Are Hummingbirds Facultatively Ammonotelic? Nitrogen Excretion and Requirements as a Function of Body Size

Ammonotely in a neotropical frugivorous bat as energy intake decreases

We tested the role of increased ammonia in urine as an energy- and/or nitrogen (N)-saving mechanism in the great fruit-eating bat Artibeus lituratus (Phyllostomidae). We compared N excretion in two groups of bats fed energy-rich (2.75 kJ g(-1) wet mass) or energy-poor diets (0.7 kJ g(-1) wet mass). Within each diet, bats were assigned to different N contents. In order to function as an energy-saving mechanism, ammonia production should increase with decreasing energy intake. To function as an N-saving mechanism, ammonia production should increase with decreasing N intake. Because we varied both diet energy density and N content, our study design allowed us to test these two possibilities simultaneously. Bats had higher food intake rate and, consequently, higher N intake rate on the energy-poor diet, but energy intake rate was lower. Most bats on the energy-rich diet were ureotelic whereas on the energy-poor diet bats were ureotelic, ammonotelic or ureo-ammonotelic. Bats fed the energy-poor diet had a higher excretion rate of ammonia and a higher percent of N excreted as ammonia. Percent N ammonia and ammonia excretion rate were inversely related to energy intake, but they were not related to N intake. By favoring ammonia production over urea, bats on the energy-poor diet may save up to 1% of their basal metabolic rate. Consumption of energy-dilute fruits by fruit bats might affect the way in which N wastes are excreted, favoring the excretion of ammonia N when food intake is accompanied by the ingestion of large volumes of water.

Maintenance nitrogen requirements of adult female ostriches (Struthio camelus)

Successful ostrich farming requires knowledge of the nutritional needs of the birds. While much information is available on the nutritional value of various feed ingredients fed to ostriches, there is little known about their specific nutrient requirements. In this study, we measured the maintenance nitrogen requirements (MNR) of ostriches by nitrogen balance. We predict, based on the previous analysis of nitrogen requirements of various species of birds, that ostriches would have a MNR of 13.6-19.1 g N/day and a total endogenous nitrogen loss (TENL) of 2.8-5.1 g N/day. Three adult female ostriches were fed five pelleted diets containing 0.6-2.3% N [4-14.6% crude protein (CP)], 17.5 kJ/g gross energy (11.4 kJ/g ME) and 30% neutral detergent fibre. Each dietary trial consisted of a 10-day adaptation period, followed by a 5-day total excreta collection period. Body mass (109 ± 3 kg) and metabolizable energy intake (20.5 ± 0.7 MJ/day) were unaffected by dietary nitrogen levels. After correcting for excreta nitrogen losses during drying, MNR was calculated to be 481 mg N/kg(0.75) /day or 16.2 g N/day (100 g CP/day), and TENL as 310 mg N/kg(0.75) /day or 10.5 g N/day. Failure to correct for the 10.9 ± 4.1% average N losses during drying would underpredict the 'true' MNR by 35% and TENL by 46%. Our estimate for MNR of ostriches predicts a dietary requirement of 6.7% protein. Our estimate of TENL was nearly twice that predicted, possibly reflecting the high fibre content of their diet.

Foraging behavior adjustments related to changes in nectar sugar concentration in phyllostomid bats

Nectar-feeding bats regulate their food ingestion in response to changes in sugar concentration as a way to achieve a constant energy intake. However, their digestive capability to assimilate sugars can limit their total energy intake, particularly when sugar concentration in nectar is low. Our experimental study evaluated the effect that changes in sugar concentration of nectar have on the foraging behavior of the nectar-feeding bats Glossophaga soricina and Leptonycteris yerbabuenae in captivity. We measured foraging behavior and food intake when bats fed at different concentrations of sucrose (5, 15, 25 and 35%wt/vol.). To compensate for low-energy intake, both bat species reduced their flight time, and increased feeding time when sugar concentration decreased. Our results suggest that nectar-feeding bats in nature confront two scenarios with complementary ecological effects: 1) bats feeding on dilute nectars (i.e. ≤15%wt/vol.) should increase the number of flowers visited per night enhancing pollination, and 2) bats feeding on concentrated nectars could spend more time flying, including long- and short-distance-flights increasing food patch exploration for use during subsequent nights, and thus enhancing plant gene flow. Further studies on foraging behavior of nectarivorous bats under natural conditions are necessary to corroborate these hypotheses.

