INSENSITIVITY OF BRAIN GROWTH TO SELECTION OF FOUR-WEEK BODY MASS IN JAPANESE QUAIL.

Abstract

Within large taxonomic groups (i.e., orders and classes), the logarithm of brain mass is related to the logarithm of body mass with a slope typically on the order of .50 to .68 (see Brody, 1945; Count, 1947; Gould, 1975; Eisenberg, 1981; Martin, 1981; Martin and Harvey, 1984). Within smaller taxonomic groups (i.e., genera and below), however, this allometric relationship commonly has a reduced slope, which can be as low as .2.4 in extreme cases (see Huxley, 1932; Weidenreich, 1941; Martin and Harvey, 1984). To explain the discrepancy between these slopes, Lande (1979) and others have proposed that selection during the differentiation of closely related taxa acts primarily on body size, and the response of brain size is due mostly to its genetic correlation with body size rather than its direct selection. If this were true, the steeper brain-body slope observed within larger taxa would then necessarily result from subsequent direct selection upon brain mass, presumably related in some way to consequences of a change in body mass. Selection on brain mass in mice has revealed a realized heritability of .64 (Roderick et al., 1976). Lande (1979) used this figure and other data to estimate the genetic correlation between brain and body mass in nonprimate mammals to be about .7. Riska and Atchley (unpubl.) have directly estimated the genetic correlation between mature brain mass and body mass at various ages in rats and mice to be between .4 and .7. In this paper, we examine the response of the brain and other body components to selection for 4-week body mass in Japanese Quail and compare the response to the brain-body mass relationship among galliform birds. If the brain-body mass relationship within species reflects a strong genetic correlation, then we would expect to see a response in brain mass even to short-term selection on body mass. Selection for 4-week body mass in Japanese Quail (Coturnix coturnixjaponica) has resulted in a more than 2-fold increase in mature (16 week) body mass (Marks and Lepore, 1968; Marks, 1978a). The rate at which mature mass is achieved also increases following selection. Marks (1978b) fitted growth data (mass versus age) for selected and control strains with logistic equations having the form W(t) = A[1 + b exp(-Kt)]-1, where W(t) is the mass of the chick at age t, A is the asymptote or mass plateau of the growth curve, and K is the rate (time1) at which the asymptote is achieved. Fitted values of K were 40% greater in selected strains than in the control strain (Marks, 1 978b). Other traits also responded to selection. Egg mass increased approximately 20% (Marks, 1979) and the mass of the yolk of the egg by about 8% (Ricklefs and Marks, 1983). The increased mass of selected chicks is accompanied by an increase in the length of the bones of the appendages, indicating a general increase in all parts of the body. Although skeletal muscles of selected chicks are larger than those of controls, amounts of protein, RNA, and DNA per gram of tissue, as well as muscle fiber diameters and activities of certain enzymes do not differ (Fowler et al., 1980). This suggests enlargement of mus-