Social enhancement for caged macaques: 93-08

Inanimate Enrichment photos93-108

 Inanimate enrichment increases
complexity of the cage environment,
thereby promoting non-injurious activities

"Providing toys to captive primates might seem an obvious and simple solution to enriching their environment, though it is absolutely clear that no inanimate playthings can compare with the presence of a compatible conspecific" [Dean, 1999].
Macaques - just like human infants - are too intelligent for not getting quickly bored by most of the commercial toys. "The attention span is limited to a day or two at most, and it is important to use any specific object only periodically, providing constant variety to keep the animals interested" [Dean, 1999; cf. Ross & Everitt, 1988; Crockett et al., 1989, 1998; Hamilton, 1991; Line et al., 1991; Weick et al., 1991; Bartecki, 1993; Cardinal & Kent, 1998; Morgan et al., 1998]. Needless to say, that a toy becomes more attractive and stimulates more sustained interest when it is shared with a companion [cf. Novak et al., 1993].

Photos 93* & 94*: Unlike most commercial toys, dry deciduous tree branches cut into 'gnawing sticks' do not lose their novelty effect, since they steadily change their configuration and texture due to constant wear and progressive dehydration [Champoux et al., 1987; cf. photo 94].

In a study of 60 pair-housed rhesus macaques who were exposed to gnawing sticks for 18 months, individuals were engaged in stick use on average 5% of the time [cf. Line & Morgan, 1991], with subadults interacting more with the sticks than adults [Reinhardt, 1990b].

The animals use the sticks for manipulating (photo 93), gnawing (photo 94), nibbling, chewing, hugging, playing and perching. Long-term use of gnawing sticks by several hundred rhesus has resulted in no recognizable health hazards [Reinhardt, 1997a; cf. Line & Morgan, 1991].

Photo 95*: Depending on the size of the individual animal, gnawing sticks have a length of 12-30 cm and a radius of 2-6. Sticks cut of sun-dried read 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 [Reinhardt, 1992e]. After 1-6 months, they usually become so small that they have to be replaced [Reinhardt, 1997].


Photo 96*: Stump-tailed macaques (M. arctoides) also benefit from read oak gnawing sticks which "not only provide some distraction [mean 6% of time] but may also be a means of dental care" [Reinhardt, 1990].

Photo 97a*: As with toys, mirrors do not evoke much interest in singly caged rhesus macaques, and subjects tend to quickly get bored with them [Kaplan & Lobao, 1991; O'Neill et al., 1997; Goode et al., 1998].
Other species, such as long- tailed macaques (M. fascicularis, as in the photo (by Richard Lynch), and pig-tail macaques (M. nemestrina) seem to show more sustained curiosity responses to mirrors [O'Neill et al., 1997].

The problem of declining use of inanimate enrichment devices usually seen with monkeys can be addressed by rotating the animals through a well-structured recreation cage larger than the standard home cage or through a specially designed playroom. The usefulness of this technique has been demonstrated in long-tailed macaques (M. fascicularis; Bryant et al., 1988; Gilbert & Wrenshall, 1989; Lynch & Baker, 1998), stump-tailed macaques (M. arctoides; Blackmore, 1989) and Japanese macaques (M. fuscata; Tustin et al., 1996); studies with rhesus macaques are still missing.



Photo 97b: Windows offer highly valued entertainment (photo by Richard Lynch). Pairs of long-tailed macaques who were transferred regularly for 1 1/2-hour periods to a playroom spent about 67% of the time looking out the windows (Lynch and Baker, 2000).

 The limited information available does not suggest that television or radio music are valued sources of enrichment for caged rhesus macaques [Line et al., 1990c; Kaplan & Lobao, 1991; Harris et al., 1999]; however, there is evidence that individuals can be trained to respond to computer-controlled video-tasks in ways that can be defined as enriching [Washburn & Rumbaugh, 1992].




Photos 98 & 99*: More important than a toy, a gnawing stick, a mirror or television is a perch for caged macaques. It no longer restricts the animals to a terrestrial life style - to which they are biologically not adapted - but opens up the vertical dimension thereby increasing the usable cage space and promoting species-typical arboreal activities such as climbing, leaping, balancing, bouncing, perching and looking-out (photo 98). Serving as a prop for exercise the perch has therapeutic value for animals suffering from cage paralysis [authors' own unpublished observation]. The perch also allows for species-typical vertical flight responses in alarming situations [Lindburg, 1971; Chopra et al., 1992] and for retreat to a dry place during the daily cage cleaning (photo 99). Access to elevated, 'safe' sites has survival value for macaques. This explains why caged animals never lose interest in a perch.

In a study with 25 adult single-caged rhesus males who were exposed to a perch for 12 months, individuals sat on their perch on average 28% of the time [Reinhardt, 1989b].

Inexpensive perches can readily be made from branches of dead deciduous trees (photo 98; Reinhardt et al., 1987c) - preferably from read oaks to forestall clogging problems of sewer drains [Reinhardt, 1992e] - or sections of polyvinyl chloride (PCV) pipes (photo 99; Reinhardt & Smith, 1988).
The diameter of a perch must be large enough so that an animal can comfortably sit on it over extended periods of time.

Rhesus monkeys are inquisitive animals who want to know what's going on outside of their cage, and they show a strong preference for sitting in the front rather in the middle or rear of the cage [Reinhardt, 1989c; Woodbeck & Reinhardt, 1991]. Therefore, perches should always be installed in such a way that they enable the occupant to sit right in front of the cage (photos 98 & 99); probably, this fosters a sense of security by giving the animal visual control over the environment outside of the cage [cf. van Wagenen, 1950; Niemeyer et al., 1998].

