Catabolic capacity of the muscles of shorebird chicks: Maturation of function in relation to body size

Newly hatched precocial chicks of arctic shorebirds are able to walk and re gulate their body temperatures to a limited extent. Yet, they must also gro w rapidly to achieve independence before the end of the short arctic growin g season. A rapid growth rate may conflict with development of mature funct ion, and because of the allometric scaling of thermal relationships, this t rade-off might be resolved differently in large and small species. We asses sed growth (mass) and functional maturity (catabolic enzyme activity) in le g and pectoral muscles of chicks aged 1-16 d and adults of two scolopacid s horebirds, the smaller dunlin (Calidris alpina: neonate mass 8 g, adult mas s 50 g) and larger whimbrel (Numenius phaeopus; neonate mass 34 g, adult ma ss 380 g). Enzyme activity indicates maximum catabolic capacity, which is o ne aspect of the development of functional maturity of muscle. The growth r ate-maturity hypothesis predicts that the development of catabolic capacity should be delayed in faster-growing muscle masses. Leg muscles of both spe cies were a larger proportion of adult size at hatching and grew faster tha n pectoral muscles. Pectoral muscles grew more rapidly in the dunlin than i n the whimbrel, whereas leg muscles grew more rapidly in the whimbrel. In b oth species and in both leg and pectoral muscles, enzyme activities general ly increased with age, suggesting increasing functional maturity. Levels of citrate synthase activity were similar to those reported for other species , but L-3-hydroxyacyl-CoA-dehydrogenase and pyruvate kinase (PK) activities were comparatively high. Catabolic capacities of leg muscles were initiall y high compared to those of pectoral muscles, but with the exception of gly colytic (PK) capacities, these subsequently increased only modestly or even decreased as chicks grew. The earlier functional maturity of the more rapi dly growing leg muscles, as well as the generally higher functional maturit y in muscles of the more rapidly growing dunlin chicks, contradicts the gro wth rate-maturity function trade-off and suggests that birds have considera ble latitude to modify this relationship. Whimbrel chicks, apparently, can rely on allometric scaling of power requirements for locomotion and the the rmal inertia of their larger mass to reduce demands on their muscles, where as dunlin chicks require muscles with higher metabolic capacity from an ear lier age. Thus, larger and smaller species may adopt different strategies o f growth and tissue maturation.