FERTILITY-CONTROL IN WILD MICE AFTER FEEDING WITH RU486 OR METHYL TESTOSTERONE

Paraffin blocks containing either no steroid, 150 mg RU486 kg(-1), 500 -mg methyl testosterone kg(-1) or 1500 mg methyl testosterone kg(-1) w ere fed to wild mice (Mus musculus) in addition to the standard labora tory diet in four large (3 m x 3 m) outdoor pens for six months over t he summer. The RU486 bait was provided for only 3 days every 18 or 21 days, whereas the methyl testosterone bait was available continuously. From a foundation stock of to mice (nine male, eleven female) in each pen, the population had increased to 253 (control), 72 (RU486), 249 ( low methyl testosterone concentration) and 103 (high methyl testostero ne concentration) at the end of six months, when 17%, 4%, 32% and 13% of the mature females were pregnant in the respective treatment groups . There was little evidence of an increase in the incidence of injurie s in the androgen-treated animals. Daily estimation of water consumpti on in the pens proved to be a good non-invasive way of monitoring popu lation growth during the course of the experiment. Intermittent feedin g with a low concentration of RU486 appeared to be much more successfu l in inhibiting reproduction than continuous feeding with a high conce ntration of methyl testosterone, and it therefore offers a new method for controlling feral mouse populations.