Ryan, E. B., Proudfoot, K. L., Fraser, D. 2012. The effect of feeding enrichment methods on the behavior of captive Western lowland gorillas. Zoo Biology 31(2), 235-241.

Three feeding enrichment treatments were tested in an outdoor yard used by six Western lowland gorillas (Gorilla gorilla gorilla). In "Yard-toss", forage was thrown by hand over one third of the yard. In "Set-up", forage and browse were hand-scattered throughout the yard. "Set-up Enriched" was similar with the addition of either a hay- and forage-filled feeder or forage-filled boomer ball(s) suspended from a climbing structure. Each treatment was presented on 5 d. Behavior was recorded for 30 min before (baseline) and 30 min after the start of each treatment. All treatments led to more foraging and less inactivity compared with baseline (P=0.05), but Yard-toss was the least effective, likely because resources were clumped and monopolized by dominant animals. In Set-up Enriched, dominant animals had the greatest increase in foraging (P=0.03), partly because they generally monopolized the suspended items, but this allowed others to forage at ground level. This separation of the animals likely explains why Set-Up Enriched led to more foraging than all other treatments (P80.05). Findings show that for these hierarchical animals, enrichment resources are most effective when distributed widely, including vertically, and that enrichment strategies must take social structure into account.