European-Style Pen Enclosures for Macaques and Other Old World Monkeys

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    In recent years, the use of European-style pen enclosures for housing non-human primates (hereafter ‘primates’) has increased significantly, to the point where this type of housing system is replacing the more traditional one-over-one caging units in some facilities globally. But what have been the drivers for this change? Certainly, regulations such as ETS 123 and Directive 2010/63/EU have been important drivers within the European Union (EU). In addition to this, guidelines on primate accommodation, care and use developed by the NC3Rs and adopted by other research funding organizations have driven improvements in NHP housing internationally; as have the changing expectations of pharmaceutical companies and other clients of contract research organizations, in line with their global animal welfare policies and corporate social responsibility policies. Moreover, in this global age, organizations want to be harmonized in their welfare standards, and not appear to be behind the curve or below par in terms of those standards. Use of large, enriched, pen enclosures can give laboratories the confidence to be more open with the public about their primate work. Finally, and most importantly, enclosures offer researchers the opportunity to improve the welfare of their primate occupants.

    The switch to enclosures rather than cages has not only been seen in the EU. In a recent, unpublished survey on primate behavioral management conducted by myself and a colleague at the Tulane National Primate Research Center, over one third of participating North American facilities reported using EU-style pen enclosures for at least some of their primates.

    Depending on circumstances (e.g. age of the animals), the minimum mandatory space allocation for macaques under the EU regulations can be five to twelve times larger than those in the ILAR Guide used in the USA and elsewhere. Double-tier cages are not permitted, and enclosures for Old World monkeys, such as macaques, must be 5.9 feet high, recognizing the importance of the vertical dimensions for primates, which flee upwards when alarmed. The minimum enclosure volume for Old World monkeys is 127 cubic feet, in which you can house up to three macaques under three years of age, or two macaques over three years of age. In this way, the EU regulations encourage pair- and group-housing and maximize the available room space.

    Achieving these space allocations for macaques and other primates at the Council of Europe has been one of the highlights of my career as a primatologist. Within the Primate Expert Group, I was able to successfully argue that minimum cage sizes should not be based on body weight alone, or the ability of the animals to make small postural adjustments in a confined space. Instead, they should be based on a suite of factors, taking into account the ecology and behavior of the various species. The ability to display a wide range of species-typical behaviors, which guidelines on primate care agree is important for the psychological wellbeing of these animals, is seriously compromised in small one-over-one cages.

    The primate guidelines from the NC3Rs were published in 2006 and revised in 2017, and have been adopted by over 20 research funding bodies. Implementation of the principles in these guidelines is a condition of receiving funds for primate research, regardless of where in the world the research is conducted; and the housing requirements in the guidelines reflect EU regulations and practices. Hence these guidelines have resulted in improvements in primate housing on funded research projects, either because studies have been moved to laboratories with EU-compliant enclosures, or because improvements have been made to the housing to bring it up to the required standard in terms of space and environmental enrichment.

    The following are some typical features of a high-quality enclosure system:

    • Floor-to-ceiling height enclosures, with high resting areas at, or above, human eye level – most primates are adapted for a 3D arboreal environment, and so structured vertical space is particularly important
    • Vertical dividers within the space that function as visual barriers, allowing the animals to better control their social interactions, and to maintain group stability
    • Horizontal, widely spaced, bars at the front of the enclosure which enable climbing, thereby increasing the usable space for the monkeys, and which improve visibility and facilitate animal interactions
    • Solid floors covered with a substrate of wood shavings for better hygiene, and to enable the scattering of fine food items for extended bouts of calm foraging behavior
    • Verandas, balconies or bay fronts to provide good visibility of the room, other animals, and of staff –which helps to reduce anxious and defensive behaviors in the animals
    • Finally, of course, the larger space permits a greater scope for enrichment, including housing animals in larger social groups with sufficient perches and shelves for all occupants

    But what evidence exists that all of this actually improves primate welfare? Historical studies found little, or no, welfare benefits from modest increases in cage size. This is to be expected if the increases are only small. For example, in one study the largest cage was only 1.5 times the size of the smallest cage. You might not expect any differences if cage complexity is not also increased – additional dead space is of little value in changing the behavior of the animals. And if the study animals were previously housed in small cages, and they have intractable behavioral abnormalities as a consequence, again, you might not expect their welfare to be improved by providing additional dead space. Unfortunately, while many labs have switched from smaller cages to larger enclosures, few have had the resources to systematically collect data on the welfare of their animals and to publish those data. However, these labs report anecdotally that primates in enclosures are calmer, less aggressive, more cooperative, and display a wider range of appropriate behaviors. Staff satisfaction has increased too; so much so that staff would not go back to their old primate housing systems.

    We do know from the scientific literature, however, that the enclosure features I’ve highlighted do improve the welfare of macaques, using predominantly behavior-based indicators. This is the case for additional enriched space, access to functional vertical space, porches/verandas, multiple perches, floor substrate and group housing.

    One concern that people have when considering moving to enclosure housing is that they won’t be able to easily isolate individual animals for veterinary or scientific procedures, or that this will be too difficult or too time consuming. But this is not an issue in practice. The key is to have flexible enclosure designs with removable panels that allow animals in the social group to be separated with ease, or to have areas within the enclosure for capture of the animals. Of course, positive reinforcement training should also be used to train the animals to cooperate with shifting or capture. If squeezebacks are necessary, then these can be installed in some of the enclosure spaces, or parts of the enclosure, to avoid having the squeezeback limit the structural enrichment provided.

    Another concern is the inability to use the cage wash. Clearly, fixed enclosures require a new style of husbandry and cleaning – it’s not just a case of changing the caging infrastructure. But this type of housing system actually remains cleaner for longer, saving on staff cleaning time, and releasing staff to spend more time on welfare-beneficial activities like the training of the animals. The floor substrate can be spot cleaned, resulting in the enclosures being hosed down less frequently. Drain blockages are easily prevented using removable drain baskets. If a total collection of urine and feces is needed for a scientific purpose, this can be done by temporarily housing the animals in a separate metabolism cage, or having one mounted within the enclosure itself, reducing the stress induced by cage change and removal from social companions.

    To summarize, the use of EU-style enclosures for primates is increasing globally in response to changes in regulations, the expectations of research funders and clients of contract research organizations, and greater awareness of the benefits for animal welfare and superior science. We should not forget that compromised welfare – such as behavioral and other abnormalities resulting from poor environments, either in the present or the past – will likely act as confounds in experimental studies. Enclosures provide greater quantity and quality of space, allow more natural behavior, and the provision of suitable environmental enrichment and socialization. All of these are known to improve primate welfare. Experience from facilities working with primates in enclosure over many years demonstrates that perceived problems can easily be overlooked. It’s important to work with manufacturers to design a custom enclosure system that optimizes primate care and wellbeing, and suits your local needs. For more information on macaque housing, care and behavior, visit the NC3Rs Macaque Website.

    This blog post is adapted from a webinar hosted by Allentown LLC that addressed the topic of enclosure housing for non-human primates. To view the webinar on-demand, including images of EU-style enclosures and supporting references, click here.

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