Role of the Urinary Bladder in Osmotic Regulation of Neonatal Lizards

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Abstract
Neonatal lizards of the viviparous species Sceloporus jarrovi possess at birth a urinary bladder that contains a large amount (14% of body mass at birth) of very dilute (36 mosm/kg) urine. After birth, no additional urine is added to the bladder, and the fluid it contains declines in volume and increases in osmotic concentration as the bladder degenerates to the vestigial organ found in adults. Neonatal lizards can reabsorb the fluid in the bladder, and, under desiccating conditions, the reabsorbed fluid serves to maintain a constant plasma osmotic pressure (mean = 289 mosm/kg). However, after the bladder is empty, plasma osmolality increases to as high as 400 mosm/kg during dehydration. Lizards evaporatively lose water at a rate of ∼36 μl·day⁻¹, but the reabsorption rate of water from the bladder is only 21 μl·day⁻¹. Thus throughout the course of dehydration, the water content of the body (exclusive of the bladder) diminishes-but to a lesser extent when the bladder contains fluid than when it is empty. In neonatal lizards, the urinary bladder appears to be useful, as it is in amphibians, as an extrarenal osmoregulatory organ that can buffer body water compartments against osmotic perturbation.