cage, running wheel, shelter, 4-5 toys changed weekly Mice addicted to cocaine ceased to display addiction-related behaviors and showed decreased activity in brain areas associated with addiction after 30 days in an enriched environment. C57/BL6 PNAS 105: 16829-30, 2008
Double cage size, nesting material, shelter, cardboard tubes, chew blocks, running wheel Mice in enriched environments with free access to anxiolytics took less meds than those in standard cages. C57BL/6J Lab Anim 40: 392-99, 2006
Larger cages, ladders, tunnels, running wheels, changed weekly Experimental immunotherapy for B-cell lymphoma decreased tumor size and spurred higher antibody counts in mice housed in enriched cages. C3H/eB J Immunother 30:517-22, 2007
Nesting material, PVC tube, exercise wheel, two nylon rings Male mouse pups born into enriched cages had higher anxiety, but weighed more and survived better than those in standard cages. C57BL/6Tac Lab Anim 38:24-34, 2009
Shelter, two tissues and paper strips for nesting material, PVC tube, chew sticks Enriched mice showed increased acute stress response (upon handling), but lower response to chronic stress (measured via long-term cortisol levels). C57BL/6J Lab Anim 41:161-73, 2007
Different types of nesting materials and shelters In an aggressive mouse strain, nesting material boosted fighting; adding shelters to the cage prevented fighting. NIH/s Comp Med 56:202-8, 2006
Interconnected cages containing either cardboard shelter or plastic shelter Mice spent more time in the cage with the cardboard shelter. BALB/cANCrlBr, C3H/HeNHsd, C57BL/6JIco Lab Anim 39:58-67, 2005
Running wheels placed into cages containing 4-5 mice The wheel increased aggression in group-housed male mice, but decreased the frequency of abnormal stereotypic behaviors. CD-1 Applied Anim Behav Sci 115: 90-103, 2008
Cages smaller and larger than normal (which in the US is 96.8 cm2 of floor space per 25-g mouse), varying types of enrichment Female mice of three strains showed less aggression and fewer stereotypic behaviors in enriched environments, but not in larger cages, suggesting cage complexity is more important than size. C57BL/6NCrl, BALB/cAnNCrl and Crl:CD1 JALAS 46:93, 2007 (Abstract PS40)
Communal nest (in which three female mice jointly care for their litters) Communally reared mice showed more social interaction and higher levels of growth factors in the brain in adulthood. CD-1 Biol Psychiatry 60:690-96, 2006

Walk into any institution, and the type of enrichment present will vary. Baumans’s institution provides rodents with some shredded paper to use as nesting material, and a tube or shelter when possible. Heiderstadt’s facility includes nesting material for mice and group housing alone for rats. The animal house where Toth works gives mice cotton balls, compressed cotton squares, mouse houses, plastic weigh boats normally used in labs to weigh reagents (that an animal could fiddle with), and other elements unless a researcher specifically opts out. Other facilities might provide a paper cup, corn husk bedding, tubing, food treats, running wheels, various commercially available toys, and huts. At some places, group housing is considered enrichment enough.

Water baths control temperature in interconnected cages, allowing researchers to determine how warm the animals prefer their environment.
Courtesy of Joe Garner

For researchers who use behavioral measures to study topics such as addiction, learning and memory, depression, or recovery from nervous system injuries, enrichment is often an experimental paradigm, not a question of animal welfare. Animals in such studies don’t just get some tissue to tear up—they complete tasks in oversized cages, with as much complexity as possible: running wheels, different places to hide, and lots of toys rotated through regularly to provide novelty. Such extreme enrichment has repeatedly been shown to affect experimental results, says Steven Dworkin, a professor of psychology studying the neuropharmacology of addiction at Western Illinois University, suggesting that even smaller changes to enrich cage environments may be affecting data as well. While he believes enrichment to improve animal welfare should be done, it must be done more carefully. “You can’t just throw [something] in [the cage] and not be concerned. We have to know empirically what it’s doing.”

Rats at Novo Nordisk are housed in larger cages that contain a hiding space and a shelf for climbing; like mice, rats are also given paper nesting material and a gnaw stick, and weekly food treats.
Courtesy of Novo Nordisk

Cunneen recalls one experiment in which a client’s mouse model required more than double the dose of an infecting agent after the animals went from an old-school, singly-housed basic environment to group housing, nesting material, and a mouse hut. “By making them less stressed or [more] able to modify their surroundings, the mice were tougher to infect,” she speculates. A study published by Wyeth researchers last year found that one type of mouse house lessened the severity of osteoarthritis in a mouse model, suggesting that the house helped increase the animals’ activity level, spurring faster recovery.10 Another 2007 study showed that an experimental vaccine for B-cell lymphoma improved survival in enriched animals by 50%, but had no effect in nonenriched animals—presumably because the latter were too immunosuppressed to be helped by the vaccine.11

But Douglas Wahlsten, a behavioral geneticist who uses environmental enrichment to study the effects of alcohol at the University of North Carolina, Greensboro, has found repeatedly that enrichment had little effect on the impact of alcohol on motor coordination. “My opinion is, [enrichment] does make a difference, but not a very large one,” he says. “It’s sort of like if you went to the gym and worked out. You’re going to get stronger, your coordination is going to get better. But the other people [who don’t work out]—are they abnormal?”

