abstract
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Genetically engineered "Supermice" harbouring multiple copies of rat growth hormone genes, attain adult body sizes almost twice that of normal mice. To determine how transgenic mice adjust resource acquisition and processing with elevated growth, dry mass budgets were conducted on Supermice (strain Tg[MT-1, rGH], Bri2) and normal Mus musculus. Rates of growth, consumption, faecal deposition, digestive assimilation, respiration, and production efficiencies were compared for both early and late growth intervals.
Younger, faster-growing mice (25-40 days old) displayed higher rates and production efficiencies than those documented for older, slower-growing mice (47-62 days old). Swprisingly, Supermice never exhibited growth rates greater than those displayed by the most rapidly growing normal controls. For transgenic animals, larger body sizes were achieved by maintaining increased growth rates into later ages. Dry mass budgets revealed that Supermice failed to alter mass-specific feeding rates to compensate for their increased growth demands, but production efficiencies were greatly enhanced instead. Superior conversion of assimilated food into biomass was obtained by diverting resources from other behavioural, reproductive, and longevity assurance systems. Shortcomings prevalent in Supermice (lethargy, reduced fecundity, decreased longevity, disturbed metabolism, and various pathological problems) have been similarly expressed in genetically engineered livestock and other organisms conforming to acromegalic states including humans and large breeds of dogs. Thus, transgenic GH mice may offer practical insights for agricultural and medical applications.
Trade-offs in the Supermouse are apparent in their "transgenic correlation structure" which represents a new alternative for testing and clarifying facets of life-history theory. It is concluded that production efficiency is a key component linking life-history features in Mus musculus, and may be a fundamental element in both environmental adjustments and evolutionary changes.