INVERSE RELATIONSHIP BETWEEN FUNCTIONAL MATURITY AND EXPONENTIAL-GROWTH RATE OF AVIAN SKELETAL-MUSCLE - A CONSTRAINT ON EVOLUTIONARY RESPONSE

In this study, we investigate whether a tissue-level constraint can ex plain the general inverse relationship between growth rate and precoci ty of development in birds. On the whole, altricial (dependent) chicks grow three for four times faster than the less dependent, more able c hicks of precocial species of similar adult mass. We suggest that an a ntagonism between growth and acquisition of mature function in skeleta l muscle constrains postnatal growth and development in most species o f birds. Altricial species, represented by European starlings in this study, hatch with skeletal muscle having low capacity for generating f orce but grow rapidly. Conversely, precocial species (northern bobwhit e quail and Japanese quail), hatch with relatively mature skeletal mus cle, especially in their legs, but grow more slowly. As development pr oceeds in all species, exponential growth rates decrease as muscles ac quire adults levels of function. Among four variables associated with muscle function, exponential growth rate (EGR) was negatively correlat ed with pyruvate kinase activity (glycolysis), potassium concentration (electrical potential), and dry weight fraction (contractile proteins ) in both pectoral and leg muscles but not with citrate synthase activ ity (aerobic metabolism) in either set of muscles. For pectoral muscle , these variables accounted for 87% of the total variation in EGR in a ll three species combined despite a twofold difference in growth rates between the starling and quail. EGRs of leg muscle (51% of variation accounted for) were less than predicted by the pectoral-muscle equatio n in quail during the early part of the postnatal period and in starli ng during the late postnatal period. This result would not contradict a growth rate-maturity constraint hypothesis if EGRs were down-regulat ed for allometric other considerations.