A New Aspect of Warfarin Resistance in Wild Rats: Benefits in the Absence of Poison

Resistance to the anticoagulant poison warfarin in the brown rat is commonly thought to incur a cost: resistant rats are prone to vitamin K deficiency, show reduced growth rates and reduced viability resulting in a lower fitness for resistant rats in a warfarin-free environment. We investigated the costs of warfarin resistance in terms of body weight, an indicator of resistance costs and a component of fitness, in two populations of wild rats containing resistant individuals from Southern England. In a warfarin-free environment, resistance remained high and contrary to predictions, the vast majority of resistant individuals were significantly heavier than susceptible animals in both populations