Animal Welfare Information Center Newsletter, Summer 1997, Vol. 8, no. 2
Viktor Reinhardt, Ph.D., D.V.M.
Animal Welfare Institute, P.O. Box 3650, Washington, DC 20007
Research laboratories generally keep nonhuman primates and other
research animals under strictly controlled light/dark rhythms to
avoid variability of research data because photoperiod (daily
exposure to light) profoundly affects animals. Light intensity
(illumination), however, has been given relatively little
consideration in the scientific literature and consequently has
been widely overlooked as an environmental factor possibly
affecting, in undesired ways, the well-being of research
subjects and the data collected from them.
Illumination in Cages Arranged in Tiers
|Illumination in cages is likely to be the most variable ambient
factor in the average animal room (1), especially when the cages
are arranged in tiers. Lower-row cages are not only farther away
from the light source, but are also located in the shade area of
the upper-row cages. Animals housed in such bottom cages live in
a crepuscular environment (fig. 1). The little light they
receive is reflected from walls, and its spectral quality is
accordingly altered. It would be unrealistic to expect uniform
lighting conditions and hence uniform research results in such a
cage arrangement. This problem has been well documented in the
rodent room in which the illumination of lower-row animals is
usually only a fraction of that of upper-row animals (1-4).
The situation is similar in the nonhuman primate room in which
monkey cages are commonly stacked in two tiers (5-15). Living in
the shade of the upper-row, animals housed in the lower-row
automatically receive much less light (8,
19). Illumination in standard lower-row cages is often so dim
(for example, 22 lux, 20) that animal care personnel have to use
flashlights for routine in-cage inspection (21).
Several authors have acknowledged this problem and have warned
that the conspicuous variation in lighting conditions between
upper-row and lower-row cages should be considered as a
contributing cause to the problem of cage-related differences in
research data that cannot otherwise be accounted for (3,
23). Indeed, the uneven distribution of light in stacked cages
may have such a profound effect on experimental results that an
animal's cage location must be mentioned in the research protocol
as a data-influencing variable. However, most authors pay little
attention to this critical detail.
Effects of Illumination on Monkeys
There are only a few studies examining the effect of light
intensity on monkeys, but the available evidence strongly
indicates that exposing monkeys to poor lighting conditions has a
destabilizing effect on their neuroendocrine system.
Heger et al. (24) assessed the fertility of breeding marmosets
(Callithrix jacchus) that were unintentionally kept in
too-dark rooms. Lighting conditions were particularly poor in
bottom-row cages where the illumination was only 20 lux. Females
living in these cages showed unusually extended ovulatory cycles,
abnormally low levels of steroid hormones, and a very low
fecundity. Increasing the brightness in the cages resulted in a
recovery of the animals' reproductive system, and breeding rates
more than doubled. Experimental studies conducted in rodents
support these findings. Reproductive variables such as gonad
weight, conception rate, litter size, and regularity of the
estrous cycle are affected negatively by low light intensity,
positively by high light intensity (1, 25-29).
The inhibitory effect of insufficient illumination on the
reproductive system is mediated by melatonin that is secreted by
the pineal gland (epiphysis) in larger quantities during times of
low rather than high illumination (30-33). Fiske and Macdonald
(34) demonstrated in macaques (Macaca fascicularis)
that melatonin treatment causes irregularities in menstrual
bleeding patterns, an increased incidence of anovulatory and
skipped cycles, a retardation of ovulation, and a shortened
luteal phase (34). Rodent studies indicate that melatonin affects
gonadal activity by inhibiting the release of hypophyseal
Professional and regulatory guidelines seemingly recognize
illumination as a variable that affects not only scientific data
but also the well-being of the research subject and the quality
of animal husbandry. Most regulations, however, seem to overlook
the fact that uniform lighting conditions, though very desirable,
are impractical in the conventional monkey room with double-tier
The International Primatological Society recommends in its
International Guidelines for the Acquisition, Care, and
Breeding of Nonhuman Primates that illumination should be
uniform and sufficient for adequate inspection of nonhuman
primates. Lighting should not be obtrusive to the well-being of
the animals (40).
