Canteen for a young lizard.

Some desert lizards are born equippd with a temporary supply of fresh water, report scientists at the University of Arizona in Tuscon. During research on lizard reproduction, Carol A. Beuchat noticed that if the 2-inch-long baby "Yarrow's spiny lizards" were held a little too tightly they would leak a large drop of clear fluid. She was surprised, because the adult Yarrow's spiny lizards have no urinary bladder and excrete a nearly dry form of waste. Beuchat, David Vleck and Eldon Braun have subsequently discovered that baby lizards of this and other desert spcies have an internal sack, or bladder, containing pure water. By a month after birth this bladder shrivels up and becomes a vestigial organ in most species.

The water in this newly discovered "canteen" comprises about 10 percent of the newborn lizard's weight. (Unlike most lizards, Yarrow's spiny lizard bears live young.) Vleck says, ". . . the bladder is essentially a reservoir, and the babies use its water [at least in the laboratory] during periods of dehydration to maintain blood and other body fluids at normal volumes and concentrations."

The role of this bladder in the lizard's natural environment has still not been determined. Vleck says that baby lizards, which have a high surface-to-volume ratio, tend to dry out faster than do adults. But he thinks it most likely that the bladder evolved originally as a means for reptilian eggs to promote water uptake m the soil. Water moves across the shell of a reptilian egg from the side with the higher water concentration to the side with the lower concentration. But if the embryo could temporarily hide some of its water in a canteen, under dry conditions the egg could continue extracting water from the soil, rather than allowing the water to flow out of the egg.

"There is an advantage to having your 'canteen' full at birth," Vleck says. But he argues that its early withering is advantageous to the adults, since a full cantee would be likely to slow a fleeing lizard.