Experimental perturbation of the development of sexual size dimorphism in the mouse skeleton.

BALB/c mice were raised for 30 days under nine treatment regimens to determine their effects on sexual dimorphism for size of the femur and of the body. Treatment groups were divided into three experiments: 1) burrowing in a high litter cage; eight times normal gravity for a total of one hour per day; a voluntary exercise control. 2) Unilateral femoral and sciatic nerve ablation; the opposite unoperated leg. 3) Swimming exercise; anabolic steroid; swimming and anabolic steroid; normal exercise control. Traits measured were body weight gain and thirteen femoral characters: moment at failure, length, cross-sectional area, anteroposterior and lateromedial diameters of the cross section, wall thickness for all four quadrants of the shaft, moments of inertia of the cross section in the anteroposterior and lateromedial directions, deflection of the shaft before failure, and total mineral content. Sexual dimorphism of the traits studied varied significantly with treatment. The greatest dimorphism was seen in the controls and the burrowing group. The least dimorphism was seen in the eight times gravity, the steroid, and the exercise and anabolic steroid groups. Reduction in the extent of sexual dimorphism was not correlated with a reduction in the loading regimen but rather seemed mediated by systemic changes in physiology brought on by several factors. Change in the amount of sexual dimorphism for size from the control reflects change in growth rates for males and females in response to different treatments. Change in growth rates of juvenile males and females will likely result in changes in the size of femoral characters and body weight in the adult. Change in the size of femoral characters affects the strength of the femur. In natural populations, a change in the body size of adult males and females could adversely affect reproductive success in groups that normally exhibit strong dimorphism, and hence, select against any conditions that reduce sexual dimorphism. Ordinarily, a significant change in sexual dimorphism--of the magnitude seen here--through natural selection, would take several generations.