Water Relations and Nitrogen Excretion in Embryos of the Oviparous Snake Coluber constrictor

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Abstract
Flexible-shelled eggs of the colubrid snake Coluber constrictor were incubated on substrates of vermiculite at water potentials of -150 kPa, -550 kPa, and -950 kPa. Eggs on the wet substrate (i.e., -250 kPa) absorbed large quantities of water and increased in mass by 54% over the first 42 d of an incubation averaging 47 d in duration. In contrast, eggs on the intermediate (-550 kPa) and dry substrates (-950 kPa) increased in mass be only 25% and 7%, respectively, over the same interval. Despite these major differences in patterns of net water-exchange between eggs and their surroundings, neither survival of embryos nor growth in dry mass of their bodies was affected appreciably by hydration of the environment. Additionally, water absorbed by eggs was not incorporated into tissues of embryos, but seems instead to have been stored in the allantoic sac. Embryonic racers detoxified most of the ammonia released in catabolism of protein by converting it to urea, which then accumulated in all of the water compartments inside their eggs (including the allantoic sac). If urea inhibits activity of metabolic enzymes in reptilian embyros in the same way that it inhibits enzymes in mammals, the uptake of water by eggs from the environment may be significant in providing a diluent for accumulating urea, thereby preventing this metabolite from reaching concentrations that could inhibit embryonic metabolism and growth.