When I see toys for mice, [such as] little balls, I always ask, what is the idea? In the wild, mice don't play with balls! —Vera Baumans

Ultimately, scientists can’t determine the effect of any environmental change without empirically checking, says Baumans. Much of what’s known about the specific impact of different types of enrichment on experiments is piecemeal and remains unpublished, she says, coming from small pilot studies done by researchers who want to check that enrichment isn’t affecting their model, or anecdotally related between animal-care staff. The fact that most enrichment data aren’t making it into the published literature is worrisome, says Toth, who argues that housing conditions should routinely be described in papers’ methods sections. What’s needed, Baumans suggests, is a major database that combines both published data and anecdotal observations, and tracks all the factors in one place. “When you have done that, we can look and say, well, with this strain, with this method—that [type of enrichment] was very helpful.”

Few biopharma companies are willing to discuss their enrichment practices, citing fears of animal rights activists. Even though an employee of Merck organized the Boston enrichment conference held last April, the company declined to comment for this article about enrichment research the company is conducting or enrichment practices it uses.

One company, though, makes a point of talking publicly about enrichment. A little over 10 years ago, Novo Nordisk, based in Denmark, began an internal review of its animal housing practices. They couldn’t find any clear guidance, so they assembled some experts—including representatives of the Danish Animal Welfare Organization—and asked them to define the most essential needs for each species the company worked with. “The number one need for most species is that you should not house them in isolation,” says Jan Ottesen, director and head of the company’s animal unit. “Which is not that difficult to do, but we hadn’t given it specific thought at the time.” Cage size was also a top priority.

Over the next few years, they worked with the animal-care staff to implement the recommendations. The end result provides rodents with cages about three times the size of those usually found in the United States, a shelf for climbing and/or a hiding place, a gnawing stick, and weekly food treats spread into the bedding to encourage foraging. Do the company’s new standards make experiments more reliable? “We believe they do, but it’s difficult to prove,” says Ottesen. “We [also] believe that the animals [are] better off—that, too, is very difficult to prove.”

It’s not from lack of trying. The company collaborated with the Centre for Applied Laboratory Animal Research at the University of Copenhagen to study the effects of their animal-care overhaul. Their ultra-enriched housing conditions did no harm—that is, none of the animal models were less representative of the disease state they approximated than they had been before—and overall, there was no jump in variability in the data obtained from enriched animals. But neither were there any measurable benefits—there was the same amount of variability, for instance. A rat will choose extra bedding and a PVC tube over a nonenriched environment, but shows no preference between basic enrichments and the extra-enriched environment Novo Nordisk provides.

Ottesen says the changes the company made to animal housing cost “several million [dollars] over several years”—an investment, to be sure, but peanuts compared to the cost to launch a single drug. Despite the expense, though, he says, the program has proven its worth even if only in boosting the company’s image as a moral entity. And even though the benefit of the extra enrichment has not been shown scientifically, he says, it was still the right move. “Most people have this mantra that it needs to be scientifically proven before you implement things,” says Ottesen. “On the other hand, it hasn’t been scientifically proven in the beginning that mice prefer to be in a small cage. So now it has to be proven they prefer to be in a large cage?”

1. S.A. Tischkau and M. Mukai, “Activation of Aryl hydrocarbon receptor signaling by cotton balls used for environmental enrichment,” JALAS , 48:357–62, 2009.
2. B.N. Gakill et al., “Some like it hot: mouse temperature preferences in laboratory housing,” Applied Anim Behav Sci , 116:279–85, 2009.
3. I. Branchi, “The mouse communal nest: investigating the epigenetic influences of the early social environment on brain and behavior development,” Neurosci Biobehav Rev , 33:551–59, 2009.
4. S.E. Hess et al., “Home improvement: C57BL/6J mice given more naturalistic nesting material build better nests,” J Am Assoc Lab Anim Sci , 47:25–31, 2008.
5. P.L.P. Van Loo et al., “Assessment of the use of two commercially available environmental enrichments by laboratory mice by preference testing,” Lab Anim , 39:58–67, 2005.
6. E.K. Kaliste et al., “Environmental modification and agonistic behavior in NIH/S male mice: Nesting material enhances fighting but shelters prevent it,” Comp Med , 56:202–8, 2006.
7. V. Marashi et al., “Effects of environmental enrichment on males of a docile inbred strain of mice,” Physiol Behav , 82:765–76, 2004.
8. J.P. Garner, “Stereotypies and other abnormal repetitive behaviors: potential impact on validity, reliability, and replicability of scientific outcomes,” ILAR J, 46:106–17, 2005.
9. M.K. Meijer et al., “Influence of environmental enrichment and handling on the acute stress response in individually housed mice,” Lab Anim , 41:161–73, 2007.
10. A. Salvarrey-Strati et al., “The influence of enrichment devices on development of osteoarthritis in a surgically induced murine model,” ILAR e-Journal , 49:E23–E30, 2008.
11. A. Benaroya-Milshtein et al., “Environmental enrichment augments the efficacy of idiotype vaccination for B-cell lymphoma,” J Immunother 30:517–22, 2007.