Animal Welfare Information Center Newsletter, Summer 1997, Vol. 8, no. 2 *************************

Lighting Conditions For Laboratory Monkeys: Are They Accurate?

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

Photo:  Conventional double-tier cage arrangement.
Figure 1.
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, 11, 12, 13, 16, 17, 18, 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, 4, 22, 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 gonadotrophins (35-39).

Illumination Guidelines

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 cage arrangement.

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 illumination (42).

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 (44).

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:

  1. 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 (47, 48).

  2. 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.

  3. 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.

  4. 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 Monkeys?

  1. 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).

  2. 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.

  3. 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, 44). This 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 reflected lighting.

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.


  1. Clough, G. (1982). Environmental effects on animals used in biomedical research. Biological Reviews 57: 487-523.

  2. Bellhorn, R.W. (1980). Lighting in the animal environment. Laboratory Animal Science 30: 440-450.

  3. Greenman, D.L., Bryant, P., Kodell, R. and W. Sheldon (1982). Influence of cage shelf level on retinal atrophy in mice. Laboratory Animal Science 32: 353-355.

  4. Weihe, W.H., Schidlow, J. and F. Strittmatter (1969). The effect of light intensity on the breeding and development of rats and golden hamsters. International Journal of Biometeorology 13: 69-7 9.

  5. Valerio, D.A., Miller, R.L., lnnes, J.R.M., Courtney, K.D., Pallotta, A.J. and R.M. Guttmacher (1969). Macaca mulatta : Management of a Laboratory Breeding Colony. Academic Press: New York.

  6. Keeling, M.E. (1974). Housing requirements -primates. In Handbook of Laboratory Animal Science Volume 1. Melby, E.C. and N.H. Altman (eds.), pp. 97-104. CRC Press: Cleveland, Ohio.

  7. Hearn, J.P., Lunn, S.F., Burden, F.J. and M.M. Pitcher (1975). Management of marmosets for biomedical research. Laboratory Animals 9: 125-134.

  8. Coid, C.R. (1976). Primates husbandry. In The UFAW Handbook on the Care and Management of Laboratory Animals, Fifth Edition. Churchill Livingstons: New York, pp 397-401..

  9. Dukelow, W.R. and T. Asakawa (1986). The squirrel monkey. In The UFAW Handbook on the Care and Management of Laboratory Animals, Sixth Edition, Churchill Livingstons: New York, pp.586-598.

  10. Harris, D. (l988). Welfare and housing of old world non-human primates (Macaca fascicularis and Papio sp.) Universities Federation for Animal Welfare: Potters Bar, UK.

  11. King, J.E. and V.R. Norwood (1989). Free-environment rooms as alternative housing for squirrel monkeys. In Housing, Care, and Psychological Well-being of Captive and Laboratory Primates. Segal, E.F., ed. Noyes Publications: Park Ridge, pp. 102-114.

  12. Scott, L. (1991). Environmental enrichment for single housed common marmosets. In Primate Responses to Environmental Change. Box, H.O., ed., Chapman and Hall: London, pp. 266-274.

  13. Field, K.J., Denny, J. and G. Kubicz (1992). Nonhuman primate socialization and environmental enrichment using a transfer tunnel. Laboratory Primate Newsletter 3 (2): 5-7.

  14. Seier, J.V. and P.W. de Lange (1996) A mobile cage facilitates periodic social contact and exercise for singly caged adult Vervet monkeys. Journal of Medical Primatology 25: 64-68.

  15. Kelley, S.T. and A.S. HalI (1995) Housing. In Nonhuman Primates in Biomedical Research. Biology and Management Bennett, B.T., Abee, C.R. and R. Henrickson (ed&t pp. 193-209. Academic Press: New York, pp. 193-209.

  16. Milhaud, C.L., Klein, M.J. and M.C. Merkel (1980). A new restraining chair for rhesus monkeys (Macaca mulatta). Journal of Medical Primatology 9: 62-70.

