Anatomical Response to Selection for Four-Week Body Mass in Japanese Quail

Abstract

We analyzed changes in masses, proportions, and water contents of components of Japanese Quail (Coturnix japonica) following long-term selection for 4-week body mass. Chicks from selected and control lines were sacrificed at 0, 1, 2, 4, 6, and 9 weeks of age and dissected into 12 components, including plumage, major muscle masses, visceral organs, and brain. Selection for 40 generations had led to more than a doubling in mature mass and a 26-28% decrease in the time required to achieve that size. The difference in growth rates was expressed only before the age of 2 weeks post-hatching, hence before chicks had achieved 50% of mature mass. Proportions of several organs and components during the growth period were not markedly affected, but the mass of the pectoral muscles of the selected chicks increased, and the mass of the brain and the wing area decreased, relative to body size. The absolute growth of the brain did not differ between lines. There were no differences between lines in the relative water contents of tissues of chicks of similar ages. These results suggest that growth rate can be increased without decreases in the apparent levels of functional maturity of tissues or changes in body proportions that lead to a decrease in the growth increment of a constraining tissue. THE fitting of sigmoid equations to growth data has allowed ornithologists to make comparisons among species based on estimates of growth-rate constants (K; Ricklefs 1968a, 1973; Dunn 1975; Case 1978). Such analyses have revealed patterns in growth-rate variation associated with adult body size, mode of development , adult anatomical proportions, and ecological circumstance (Ricklefs 1979b). There is, however, no general agreement on the selective factors and biological constraints that underlie these patterns (Ricklefs 1969, 1979b; Case 1978; O'Connor 1978; Dunn 1980). Several hypotheses that account for variation in growth-rate constants among species are plausible: (1) Growth rate may be optimized with respect to opposing selective factors of mortality and brood size, the latter mediated by the relationship of chick energy requirement to growth rate (Lack 1968, Case 1978, Ricklefs 1984a). Factors favoring more rapid growth include predators, adverse weather, and sibling competition (Werschkul and Jackson 1979). (2) Among species whose young are self-feeding (i.e. precocial species) or that have broods of only one chick (i.e. most pelagic species), mortality factors may select increased growth rate up to a value determined by the availability of energy or essential nutrients in food (Ricklefs 1984a). Nutritional limits to growth rate in poultry are suggested by the effects of protein levels on growth (Marks 1978c, Clark et al. 1982) and heritabilities and selection responses of body mass (Marks and Britton 1978).