Dennis J. Chmiel Jr. & Michael Noonan
As published in Laboratory Animals, 1996, 30, 97-101.
In an effort to identify suitable stimulus objects which could be placed into standard
laboratory cages in order to provide rats with a degree of environmental enrichment, the
preference of rats to spend time near fifteen diverse objects was measured in a
free-choice paradigm. Rats showed no preference for objects such as pipes and partitions
which we had reasoned might satisfy a wall-hugging tendency. They also showed no
preference for objects which we had reasoned to be potentially interesting as manipulanda.
The rats did show reliable preferences for spending time with some, but not all, chewable
objects. A block of wood predrilled with holes was the most attractive, and we cautiously
recommend that researchers consider providing laboratory rats with such an object to allow
them the opportunity to exercise a fundamental, species-typical behavior¾ chewing.
The welfare of animals caged as subjects in research laboratories has been given
considerable recent attention (cf. Murphy, Rowan & Smeby, 1991). It has been argued
that when animals are used in research, steps should be taken to maximize their well-being
within the limits imposed by experimental design (Guttman, Mench & Simmonds, 1989).
Objection has been raised to the common practice of housing animals in nearly-featureless,
box-shaped cages, because such conditions provide animals with little or no opportunity
for mental stimulation or for the exercise of species-specific behaviors (cf.
Wemelsfelder, 1984). Thus, there has been a growing interest in the testing of various
aspects of cage environments (e.g., Arnold & Estep, 1993; Blom et al, 1993; van
Rooijen, 1984), and the development of stimulus objects which can be placed into cages
with research subjects so that they are provided with an element of environmental
enrichment (e.g., Huls, Brooks & Bean-Knudsen, 1991).
To date, this effort has primarily focused on improving the conditions of primates
(e.g., Lambeth & Bloomsmith, 1992; Reinhardt & Smith, 1988; Ross & Everitt,
1988), both because of the focus of animal-welfare advocates on these animals and because
they are among the more intelligent and cerebrally more complex animals which are utilized
(cf. Bramblett, 1989). It is rodents, however, that make up the majority of the mammals
utilized as research subjects, and their welfare should also be addressed. Millions of
laboratory rats are used in research projects whose integrity would not be affected if the
housing conditions of the subjects were altered to allow for a degree of behavioral
In our experience, researchers are generally concerned with the welfare of their
research subjects, and are open to the possibility of taking steps to improve the housing
conditions of their animals (cf. Adams, 1981; Guttman, 1990). Given the large investment
which most scientific institutions have in their stocks of standard laboratory rat cages,
and the general utility which such cages provide, it is worth exploring whether conditions
can be improved within the framework of existing structures. At issue is whether objects
could be placed into standard laboratory rat cages to provide enrichment to the animals.
Such objects would have to be safe, economical, easily cleaned, and, most importantly,
suitable to the purpose. That is, it would need to be "species appropriate"
(Line, 1987) and engage the rat in one or more beneficial ways (e.g., by evoking
exploratory behavior, by evoking naturalistic behaviors, by providing mental stimulation).
Using the logic that, when given a choice, an animal will choose conditions which
optimize its comfort level (Dawkins, 1980), we tested our subjects individually in two
adjoining cages with various test objects confined to one of the two cages. Each animal
was free to move between the two cages so that it had the opportunity to choose to be
with, or away from, a given object. The amount of time that each rat spent with a given
stimulus was taken as its preference for that object; that is, the objects
desirability from the rat's perspective.
Apparatus. The test enclosures were constructed by bolting two suspended-type,
laboratory-rat cages (41x25x19 cm each) to each other on their rear vertical walls. The
rear and side walls of these cages were made of sheet metal, the bottoms and fronts were
made of wire mesh, and the tops (constructed for this experiment) were made of transparent
Plexiglas. A 7 cm diam. hole was cut through the adjacent rear walls of the adjoining
cages to provide opportunity for the rat to move freely from one to the other. Fresh water
and rat chow (Agway brand Prolab Rat Diet) were provided ad libitum through food
hoppers and water bottles attached to both of these adjoining cages. The joined cages were
suspended above a standard litter tray on a framework which allowed them to tip slightly
toward one or the other side as the rat moved from one cage to the other. Micro-switches
mounted on the framework opened and closed with each tip of the cage structure, and their
output was fed to a computer which continuously recorded the amount of time each side of
the enclosure was tipped down.
Subjects. The subjects were male Long-Evans rats weighing between 476 and 750
grams. (Full grown adult male rats were used to insure positive cage tipping when the rat
moved back and forth.) Prior to this investigation, the subjects had been housed
individually in standard 41x25x19 cm suspended-wire cages. Ten rats were tested for each
stimulus object in two cohorts of five rats simultaneously undergoing identical tests in
five test enclosures. The same ten rats were tested on all stimulus objects, except that
one of the rats in the second cohort became ill and was replaced part way through the
testing sequence. Except during the bright-light test described below, the rats were
housed on a 12:12 white:red light cycle. Ambient room temperature was maintained
throughout the study at 22 deg C.