  17. Abee, C.R. (1985). Medical care and management of the squirrel monkey. In Handbook of Squirrel Monkey Research, Rosenblum, L.A. and C.L.Coe (eds.), Plenum Press: New York, pp.447-488.

  18. Reinhardt, V. (1989) Evaluation of the long-term effectiveness of two environmental enrichment objects for singly caged rhesus macaques. Lab Animal 18(6): 31-33.

  19. Weed, J.L., Baker, S.C., Harbaugh, S.W. and J. Erwin (1995). Innovative enclosures for laboratory primates: Evaluation of a "breeding condominium." Lab Animal 24(7): 28-32.

  20. Reinhardt, V., Zweifel, D. and D. Pape (1992). Improving the microenvironment of caged laboratory macaques. Animal Technology 43: 179-183.

  21. Reinhardt, V. (1994). Comments on the proposed revision of the Guide for the Care and Use of Laboratory Animals. Institute of Laboratory Animal Resources Public Forum April 2nd, 1994, St Louis, Missouri.

  22. Lockard, R.B. (1963) Self-regulated exposure to light by albino rats as a function of rearing luminance and test luminance. Journal of Comparative Physiology and Psychology 56: 558-564.

  23. Ott, J.N. (1974) The importance of laboratory lighting as an experimental variable. In Environmental Variables in Animal Experimentation, Magalhaes, H., ed., Bucknell University Press: Lewisburg, Pennsylvania, pp. 39-57.

  24. Heger, W., Merker, H.J. and D. Neubert (1986) Low light intensity decreases the fertility of Callthrix jacchus. 14: 260.

  25. Schidlow, F. (1967) Fortpflanzung und Wachstum von Ratten bei Kunstlicht verschiedener lntensitäten. Dissertation: Universität Zürich.

  26. Strittmatter, F. (1968) Fortpflanzung und Wachstum von Goldhamstern bei Kunstlicht verschiedener lntensitäten. Dissertation: Universität Zürich.

  27. Hautzinger, G.M. and B.A. Piacsek (1973). Influence of duration, intensity, and spectrum of light exposure on sexual maturation of female rats. Federation Proceedings 32: 213.

  28. Vriend, J. and J.K. Lauber (1973). Effects of light intensity, wavelength and quanta on gonads and spleen of the deer-mouse. Nature 244: 37-38.

  29. Brainard, G.C., Vaughan, M.K., and R.J. Reiter (1986). Effect of light irradiance and wavelength on the Syrian hamster reproductive system. Endocrinology 119:648-654.

  30. Cardinali, D.P. and R.J. Wurtman (1975). Control of melatonin synthesis in the pineaI organ. In Frontiers of Pineal Physiology, AItschuIe, M.D. (ed.), MIT Press: Cambridge, Massachusetts, pp. 12-43.

  31. Brainard, G.C., Richardson, B.A., Petterborg, L.J. and R.J. Reiter (1982). The effect of different Light intensities on pineal melatonin content. Brain Research 233:75-81.

  32. Brainard, G.C. (1989). Illumination of laboratory animal quarters: Participation of light irradiance and wavelength in the regulation of the neuroendocrine system. In Science and Animals: Addressing Contemporary Issues. Guttman, H.N. and J.A. Mench, eds., Scientists Center for Animal Welfare: Greenbelt, Maryland, pp. 69-74.

  33. Lynch, H.J., Deng, M.H., and R.J. Wurtman (1985). Indirect effects of light: Ecological and ethological considerations. Annals New York Academy of Sciences 153: 231-241.

  34. Fiske, V.M. and B.J. Macdonald (1973). Melatonin and reproduction. Proceedings of the Fourth International Congress of Endocrinology 880-853.

  35. Fraschini, F, Mess, D. and L. Martini (1968). Pineal gland, melatonin, and the control of LH secretion. Endocrinology 82: 919-922.

