71.3 Jan. 7 Thyroid hormones increase rates of resting metabolism and skeletal growth in hatchling archosaurs OWERKOWICZ, T.; University of Adelaide, South Australia email@example.com
Palaeontologists suggest that bone microstructure of fossils can be used to infer whether an extinct animal was endo- or ectothermic. This is based on the assumption that resting metabolic rate (RMR) is a major determinant of bone growth in vertebrates. While endotherms show higher rates of bone formation and remodelling than similar-sized ectotherms, no experimental evidence exists to support this correlation. Thyroid hormones may provide a link between RMR and growth rate. Endotherms have higher plasma levels of free thyroid hormones (fT3, fT4) than ectotherms, and RMR is elevated in hyperthyroidism. Thyroid hormones are also known to accelerate bone formation and resorption, which underlie skeletal development. I tested this hypothesis by altering plasma fT3 and fT4 levels in hatchlings of the saltwater crocodile (Crocodylus porosus) and emu (Dromaius novaehollandiae). Experimental animals received daily doses of T3 and T4, and controls received saline. Normal thyroid gland activity was suppressed with methimazole in experimental emus. For 12 weeks, growth was monitored and animals were injected with fluorescent dyes to label new bone. Rates of oxygen consumption were measured with open-flow respirometry at the end of each experiment. Both RMR and growth rate were elevated by a combination of T3 and T4. Growth rate in crocodiles increased as plasma fT3 and fT4 levels rose, and the growth response was dose-dependent. Growth rate in emus slowed as plasma fT3 and fT4 levels decreased, but accelerated again with higher doses of T3 and T4. Faster-growing hatchlings formed new bone with higher vascular density than their slower-growing siblings. This suggests that both RMR and skeletal growth rate are dependent on plasma levels of thyroid hormones, and may explain why endotherms grow faster than ectotherms.