Swings are less suitable than perches to enhance cage space complexity. When given the choice, rhesus monkeys clearly prefer perches over swings, presumably because perches - unlike swings - are fixed structures permitting relaxed posturing rather than unstable balancing in a cage that is too small for accurate adjustments of body movements [Kopecky & Reinhardt, 1991; cf. Dexter & Bayne, 1994; Phillippi-Falkenstein, 1998].


Photos 100 & 101: When being exposed to a branch and a PVC pipe, identically installed and identically sized (photo 100), singly housed rhesus show no significant preference but spend equivalent amounts of time with each type of perch [Reinhardt, 1990e]. Branches also serve as attractive objects for gnawing (photo 101), but this renders them less durable, requiring regular replacement.

Photos 102*-104*: PVC pipes have the advantage over branches of being evenly straight. This makes it possible to install them not only in standard cages but also in squeeze-back cages (photo 102): The perch rests in a socket on the front wall of the cage (photo 103) and in a stainless steel sleeve in the squeeze back (photo 104), which can now be freely moved over the whole length of the perch [Reinhardt & Pape, 1991; Reinhardt et al., 1991a].

Photo 105a*: In order to be useful, a perch not only has to be installed in a way that it allows the animal to sit on it close to the front of the cage without touching the ceiling, but it also has to be high enough so that the animal can use the space under it as needed (right graph; Reinhardt & Reinhardt, 1999a). A perch is not properly placed when it blocks part of the minimum floor space needed by the occupant (left and center graph) "to make normal postural adjustments with freedom of movement" [USDA, 1991].
  Photo 105b: The height of the minimum-size standard cage is insufficient to permit the appropriate placement of a perch (photo by Matt Rossell).


  Photo 106: All macaque species - not only rhesus - withdraw in alarming situations by taking to the vertical dimension of their environment, and "spend most of the day in elevated locations and seek the refuge of sleeping trees at night" [NRC, 1998; cf. Roonwal & Mohnot, 1977; Wheatley, 1980].

All macaque cages should be provisioned with at least one perch that enables the occupant(s) - here a stump-tailed macaque - to sit well above the floor of the cage.

Photo 107*: Structural enhancement of the vertical dimension is particularly important for animals who are confined in lower-row cages, and hence forced to lead a terrestrial life style for which they are not adapted. They sit on perches signifi- cantly more often than upper-row caged animals [Reinhardt, 1989b; Woodbeck & Reinhardt, 1991; cf. Shimoji et al., 1993], probably to get at least a short distance away from the 'unsafe' horizontal dimension of the room.
But even with access to perches, lower row-caged animals are unable to retreat to a 'safe' place above the human predator who periodically captures them and subjects them to life-threatening situations and distressing procedures. They "might per- ceive the presence of humans above them as particularly threatening" [NRC, 1998].
The lower-row cage not only is a potentially distressing living environment for macaques which my introduce an uncontrolled extraneous variable into research data [Reinhardt & Reinhardt, 2000b], but it is also an unacceptably dark living environment for them [Reinhardt, 1997; Reinhardt & Reinhardt, 1999b].





Photos 108 & 109*: Macaques are diurnal animals. In the traditional double-tier system, however, monkeys of the bottom row live in the crepuscular shade area of the upper row. Illumination is often so poor that animal care staff routinely uses flashlights to identify animals and check their well-being [Reinhardt, 1997].

No systematic research has yet been conducted to assess the impact of the cave-like lower row housing environment on the occupants' well-being. A preliminary study conducted with a small number of long-tailed macaques, however, indicates that subjects kept in relatively dark lower-row cages exhibit more abnormal behaviors and are more active than subjects housed in significantly brighter upper-row cages [Schapiro et al., 2000].

It stands to reason to suggest that improving the species-inadequate lighting conditions of lower row-caged macaques would be an effective environmental enrichment strategy, which may foster the behavioral health and the general well-being of the animals.

It is perhaps no coincidence that federal law stipulates that that "lighting must be uniformly diffused and provide sufficient illumination ... to ensure the animals' well-being" [USDA, 1991]. Needless to say, this cannot be accomplished in a double-tier caging system, where 50% of the animals are kept in a shady environment.

It is surprising that cage location of research monkeys is rarely mentioned in scientific articles [Reinhardt & Reinhardt, 2000b], although the environment of upper and lower row-housed animals markedly differs both in terms of illumination and living dimension. Not accounting for these variables may increase variability of data and, consequently, the number of experimental animals needed to obtain statistically acceptable results.
It should be remembered that the double-tier caging arrangement - which is still the prevailing caging system for macaques [Rosenberg & Kesel, 1994] - was introduced in the 50's as an emergency situation to accommodate hundreds of thousand of monkeys used for the development of vaccines. Actually, "the original cages used for housing monkeys individually were modified [stacked] chicken or turkey cages constructed of galvanized wire" [Kelley & Hall, 1995; cf. Stone, 1962]. Today, the animals are stuck in this out-dated system not because there is an emergency, but because it saves money to house twice the number of animals in double rows instead of in single rows.

Keeping macaques in a single-tier rather than double-tier system in high cages equipped with perches would be an important refinement of husbandry and research methodology. Such housing would (a) enable all animals of a room to access the 'arboreal dimension' of their enclosure and retreat to 'safe vantage points' above the human predator, (b) offer all animals of a room the option of uniform illumination, and (c) provide more favorable conditions for professional animal care.


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