Analysis of the feeding habits of the swallow-tailed hummingbird, Eupetomena macroura (Gmelin, 1788), in an urban park in southeastern Brazil

The aim of this work was to observe and describe the feeding habits and available food resources of the swallow-tailed hummingbird, Eupetomena macroura. The study was carried out in a municipal park located in the city of Taubaté, in the state of São Paulo. The observations took place between December 2003 and October 2004, recording the following variables: 1) the plant species visited for feeding and territorial defense; 2) the kinds of food resources; and 3) the kinds of flight to procure and obtain food. E. macroura visited 12 plant species. For territorial defense, Mangifera indica was the most visited, whereas Malvaviscus arboreus was most visited for feeding. The foliage was the plant part that received the most frequent visits. In order to obtain nectar, the only species visited was M. arboreus; to obtain arthropods, the species most visited were Mangifera indica and Hymenaea stilbocarpa. In the dry season, the hummingbirds visited flowers, whereas in the rainy season they visited leaves to acquire food. The arthropod groups most frequently found on leafy branches were Homoptera and Psocoptera. Finally, the results of the type of flight analysis showed that flight used to capture food was more often observed than were flights to search for food. In conclusion, these observations suggest that E. macroura shows plasticity in feeding behavior, which can help it to persist in urban areas.

Sucrose hydrolysis does not limit food intake by Pallas's long-tongued bats

Nectarivorous bats include very dilute nectar in their natural diet, and recent work with Pallas's long-tongued bat Glossophaga soricina showed that sugar (energy) intake rate decreased at dilute sucrose solutions. However, chiropterophillous nectar is composed mainly of the hexoses glucose and fructose. Because bats fed hexose nectar would save the delay of hydrolyzing sucrose, we hypothesized that sugar intake rate should be higher on this diet than on sucrose nectar. We compared intake response in Pallas's long-tongued bats offered 1 : 1 glucose-fructose (hexose) and sucrose diets at 5%, 10%, 20%, 30%, and 40% (mass/volume) sugar solutions. We also tested the hypothesis that sucrose hydrolysis limits food intake in bats. Intake response was the same in bats fed both types of diet: sugar intake rate was lower in dilute solutions and then increased with sugar concentration. Similar intake responses in both diets indicate that sucrose hydrolysis alone does not limit food intake and support the idea that the burden of processing excess water in dilute solutions plays a major role.

Acute poisoning of silver gulls (Larus novaehollandiae) following urea fertilizer spillage

Two episodes of accidental urea toxicosis are described in wild silver gulls (Larus novaehollandiae) following spillage of fertilizer grade urea at a commercial shipping facility near Perth, Western Australia. In both cases, urea spillage had been seen to contaminate freshwater wash-down pools on the wharves where ships were being unloaded and gulls were seen to be drinking and washing in the pools nearby the spillages. Affected birds were found moribund or dead. Necropsy and histopathological findings were non-specific and consisted of mild to moderate congestion of visceral organs and brain. Analysis of a water sample collected during Case 1 revealed a very high urea concentration of 4.124 mol/l (pH 5.5), and fluid from the proventriculus of two birds had urea concentrations of 382 and 308 mmol/l, respectively. Nine birds were examined during the second episode (Case 2) and, from heparinized heart blood samples collected (n = 5), the mean plasma urea (288 +/- 92.0 mmol/l), ammonia (43.9 +/- 34.2 mmol/l) and uric acid (7.45 +/- 1.99 mmol/l) concentrations were markedly elevated above the reference ranges for all bird species. Proventricular contents (n = 7) similarly contained high concentrations of urea (394 +/- 203 mmol/l) and ammonia (9.3 +/- 15 mmol/l). The probable mechanisms of urea and ammonia toxicity in these birds are discussed.

