A Cost of Resistance in the Brown Rat? Reduced Growth Rate in Warfarin-Resistant Lines

Warfarin resistance in the brown rat is known to be associated with deficient vitamin K metabolism and reduced viability, especially in young males. We investigated the possibility that warfarin resistance may impose another fitness cost by reducing the energy available for growth. Lines homozygous for Welsh and Scottish resistance alleles had been established on a common genetic background and were compared with a homozygous susceptible line (93-98% genetic similarity). Weights of 1686 young male rats at ages 22, 28 and 50 days were related to resistance genotype and litter size. At 22 days, mean weights of lines were very similar. There was a significant reduction in weight of one resistant line at 28 days (P < 0.05), and at 50 days the reduction in weight in both resistant lines was highly significant (P < 0 00001). It is suggested that reduced growth rate may involve abnormal vitamin K-dependent calcium metabolism rather than just a blood clotting disorder. One fitness consequence of reduced growth rate in resistant homozygotes is the relationship of social dominance to body size. Key-words: Cost of resistance, growth rate, osteocalcin, Rattus norvegicus, social dominance, warfarin resistance, vitamin K metabolism