Phenotypic variation in strength among eleven inbred strains of rats

As a first step toward the long-range goal of identifying the genes that determine strength, we subjected 11 inbred strains of rats to three tests of muscular strength. The tests consisted of measuring (1) the force exerted by the rat as it was pulled by the base of the tail off a grid on the pan of a top-loading electronic balance (scale test); (2) the length of time the rat hung from a 2.5-mm-diameter U-shaped wire (wire-hanging test); and (3) the length of time the rat hung from a vertically oriented grid consisting of 4-mm-diameter rods (grid-hanging test). Six rats of each gender from each strain were tested at 12 weeks of age, once/day for 5 consecutive days. For the two tests that required use of all four limbs (the scale and grid-hanging tests), one strain performed best (DA). In contrast, on the test that required primarily the use of the front limbs (wire-hanging test), the DA was the lowest performing strain and the F344 rats the best. This differential ranking suggests that the tests selected for variance in the morphological distribution of strength among the strains. There was a 1.5- to 5.2-fold divergence observed between the males of the highest and lowest strains on the scale test and grid hanging tests. This large divergence provides the opportunity to search for intermediate phenotypes and quantitative trait loci that contribute to the different performances of the strains on these strength tests.