Metabolic and Digestive Responses to Artificial Selection in Chickens

Abstract

Abstract,-Compared to ancestral wild jungle fowl, domestic broiler chickens have been consciously selected for large body size, relatively large pectoral muscles, rapid growth, and high feed efficiency. Hence intraspecific comparisons of these two strains could help identify consequences of unconscious artificial selection, trade-offs in energy allocation, and factors limiting energy budgets. We therefore compared our measurements of many corresponding parameters in both strains: growth rate, energy intake, digestive efficiency, metabolic rate and its components, organ masses, and intestinal brush-border nutrient transporter and hydrolase activities and capacities, as functions of age and body mass in zero-to nine-week-old chicks.Both strains prove to have the same digestive efficiency, Compared to equal-sized jungle fowl, broilers have higher daily energy intake and activity costs, Broilers have relatively longer and wider, hence heavier, small intestines, and their other gut compartments are also relatively larger. Offsetting these increases, broilers have relatively smaller brains and leg bones, these being much less important to a captive bird than to a wild bird exposed to predators. Broilers have generally lower intestinal transporter activities, but relatively higher transporter capacities because of their larger guts.Among domestic chicken strains, comparison of broilers with layers, the former having been consciously selected for much higher growth rates, yields generally similar conclusions. Thus, as recognized in broad outline by Darwin, domestication provides clear examples of conscious selection, of unconscious selection for traits prerequisite to the consciously selected traits, and of unconscious selection against traits rendered less important or competing for space or energy.Key words.-Conscious selection, domestication, energy budget, growth rate, intestinal transporters, metabolic rate, organ mass, unconscious selection.Received September 27, 1994, Accepted August 22, 1995 Changes of wild populations under domestication illustrate so well the effects of selection that Darwin (1859) devoted the first chapter of his book The Origin of Species to the subject. Many subsequent comparisons of animal domesticates with their wild ancestors have discussed effects of artificial selection on anatomical traits and behavior. In the present manuscript we instead discuss effects on the physiology of growth, metabolism, energy budgets, and digestion. As is well known, these processes differ widely among taxa (Kleiber 1965;Heusner 1985). Artificial selection provides us with an opportunity to identify physiological factors immediately responsible for variation in energy budgets, without the inevitable confounding effects of phylogeny. It may prove easier to identify those physiological factors by comparing fast-and slow-growing strains of the same wild ancestral species, expected to differ in only a modest number of respects, than by comparing more distantly related taxa. For example, such intraspecific compar...

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