Howell, S. M., Fritz, J., Murphy, J. et al. 1998. Vertical poles with cow bells: An enrichment device for chimpanzees (Pan troglodytes). The Newsletter 9(3,4), 3-5.

Adding a simple cow bell can increase use of vertical climbing poles and may have resulted in increased locomotor activity for [group-housed] captive chimpanzees. These data were collected in 1996/97 and the cow bells are still 'ringing' two years later.