Nitrogen Excretion by Turkey Vultures

-The proportions of nitrogen (N), as urate-N (76.1%), NH3-N (16.4%), and urea-N (7.5%) in the urine of fasted Turkey Vultures, are similar to those found in the urine of domestic chickens and ducks, the only other birds for which comparable data are available. Vultures show an adaptive increase in the percentage of urinary N excreted as urate (up to 86.8%) and a decrease in NH3-N (down to 8.9%) and urea-N (down to 4.3%) in the period after feeding on meat. These alterations in the proportions of nitrogenous wastes increase the efficiency of N excretion; fed birds excrete 4.8 times more N per milliliter of urinary water than fasted birds. The efficiency of N excretion during the 1.5-h period after feeding is comparable to that of the most efficient mammalian excretory systems. The elimination of urate (i.e., uric acid or any other urate compound) as the major nitrogenous excretory compound of birds generally is considered to be an adaptation for water conservation. Needham (1938) described the benefits of urate as a nitrogenous end product in cleidoic eggs; the potential osmotic advantage of urate excretion to adult birds is discussed in most current avian physiology texts (e.g., Sykes 1971, Shoemaker 1972, Sturkie 1976). The low solubility of urate in water (Peters and van Slyke 1946) and its ability to form colloids (Young and Dreyer 1933) might cause less osmotic water to be lost in urate excretion than might occur with osmotically active nitrogenous wastes. The advantages of urate excretion might be evident in two ways in species adapted to either water shortage or high demands for nitrogen (N) excretion relative to water availability: (1) by excretion of a consistently high proportion of urate or (2) by transient increases in the proportion of N excreted as urate, in response to either decreased water availability or high N intake. Regarding the first of these possibilities, previous studies of the proportions of nitrogenous excretory compounds have used only domestic chickens and ducks (e.g., Table II, Shoemaker 1972). While urate accounted for the largest proportion of the waste N in all studies, the percentages of urate-N ranged from 60 to 87% in chickens (see reviews by Sykes 1971, Shoemaker 1972). The only study on ducks (Stewart et al. 1968) reported a low proportion of urateN (54-57%), and it is tempting to speculate that this is a reflection of an abundant water supply with little selective pressure to conserve water. Studies of wild species with different drinking and feeding habits, and from different habitats, are needed to determine whether patterns of nitrogen excretion show long-range evolutionary adaptations. McNabb and McNabb (1975a) found no evidence of adaptive changes in the proportion of urate-N excreted by chickens when water availability was reduced to the minimum required for weight maintenance. While water was limited, chickens ate less high protein food rather than increase urate-N excretion. Moreover, studies such as that of Teekell et al. (1968) show no increase in the proportion of excreted urate over the range of 0-14% dietary protein. In contrast, Sykes (1971) presented data which suggest that fed chickens excrete a higher proportion of urate than fasted chickens. However, he gave only mean values from unpublished work, without statistical analysis, and as he did not indicate if water was available, these data cannot be evaluated adequately. Our objective was to investigate nitrogen excretion in a carnivorous species, the Turkey Vulture (Cathartes aura). Vultures feed on carrion which may be far from water. Thus, when feeding under natural conditions, they consume much nitrogen but are limited to that water available in the food. We compared the proportions of urinary N