This is a proposal to investigate the physiological mechanism that links the metabolism and growth of embryonic turtles to the water exchanges occurring between their eggs and the environment. Embryonic turtles form large quantities of urea as the end- product in protein catabolism, and this urea accumulates in all fluid compartments inside their eggs over the course of incubation. The rate of increase in concentration of urea is higher among eggs in negative water balance (i.e., in dry environments), but the rates of metabolism and growth by embryos are lower in the former than in the latter. Research described here will focus on the possibility that urea, which is known to inhibit key metabolic enzymes at low concentrations in vitro, plays, a critical role in suppressing metabolism in embryos with access to limited supplies of water. A pair of experiments will determine (1) whether manipulating concentration of urea inside eggs affects the size of hatchling turtles, and (2) whether the effect of urea is mediated by a suppression of embryonic metabolism and growth. These experiments comprise an important test for a novel theory purporting to explain why animals regulate the osmotic and ionic composition of their body fluids. The research will also elucidate an ecologically important mechanism whereby embryonic turtles balance their demand for water with availability of water. Many species of turtles produce flexible-shelled eggs that exchange water relatively freely with the environment inside their subterranean nests. Eggs incubating in moist environments consequently absorb large amounts of water during development, whereas eggs incubating in relatively dry settings lose water to their surroundings. Interestingly, embryos developing in moist environments (and having access to relatively large quantities of water) have higher rates of metabolism and growth than embryos developing in dry conditions (and having access to smaller amounts of water). Research to be performed under auspices of this grant will ask how the availability of water to developing embryos affects their metabolic waste, differs in concentration in blood and other body fluids from embryonic turtles developing wet and dry environments, and that high concentrations of urea cause and inhibition of metabolism and growth.
