Lithium and cyanide-induced conditioned food aversions in brushtail possums

Bait shyness through conditioned food aversion (CFA) by the common brushtail possum (Trichosurus vulpecula) may reduce the effectiveness of sodium cyanide (NaCN) poisoning operations in New Zealand. Thus, we compared the relative effectiveness of 2 toxins (NaCN and lithium chloride [LiCl]) in establishing CFA in this species. Once a week, for 30 minutes, we presented 27 individually caged wild possums with a novel raspberry-flavored food. Within 74 minutes of feeding they received an intraperitoneal injection of either 414 mg/kg sodium chloride, 200 mg/kg LiCl, or 4 mg/kg NaCN (weeks 0, 1, and 2), or 300 mg/kg LiCl (weeks 0 and 1). We presented flavored food alone at weekly intervals for 8 more weeks. Following the first injection, possums reduced (P < 0.05) intake of flavored food, relative to the saline control, for 2 weeks (NaCN group) or for 9 weeks (both LiCl groups). The demonstration of a NaCN-induced CFA in some possums contrasts with previous failures to establish aversions in laboratory rats (Rattus norvegicus), and suggests that modifications to current toxin-control practices for possums are required to limit the development of bait shyness.