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Abstract
Rhodamine B (RB) is a dye that becomes incorporated into the structure of growing hair of animals that ingest it, appearing as an orange‐red fluorescent band detectable under a fluorescent‐light microscope. This marker was evaluated as a means of assessing bait acceptance by stoats (Mustela erminea L.). Eleven wild‐caught captive stoats were each fed a broken hen egg injected with 25 mg of RB on two occasions, 5 weeks apart. This was equivalent to 62–108 mg kg–1, depending upon stoat weight, on each occasion. At least three facial whiskers were collected from each dosed stoat on each of two sampling dates (giving a total of at least six whiskers from each stoat). The sampling dates varied from 1 to 17 weeks after first dosing. Whiskers were also collected from one of the dosed stoats that died of other causes 19 weeks after first dosing, and from four stoats not dosed with RB. All 11 of the stoats fed RB had at least one fluorescent band in at least one of the sampled whiskers. None of the four stoats not fed RB had fluorescent bands in their whiskers. The marking persisted in all dosed stoats for at least 6 weeks, and in one dosed stoat for at least 19 weeks after dosing. However, only 56% of the 91 whiskers inspected from the dosed stoats had fluorescent bands, and only 9% of the whiskers had two fluorescent bands, representing the two doses of RB. The distance between the two fluorescent bands indicated a mean whisker growth of 0.6 mm day‐1. The distance from the base of the whiskers to the base of the fluorescent bands was broadly related to the time after ingestion of bait containing RB. However, the variation was too great for distance along the whisker to be reliably used as a quantitative measure of time after bait ingestion. The technique can be used to assess bait acceptance in the field provided all stoats are sampled within c. 4–6 weeks of baiting, and at least 6–9 whiskers are sampled from each stoat.