Financial and labor costs often limit the implementation of environmental enrichment options, especially when enrichment should be provided for a large number of individual animals.
The Wisconsin Gnawing Stick was developed in 1987 to serve as an affordable inanimate enrichment option for caged macaques. The sticks simply consist of branch segments of dead deciduous trees. Depending on the size of the individual animal, they have a length of 12-30 cm and a radius of 2-6 cm. Sticks cut of red oak (Quercus rubra) branches are particularly suitable because they gradually wear into flakes that are so small that even large quantities pass through sewer drains without clogging. Branches of many other common tree species (white oak, black locust, box elder, black cherry, weeping willow, silver maple) disintegrate into relatively large strips that tend to get stuck in drains, thereby causing clogging problems.
Branches of dead trees are obtainable for no or little expense, and they can be easily cut with a bow saw into adequately sized segments. The gnawing sticks are placed into the cages without any attachment. They are cleaned with warm water daily and disinfected once every 2 weeks during the routine cage sanitation procedures.
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| Figure 1 | Figure 2 |
Loose branch segments elicit the following behaviors in macaques: manipulating, gnawing, nibbling, chewing, hugging, dragging, rolling, playing, and perching (fig. 1). The opportunity to gnaw a chewable natural material not only counteracts boredom, but is also likely to benefit the animal's dental health (Brinkman 1996, Reinhardt 1990a, Reinhardt 1990b). Because of gradual wear and progressive dehydration, the sticks steadily change their texture and configuration, thereby retaining some novelty. After 1-6 months, they usually become so small that they have to be replaced (figure 2).
In a pilot study, 25 adult, singly-caged rhesus macaques (Macaca mulatta) were each exposed to a regularly replaced gnawing stick for 12 months and observed thereafter. The most recently replaced stick was 1 month old. It revealed traces of wear in 96 percent of cases and was actively used by the animals 0 to 20.6 percent of the time with a mean of 3.3 percent (Reinhardt 1989).
In a later assessment, 60 pair-housed rhesus macaques of different age classes (42 adults 9-30 years old, 18 sub-adults 3.5-4 years old) were exposed to gnawing sticks for 18 months and observed thereafter. Each pair had continuous access to two sticks that had been replaced 1 week before the test observation. The sticks showed traces of wear in 100 percent of cases. Individuals were engaged in stick use on average 4.8 percent of the time, with subadults spending significantly more time with the sticks than adults (9.5 percent versus 2.8 percent) (Reinhardt 1990b). In a comparative study with 20 adult pair-housed stumptailed macaques, individuals were actively engaged with their gnawing sticks 2.2 percent to 28.2 percent of the time with a mean of 5.7 percent (Reinhardt 1990a).
In addition to perches and social companionship, red oak gnawing sticks were implemented at the Wisconsin Regional Primate Research Center as basic cage enrichment in 1989. All caged rhesus macaques (more than 700 animals) and all caged stumptailed macaques (approximately 36 animals) have continual access to gnawing sticks since that time. This simple and inexpensive enrichment technique provides the animals with sustained species-adequate stimulation for the expression of species-typical behaviors. Long-term exposure to the sticks has resulted in no recognizable health hazards (compare with Line and Morgan 1991).
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