alternative CTA agents to 17a ethinyl oestradiol for field use; the use of a second dose of these compounds to improve longevity of the CTA warrants further study. q 2000 Elsevier Science
B.V. All rights reserved.
Keywords: Conditioned taste aversion; 17a Ethinyl oestradiol; Cinnamamide; Thiabendazole; Rat; Vertebrate control
1. Introduction
Ž .
A conditioned taste aversion CTA is acquired through an association between the Ž
. taste of a food and a feeling of illness experienced after ingestion Garcia et al., 1974 .
This association is not under conscious control, and is believed to be made in the area Ž
. postrema of the brain stem Borison and Wang, 1953; Bernstein et al., 1986 . Once such
an association has been made, the consumer will avoid eating food with that taste again, Ž
. sometimes experiencing nausea on re-encountering it Gustavson et al., 1974, 1976 .
This is a natural response that has evolved to prevent poisoning, and exists in many Ž
. animals including some invertebrates Gustavson, 1977 . CTA can be induced deliber-
ately by providing an animal with a food treated with an undetectable malaise-inducing chemical. The chemical must be undetectable for the target individual to form the
Ž .
association between the real taste of the food rather than adulterated food and illness. CTA has the advantage that a single dose can be sufficient to prevent consumption of a
Ž particular food for many months without having to re-dose the animal Gill et al., in
. prep. . Harnessing the CTA response has the potential to provide a powerful, non-lethal,
environmentally sensitive means of reducing problems of predation, crop damage, and nuisance caused by vertebrates.
The nature and dose of the illness-inducing chemical is critical to the success of this technique. For field use, a CTA compound should induce a robust, long-lasting CTA
Ž .
after one or possibly two oral dose. At this effective dose, it must cause neither chronic illness nor persistent detrimental effects in the target or any non-target species at risk of
exposure. The formulated compound must also be undetectable, physically stable in the bait substrate, and sufficiently delayed in action to allow the target animal to finish its
Ž .
meal Nicolaus et al., 1989a A compound which induces a robust CTA and meets most of these criteria is 17a
ethinyl oestradiol, a synthetic oestrogenic hormone. It appears to be undetectable by Ž
mammals e.g., rats Rattus norÕegicus, foxes Vulpes Õulpes, and raccoons Procyon .
lotor when added to baits at a dose of F 4 mgrkg body weight, and a single dose can Ž
generate a long-lasting aversion to a given food Nicolaus et al., 1989a,b; Semel and .
Ž Nicolaus, 1992 . This dose is well below the compound’s oral LD
in the rat 1200
50
. mgrkg but as a synthetic reproductive hormone, the compound has potential to affect
Ž reproductive processes and systems Jean, 1968; Yanagimachi and Sato, 1968; Yasuda
. et al., 1981 . Although no such effects have been reported in trials with captive or wild
vertebrates, it is unlikely that the compound would be acceptable for field use, especially where the target species is protected or endangered, or where baiting coincides with
reproductive stages.
Many compounds have been assessed for their efficacy in generating CTA for vertebrate control. Most fail because they are either ineffective or unstable when
Ž administered orally, e.g., apomorphine Pharmaceutical Society of Great Britain, 1979;
. Ž
Conover, 1989 ; are unsafe at the effective dose, e.g., carbachol Nicolaus and Nellis, .
Ž .
1987; Gill et al., in prep. and emetine Conover, 1989 ; or because they are too Ž
. detectable at levels required to generate an aversion, e.g., lithium chloride Burns, 1980 .
Safe and effective alternatives are needed. Two compounds, cinnamamide and thiabendazole, warrant further study. Cinna-
mamide is a synthetic derivative of cinnamic acid, a naturally occurring plant secondary compound. It is repellent to both birds and mammals, acting through its taste and smell
Ž and through a post-ingestional effect at higher doses e.g., Watkins et al., 1995; Gurney
. et al., 1996; Gill et al., 1997 . When its taste and odour were masked, a single oral dose
of 160 mgrkg induced a CTA to sweet water in captive house mice Mus musculus which, under two-choice testing against plain water, lasted throughout a 64-day trial
Ž .
Ž .
Watkins et al., 1998 . This dose is 10 of the oral LD for mice 1600 mgrkg and no
50
long lasting ill effects were noted. Thiabendazole is approved for use as a systemic fungicide and an anthelmintic for mammals. Timber wolves fed a single dose of 55–80
mgrkg thiabendazole avoided some meat-based foods, especially when they had not Ž
. been food-deprived Zeigler et al., 1983 . New Guinea wild dogs and dingoes fed two
Ž doses of 40–80 mgrkg in lamb meat refused to eat untreated lamb meat Gustavson et
. al., 1983 , and black bears reduced their damage to beehives after consuming beeswax
Ž .
and ‘slum gum’ bait containing 160 mgrkg Polson, 1983 . The doses of thiabendazole Ž
used in these experiments are - 6 of its lowest documented oral LD in rats 3100
50
. mgrkg .
We compared the ability of cinnamamide, thiabendazole, and 17a ethinyl oestradiol to induce a CTA to a novel food in the laboratory rat. Novel foods were used because
they more readily induce CTAs than do familiar foods that previously have produced no Ž
. ill effects Revusky and Bedarf, 1967; Wittlin and Brookshire, 1968 .
2. Materials and methods
