Behavioural resistance towards poison baits in brown rats, Rattus norvegicus

Brown rats are unexpectedly difficult to control in some areas. In an attempt to discover why, rat control treatments were carried out on two farms in Hampshire (UK), using a non-anticoagulant poison, calciferol, to which there is no physiological resistance. Rats were radio-tracked before and during the period in which they had access to poison. Although some rats succumbed, others (at least 20-50%) survived despite intimate and repeated access to the poisoned bait. Three categories of hypotheses that might explain the survival of these rats are evaluated; these involve physiological, ecological and behavioural factors. The hypotheses are not mutually exclusive, but it is concluded that the predictions of the behavioural hypothesis most closely fit the facts: the control failed because, despite having easy access to the bait, behavioural traits prevented the rats from eating a fatal dose of poison. Behavioural resistance is defined as a relative measure of a rat's reluctance to eat palatable poison to which it has easy access and those in the Hampshire study area appeared to be behaviourally resistant. Mechanisms leading to behavioural resistance could be an enhanced capacity for (a) associative conditioning, (b) neophobia or (c) abilities to detect and recognise poison. Each of these mechanisms might suffice alone, but any combination of them might work synergistically. Rats that survived the treatments had significantly smaller pre-treatment linear home ranges than rats that died. There were also indications that more females survived than males. Of the possible mechanisms that might result in behavioural resistance, enhanced neophobia was a necessary and possibly sufficient explanation for the high survival of the rats on these farms. © 1993.