The U. S. Department of Agriculture specifies in the animal
welfare regulations that lighting for nonhuman primates must be
uniformly diffused and provide sufficient illumination to aid in
maintaining good housekeeping practices, adequate cleaning of
cages, satisfactory inspection of animals, and to ensure the
animals' well-being (41).
The Canadian Council on Animal Care stipulates in its Guide
to the Care and Use of Experimental Animals that lighting
of laboratory animals should provide good visibility and uniform
The National Research Council of the United States implicitly
acknowledges in its latest revision of the Guide to the Care
and Use of Laboratory Animals that it is difficult to create
uniform lighting conditions when animals are housed in stacked
cages. The new Guide therefore no longer recommends that
lighting should be uniformly diffused (43) but simply that
lighting should be diffused throughout an animal holding area to
provide sufficient illumination for the well-being of the animals
and to allow good housekeeping practices (44). The
Guide explicitly notes that illumination should be
sufficient for adequate inspection of all animals
including those kept in the bottom-most cages in racks
The Council of the European Communities states in its
Guidelines for Accommodation and Care of Animals that it
is necessary to provide adequate lighting to satisfy the
biological requirements of animals and to provide a satisfactory
working environment (45).
The prevailing monkey cage arrangement makes adequate, that is
uniform, illumination impossible, because the cages of upper-rows
block the light from entering the cages of lower-rows. Animals
housed in lower-rows are usually subjected to an artificial
night/dawn rhythm without exposure to reasonably bright light.
This situation is unacceptable for several reasons:
- Monkeys--with the exception of one species (owl monkey,
Actus trivirgatus)--are diurnal animals who are
naturally more active in a bright environment such as the
upper-row cage, than in a twilight environment such as the
lower-row cage (12, 46).
Keeping them in the dim, cavelike
ambiance of the typical lower-row cage (fig. 1) cannot be
conducive to their general well-being. Human primates would be at
risk of developing depressive moods under like conditions
- Poor illumination may cause an imbalance of the
neuroendocrine control system of reproduction and possibly also
of other body functions (49-51). Physiological data collected
from monkeys who live under insufficient lighting conditions have
therefore only limited scientific value.
- Housing experimental subjects in cages of different light
intensities (fig. 1) is introducing a dependent variable that
increases the number of test animals needed for statistically
significant results (52). Refined illumination techniques that
diminish or eliminate sources of variability could reduce the
number of animals required in research protocols, particularly
those dealing with photosensitive processes such as reproduction.
- The work environment for attending animal care personnel is
unsatisfactory when the illumination of cages is so poor that
professional cleaning, adequate inspection, and correct
identification of the occupant(s) can be achieved only with
flashlights (fig. 1).
What Can Be Done to More Evenly Illuminate Caged
- Replacing the solid, light-impermeable side panels/walls of
lower-row cages with light-permeable panels makes the cage
interior more accessible to light (20).
- Mounting the light fixtures over the midline of the aisles
rather than over the top cages increases the amount of light
entering lower-row cages.
- Arranging the two cage rows in such a way that the
lower-row is partially standing out under the upper-row increases
the shade-free illumination field of the lower-row cages.
These refinement options are compromise solutions that can
markedly increase the light intensity in lower-row cages, thereby
enhancing the quality of the caged animal's living environment.
It should be remembered, however, that light intensity decreases
with the square of the distance from the light source (53). This
physical law makes it impossible to guarantee uniform lighting
conditions because the distance from the light source is
irrevocably much longer for animals living in lower-row cages
than for animals living in upper-row cages close to the light
fixtures in the ceiling.
It has been proposed to rotate cage positions relative to the
light source to reduce inappropriate illumination(3,
management practice allows half the animals of a room to
temporarily move from dark into bright cages, while the other
half of them has to move into dark cages in exchange. The
inherent inadequacy of the illumination conditions in lower-rack
cages is not addressed by this technique.
Caging all animals at the same level of the room is the only
solution to the problem of uneven lighting conditions:
a) The living environment of all animals can be arranged at the
same distance from the light source.
b) All animals receive the same quality of direct rather than
c) The illumination field of the whole room is free of
shade-casting cage structures.
I am very grateful to Annie and Catherine Reinhardt for reading
this manuscript and offering very constructive comments.
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This article appeared in the Animal Welfare Information Center
Newsletter, Volume 8, Number 2, Summer 1997
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