Evidence for a trade-off between growth and behavioural activity in giant "Supermice" genetically engineered with extra growth hormone genes

The "Supermouse," transgenic strain (Tg(MT-1,rGH),Bri2), contains multiple copies of rat growth hormone genes. Supermice exhibit doubled growth rates and body sizes but no increase in mass-specific feeding rates or assimilation efficiencies. Extra growth is achieved entirely by means of increased production efficiency by diverting energy from other processes. Reduction of behavioural activity is one potential mechanism for diverting energy to growth. To test this hypothesis we compared the behavioural time budgets (resting, sleeping, locomotion, wheel running, feeding, drinking, and grooming) of transgenic and normal male mice. The activities of individual Supermice and normal mice in artificial enclosures were videotaped and compared over 24-h periods. The duration of each activity was recorded and compared. Transgenic mice slept 126% as much as their normal counterparts (an increase of 3.4 h/d), and locomotion and wheel running combined were only 53.8% that of normal mice (a decrease of 2.95 h/d). Supermice spent only 77% as much time drinking and 69% as much time grooming as normal mice. No differences in the duration of feeding were found. The evidence suggests a direct trade-off between growth rate and behavioural activity consistent with the "principle of allocation" paradigm for life-history evolution.