Thyroid Hormones Reduce Incubation Period without Developmental or Metabolic Costs in Murray River Short-Necked Turtles (Emydura macquarii)*

AbstractMetabolic processes are affected by both temperature and thyroid hormones in ectothermic vertebrates. Temperature is the major determinant of incubation length in oviparous vertebrates, but turtles can also alter developmental rate independent of temperature. Temperature gradients within natural nests cause different developmental rates of turtle embryos within nests. Despite temperature-induced reductions in developmental rate, cooler-incubated neonates often hatch synchronously with warmer siblings via metabolic compensation. The physiological mechanisms underlying metabolic compensation are unknown, but thyroid hormones may play a critical role. We applied excess triiodothyronine (T3) to developing eggs of Murray River short-necked turtle (Emydura macquarii)—a species that exhibits metabolic compensation and synchronous hatching—to determine whether T3 influences developmental rate and whether changes to incubation period incur metabolic costs. We measured heart rate, oxygen consumption and inc...