  36. Wurtman, R. (1969). The pineal and endocrine function. Hospital Practice 4:32-

  37. Ying, S.Y. and R.O. Greep (1973). Inhibition of ovulation by melatonin in the cyclic rat. Endocrinology 91: 333-335.

  38. ColIu, R., Fraschini, F. and L. Martini (1971). Blockade of ovulation by melatonin. Experientia 27: 844-845.

  39. Wurtman, R.J. (1975). The effects of light on man and other mammals. Annual Reviews of Physiology 37: 467-483.

  40. International Primatological Society (1989). International guidelines for the acquisition, care and breeding of nonhuman primates. Primate Report 25: 3-27.

  41. U.S. Department of Agriculture (1991). Title 9, CFR (Code of Federal Regulations), Part 3. Animal Welfare; Standards; Final Rule. Federal Register 56: 6426-6505.

  42. Olfert, E.D., Cross, B.M. and A.A. McWilliam (eds.) (1993). Guide to the Care and Use of Experimental Animals, Volume 1 (2nd Edition). Canadian Council on Animal Care: Ottawa, Ontario.

  43. National Research Council (1985) Guide for the Care and Use of Laboratory Animals (NIH Publication No. 85-23) United States Department of Health and Human Services, National Institutes of Health: Bethesda, Maryland.

  44. National Research Council (1996) Guide for the Care and Use of Laboratory Animals. National Academy Press: Washington, DC.

  45. European Economic Community Council (19861 Directive 86/609 on the Approximation of Laws, Regulations, and Administrative Provisions Regarding the Protection of Animals Used for Experimental and Other Scientific Purposes, Annex 11 Guidelines for Accommodation and Care of Animals. Official Journal of the European Communities L 358: 7-2 8.

  46. Box, H.O. and B. Rohrhuber (1993). Differences in behaviour among adult male-female pairs of cotton-top tamarins (Saguinus oedipus) in different conditions of housing. Animal Technology 44: 19-30.

  47. Lewy, A.J., Kern, H.A., Rosenthal, N.E. and T.A. Wehr (1982). Bright artificial light treatment of a manic-depressive patient with a seasonal mood cycle. American Journal of Psychiatry 139- 1496-1498.

  48. Kripke, D.F. (1985). Therapeutic effects of bright light in depressed patients. Annals of New York Academy of Sciences. 453: 270-281.

  49. Ross, S., Kessler, C., Nagy, Z.M. and J.P. Scott (1966). Effects of illumination on wall-leaving behaviour and activity in three inbred mouse strains. Journal of Comparative and Physiological Psychology 62: 338-340.

  50. HoIIwich, K. (1979). The Influence of Ocular Light Perception on Metabolism in Man and in Animal. Springer-Verlag: New York.

  51. Weihe, W.H. (1976) The effect of light on animals. Laboratory Animal Handbook 2: 63-76.

  52. Brockway, B.P., Hassler, C.R. and N. Hicks (1993). Minimizing stress during physiological monitoring. In Refinement and Reduction in Animal Testing. Niemi, S.M. and J.E. Willson, eds., Scientists Center for Animal Welfare: Greenbelt, Maryland, pp 56-69.

  53. Kaufman, J.E. and J.F. Christensen (1984). IES Lighting Handbook Reference Volume. Illuminating Engineering Society of North America: New York.

This article appeared in the Animal Welfare Information Center Newsletter, Volume 8, Number 2, Summer 1997

Go to:
Contents, Animal Welfare Information Center Newsletter
Top of Document

The Animal Welfare Information Center
U.S. Department of Agriculture
Agricultural Research Service
National Agricultural Library
10301 Baltimore Ave.
Beltsville, MD 20705-2351

Phone: (301) 504-6212
FAX: (301) 504-7125
Contact us:

Policies and Links

USDA logo ARS logo NAL logo
December 3, 1997
This page's URL is