Pen trial of estrogen-induced conditioned food aversion to eggs in raccoons (Procyon lotor)

Abstract

Aversive conditioning is a promising but unproven non-lethal approach to reducing mammalian depredation on the eggs of ground-nesting birds, terrapins and sea turtles. This research tested the efficacy of oral estrogen concealed in a bland carrier as an aversive agent for wild-caught raccoons (Procyon lotor) under controlled conditions. Nine treatment group raccoons were given six estrogen-injected eggs every other day during a 14-day treatment phase, and then given a combination of two estrogen-injected eggs, two fresh eggs, and two carrier-only injected eggs every other day during a 14-day challenge phase. Nine control group animals received six carrier-only injected eggs every other day during the treatment phase, and then two fresh eggs and four carrier-only injected eggs every other day during the challenge phase. All treatment animals exhibited a conditioned food aversion (CFA) after 1–8 egg feedings (15–116 mg of estrogen per kilogram of body mass). All later sampled at least a few eggs, but they consumed fewer eggs than the control animals during both the treatment phase (p < 0.001) and challenge phase (p < 0.001). No raccoon could distinguish treated from untreated eggs during the challenge phase (p = 0.740); the treatment was undetectable by visual or olfactory cues. We observed no conspicuous changes in the feeding activity, behavior or demeanor of the treatment animals. Treatment and control animals ate (p = 0.629) and drank (p > 0.05) comparably. Treatment animals gained less mass than control animals (p = 0.013), but there was no apparent relationship between estrogen intake and mass change (p = 0.912). Testes of treatment males were similar in volume and mass (p = 0.712) to those of control males. Treatment animals experienced higher frequencies of abnormal feces (p < 0.005) and dermatitis (p = 0.001) than control animals. A treatment female died during the trial from an aborted late-term pregnancy, probably induced by the estrogen. Necropsies revealed no obvious tissue or organ damage from estrogen exposure. The conditions of this pen trial provide a conservative test of the potential for using an estrogen-induced CFA as a management tool for reducing egg consumption in the wild. Ingestion of 20–80 mg kg−1 of estrogen delivered over 1–4 days would be sufficient to bring about a reduction in egg predation using this method. A full-scale field trial of estrogen is likely to be productive under circumstances where all of the target population is subject to treatment.