Water Content, Thermogenesis, and Growth Rate of Skeletal Muscles in the European Starling

-We examined the relationship between function, water content, and growth rate of skeletal muscles in the European Starling (Sturnus vulgaris). Function was measured as the ability of nestlings to increase their rate of oxygen consumption in response to cold stress. Water content and growth rate of pectoral and leg muscles were determined for tissues dissected from a series of known-age nestlings. The maximum metabolic response to cold stress increased from 0 at 4 days of age to 4.5-5.5 cm3 O2g'-'h-' at 15-16 days. During this period, the mass of the leg muscles increased 5-fold and that of the pectoral muscles, 12.5fold. The water fraction (WF) of the pectoral muscles decreased from about 0.86 to 0.72, while that of the leg muscles decreased from 0.83 to 0.72. Of the variatlon in metabolic response to cold stress per gram of muscle, 89% could be related to the WF of skeletal muscle by a linear relationship. Over the ages surveyed, the metabolic response varied from 0 at WF = 0.85 to an increase above resting metabolism amounting to 60 cm3 02 gram of pectoral and leg muscle-' h-' at WF = 0.72. Also during this period, the growth rate of both muscle masses decreased with age and proportion of water, from a mass-specific growth rate of 0.5/day at WF = 0.85 to near 0 at WF = 0.72. If these relationships represent a balance between growth rate and functional maturity in developing tissues, then even small changes in function, associated with a change in water content of only a few percent, could have large consequences for the growth rate of the individual. Received 30 August 1983, accepted 30 November 1984. IN comparisons among species of birds of the same size, the growth rate of chicks varies over an approximately 10-fold range (Ricklefs 1979b). Much of this variation is associated with the mode of development. Chicks of precocial species, which are relatively independent of their parents from an early age, grow more slowly than do those of altricial species, which depend upon their parents for food and warmth during much of their development. Such comparisons, and the observation that mass-specific growth rates of individuals slow as the individual matures functionally, led Ricklefs (1969, 1973, 1979a) to suggest that growth rate is inversely related to functional maturity at the tissue level. As a tissue differentiates and begins to function at an adult level, its proliferation by cell division and growth by cell enlargement decrease. Skeletal muscle provides a model for this constraint in that as mesenchyme cells (myoblasts) of the embryonic muscles differentiate, they fuse to form muscle fiI Present address: Department of Zoology, University of Washington, Seattle, Washington 98195 USA. bers and cease to proliferate (Holtzer 1970, Cameron and Jeter 1971). Associated with the maturation of tissues is a decrease in content of water (Ricklefs 1967, 1979a; Bilby and Widdowson 1971; O'Connor 1977; Dunn and Brisbin 1980). In comparisons among species and in comparisons among individuals during development, the water content of a tissue has frequently been used as an index to its differentiation and functional maturity. But in spite of the general association between water content, function, and growth rate, these relationships have not been determined quantitatively. Indeed, Marsh and Wickler (1982) have questioned the use of water content as a suitable index of muscle function. We examined the relationship between function, water content, and growth rate of skeletal muscles in the European Starling (Sturnus vulgaris). Skeletal muscle was chosen for two reasons: first, its pattern of development has been suggested to underlie differences in growth rate (1/time) among species of birds (Ricklefs 1979a, b); second, the function of skeletal muscle can be determined directly by simple physiological measurement. During development, increase in 369 The Auk 102: 369-376. April 1985 This content downloaded from 157.55. 39.25 on Tue, 02 Aug 2016 06:21:48 UTC All use subject to http://about.jstor. org/terms 370 RICKLEFS AND WEBB [Auk, Vol. 102