SIZE AND AGE STRUCTURE OF RURAL POPULATIONS OF RATTUS-NORVEGICUS CONTAINING INDIVIDUALS RESISTANT TO ANTICOAGULANT POISON WARFARIN

Some populations of brown rats contain individuals resistant to the anticoagulant poison Warfarin. In the case of rats from mid-Wales [United Kingdom], resistance is a dominant character controlled by an autosomal gene; it is associated with a nearly recessive character related to vitamin K requirement. Strong selection operates whether or not a population is subject to Warfarin poisoning. In order to make predictions about how such selection will influence natural populations the rats inhabiting an area of farmland in the Welsh country of Powys were studied from April 1972-April 1974. Three field populations on 2 farms were investigated by capture-recapture methods. One population in the farm buildings was also sampled less systematically after treatments with an acute poison. The 3 field populations occurred in hedges, along watercourses and in small areas of woodland. There was evidence that some rats avoided capture while others became trap-addicted. Of the 3 field populations only 1 provided sufficient data for a thorough capture-recapture analysis. Monthly estimates of size of this population ranged from 11-126 for males and 9-63 for females. There was a peak of numbers in late summer and autumn. Strong selection must cause many changes in the frequency of Warfarin resistance. Genetic drift due to factors such as the chance establishment of a transient male in one of the mosaic of small populations in the lowlands of the Welsh-English border must also influence the level of resistance.