Procedure. Fifteen objects were tested and they are listed in Table 1 in the order
in which they were presented to the first cohort. The objects were chosen because they
were reasoned to satisfy one or more of the following criteria: (a) potentially
interesting as manipulanda, (b) potentially satisfying to a wall-hugging
("thigmotaxic") tendency, (c) potentially suitable for chewing. In addition,
taking advantage of the well known tendency for rats to avoid bright lights (or
conversely, the preference of rats for dark enclosures), to assure ourselves that our
apparatus reliably measured rats preferences, we also tested the presence of a
bright light above one of the two adjoining cages. Each stimulus was tested over an 8-day
period in which the object was placed on one side of the enclosure for the first 4 days
and then moved to the other side of the enclosure for the remaining 4 days. Each 8-day
testing period immediately followed the preceding one without pause. The side receiving
the object first was the same for each rat in a cohort and was counter-balanced between
cohorts. The total time spent on one side of the apparatus was recorded for each four-day
period, and served as the dependent variable in a 1-between (cohort), 1-within (side of
stimulus) analysis of variance conducted separately for each stimulus object. That is, a
repeated-measures ANOVA was used to compare the amount of time spent on a given side when
the stimulus was present on that side with the amount of time spent on the same side when
the object had been moved to the opposite side.
There were no significant differences between the two cohorts for any of the stimuli
tested, and their data were therefore combined. For each object, Table 1 presents the F
statistic associated with the main effect of side-of-stimulus along with the associated
The strongest effect occurred when the light bulb was the stimulus. On average over the
8-day test, the rats spent 78% of their time on the darker side of the apparatus. The rats
also showed lesser, but statistically significant, attractions to wooden blocks with holes
(60% of their time on average), golf balls (56%), and small wooden balls (54%). None of
the other objects influenced the rats' movements with statistical reliability.
Despite their well known reputation for preferring to remain near walls and avoid open
spaces, our rats were not reliably attracted toward those stimuli which we had presumed
would satisfy such a tendency (stationary L-shaped walls, Y-shaped plumbing fixture, steel
soup cans). Indeed, the large soup cans were associated with a non-significant tendency to
avoid them. It is possible that the instability inherent in the cans tendency to
roll, combined with their large size, evoked a degree of fear on the part of the rats. Our
test also revealed no reliable attraction on the part of the rats to those objects which
we had conceived of as potentially interesting manipulanda (caged peach pit, sandwiched
mango seed, and acrylic balls and blocks).
By contrast, two objects which did prove to be reliably attractive were among those
which we had conceived of as potentially of interest for chewing (wood block with holes,
and small wood ball). In addition, one object which we had originally chosen as a
potentially interesting manipulandum, but which turned out to be chewed extensively (golf
ball) also proved to be attractive to the rats. (For most of our rats, the golf balls were
gradually reduced to small bits which fell through the mesh into the litter trays during
the course of each 4-day period.) Even the difference in attractiveness between the
acrylic cubes and the acrylic balls (both non-significant; but nearly significant for the
blocks while clearly not for the balls), appears to be due to the rats ability to
gain purchase with their teeth on the edges and corners of the cube contrasted with their
inability to do so with the balls. Cognizant of the fact that Rodentia translates
from the Latin as "gnawers", we should probably not be surprised by these
findings. Evidently, rats like to chew, and, if given the opportunity, they seek out and
spend time with objects which they can chew on. This finding is consistent with similar
findings recently obtained with rabbits (Huls, Brooks & Bean-Knudsen, 1991), and
guinea-pigs (Scharmann, 1991).
Nonetheless, even on this issue, caution seems warranted in making assumptions about
what would constitute a suitable object for chewing. All wooden objects are presumably
chewable, but not all of the wooden objects which we tested were equally preferred by the
rats. The different wooden objects were derived from wood cut from different species of
trees, and we wonder if differences in the types of wood (hardness, palatability, etc.)
might have made a contribution. We think it will be worthwhile to test this possibility
directly in future work by measuring the relative attractiveness of wooden blocks of
identical size and shape cut from different species of trees.
We are sympathetic to the complaint that laboratory rats typically endure barren
environments which do not optimize their well being. We feel that the insertion into
existing cages of potentially enriching objects merits serious consideration, and is a
practice which we would cautiously encourage. We feel that our results provide guidance
for the development of suitable, rat-preferred, stimulus objects. Our results appear to
rule out such well-intentioned objects as cans, pipes and non-chewable balls. We also have
had no success at developing potentially attractive manipulanda. Based on our results, we
do recommend that rats be provided with an object to chew on. A small block of wood
drilled with holes is presently suggested by our data to be ideal. Hygiene can be a
concern with wooden objects, but if the block is chosen of a sufficiently small size, the
concern may be minimized. In our experience, wood is chewed into small bits that are
discarded with the feces before it has been in the cage long enough to become
We realize that our study is only a beginning and that measuring time spent with
objects provides only limited information (Duncan, 1978). We recognize that our method
could not have revealed whether a given stimulus provoked a cyclical pattern of time
spent, such as might occur if there were an initial period of avoidance due to neo-phobia
followed by an attraction due to curiosity. Only direct observation of the animals will
reveal details of the particular responses of the animals to given stimuli. We know too
that the long-term effects of exposure to any object would need to be carefully evaluated,
and recognize that a given object may benefit, or harm, a rat in ways that are not
revealed by the time data used in the present study. Nonetheless, if these concerns can be
satisfactorily addressed in future investigations, the provision to laboratory rats of a
block of wood for chewing will at the very least occupy their time, and will allow them to
exercise a behavior which is fundamental to their nature. We hope that this would offer
some relief from the considerable boredom which they are inferred to endure in captivity
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