Researchers Evan MacLean and Sheila Roberts Prior of Duke University reveal what's wrong with the traditional double-tier primate caging system.
Although animals living in the lower row of double-tier cages often endure unfavorable living conditions, this housing system remains the status quo for laboratory primates. Relegating monkeys to lower cages is ecologically inappropriate because their natural tendency is to seek higher ground for sleep, rest and refuge from threatening situations. Confining primates to the ground level prevents them from expressing this instinctually motivated behavior, and animals may consequently experience unnecessary fear and anxiety. Additionally, lighting conditions in the lower row are dramatically more poor than those in the upper row. Most primates are diurnal animals, so it is important to provide them with access to a well-illuminated space.
Although the environmental differences between the upper and lower row have been well documented, we know relatively little about how primates respond to this variation. Several reports have documented that captive primates spend the majority of their time in the uppermost section of their enclosure. However, it is difficult to determine whether this preference is for higher elevation, better illumination or both. Understanding how each of these factors influences monkeys' subjective well-being will inform questions of how best to house laboratory primates.
In a recent study funded in part by a grant from the Animal Welfare Institute and the Johns Hopkins Center for Alternatives to Animals Testing, our team investigated how rhesus and long-tailed macaques allocate their time between the upper and lower rows of a double-tier cage when illumination at both levels of the cage is manipulated. In a baseline condition, we examined where monkeys spent their time under normal lighting conditions.
In a second condition, we tested where monkeys spent their time when the illumination levels were reversed. We hypothesized that if elevated space was more important to the animals than sufficient illumination, the monkeys would spend the majority of their time in the upper row across both conditions. However, if sufficient illumination was more important than elevated space, we expected the monkeys would spend the majority of their time in the upper row during the baseline condition, but reverse this preference with reversed lighting.
Across both conditions, monkeys showed a strong preference for the upper-row cage, indicating that elevation is more important than illumination in guiding location preference. Although monkeys did increase the amount of time they spent in the lower row during periods of reversed lighting, the trend was not significant. Nonetheless, we do not interpret this result as evidence that sufficient lighting is not important to captive monkeys. Rather, we believe that the monkeys' consistent preference for the upper row reflects the paramount importance of access to elevated space. We question whether we would have observed a greater preference for illumination if our experimental conditions permitted activities for which illumination is likely to be important, such as grooming.
While there are still questions regarding the importance of light, our results support the notion that elevated space is among the most important features of a captive primate's environment. Accordingly, we encourage housing primates in upper-row cages whenever possible. If circumstances require some animals to be housed in the lower row of double-tier cages, we recommend providing regular access to tall, well-illuminated exercise cages equipped with high resting surfaces. At our facility, we rotate access to exercise cages daily to ensure all animals housed have regular access to the benefits of an elevated, well-illuminated living environment. This allows animals to express their instinctive need for access to the arboreal "safe" dimension of their living quarters.
An improved cage for monkeys provides access to both the top and bottom levels, as well as species-adequate illumination. Unfortunately, the typical method for housing primates stacks two cages on top of each other. Those in the bottom cage are forced to live close to the ground in a crepuscular environment to which they are not biologically adapted.