Do Nectar- and Fruit-Eating Birds Have Lower Nitrogen Requirements Than Omnivores? an Allometric Test

We used an allometric approach to compare the minimum nitrogen requirements (MNR) and the total endogenous nitrogen loss (TENL) of nectar- and fruit-eating birds with those of omnivorous birds. These two parameters were 4× higher in omnivores than in nectarivores and frugivores. In nectarivorous-frugivorous birds, MNR was 152.8 mg N kg−0.76 day−1; in omnivorous birds, it was 575.4 mg N kg−0.76 day−1. Similarly, TENL was 54.1 mg N kg−0.69 day−1 in nectarivores-frugivores, and 215.3 mg N kg−0.69 day−1 in omnivores. The residuals of the allometric relationships between TENL and MNR and body mass were positively correlated, which suggests that a large proportion of the interspecific variation in MNR is explained by variation in TENL. Although our results show that nectar- and fruit-eating birds have low nitrogen requirements, the mechanisms that these animals use to conserve nitrogen remain unclear.

¿Tienen las Aves Nectrarívoras y Frugívoras Requerimientos de Nitrógeno Menores que las Omnívoras? Una Prueba Alométrica

The influence of ambient temperature and the energy and protein content of food on nitrogenous excretion in the Egyptian fruit bat (Rousettus aegyptiacus)

The diets of frugivorous and nectarivorous vertebrates contain much water and generally have high energy but low protein contents. Therefore, we tested the prediction that to save energy under conditions of high energy demands and high water intake, frugivorous Egyptian fruit bats (Rousettus aegyptiacus) will increase both the absolute quantity and the proportion of ammonia in their urine. We also examined whether such changes occur when protein intake is low and water intake is high. We did three feeding trials. In trials 1 and 2, bats were fed one of four liquid diets containing constant soy protein concentrations but varying in sucrose concentration and were kept at ambient temperatures (T(a)) of 30 degrees Celsius and 12 degrees Celsius, respectively. In trial 3, bats were kept at Ta=12 degrees Celsius and fed one of four liquid diets with equal sucrose concentrations but varying protein concentrations. In trial 1, food intake at a sucrose concentration of 256 mmol/kg H(2)O was initially high but decreased to a constant rate with further increases in sucrose concentration, while in trial 2, food intake decreased exponentially with increasing sucrose concentration. As predicted, at 12 degrees Celsius with varying sucrose concentration, both the absolute quantity and the fraction of ammonia in the bats' urine increased significantly with food intake (P<0.02), while the absolute quantity of urea and the fraction of urea nitrogen excreted decreased significantly with food intake (P<0.03). Varying sucrose concentration had no significant effect on nitrogen excretion at Ta=30 degrees Celsius. Varying protein concentration had no significant effect on nitrogen excretion at Ta=12 degrees Celsius. We suggest that Egyptian fruit bats can increase ammonia excretion in response to increased energetic demands, and we calculate that they can save energy equal to approximately 2% of their daily metabolic rate by doing so.

Ammonia Excretion Increased and Urea Excretion Decreased in Urine of a New World Nectarivorous Bat with Decreased Nitrogen Intake

We determined the effect of water and nitrogen intake on nitrogenous waste composition in the nectarivorous Pallas's long-tongued bat Glossophaga soricina (Phyllostomidae) to test the hypothesis that bats reduce excretion of urea nitrogen and increase the excretion of ammonia nitrogen as nitrogen intake decreases and water intake decreases. Because changes in urine nitrogen composition are expected only in animals whose natural diets are low in nitrogen and high in water content, we also measured maintenance nitrogen requirements (MNR). We hypothesized that, similar to other plant-eating vertebrates, nectarivorous bats have low MNR. Our nitrogen excretion hypothesis was partly proved correct. There was an increase in the proportion of N excreted as ammonia and a decrease in the proportion excreted as urea in low-nitrogen diets. The proportion of N excreted as ammonia and urea was independent of water intake. Most individuals were ureotelic (n = 28), and only a few were ureo-ammonotelic (n = 3) or ammonotelic (n = 2). According to our nitrogen requirement hypothesis, apparent MNR (60 mg kg(-0.75) d(-1)) and truly digestible MNR (54 mg N kg(-0.75) d(-1)) were low. A decrease in urea excretion in low-nitrogen diets may result from urea recycling from liver to the gut functioning as a nitrogen salvage system in nectarivorous bats. This mechanism probably contributes to the low MNR found in Pallas's long-tongued bats.

Comparative renal physiology of exotic species

In this article the osmoregulatory, acid-base homeostasis, and excretory functions of the renal system of invertebrates and vertebrates are reviewed. The mammalian renal system is the most highly evolved in terms of the range of functions performed by the kidneys. Renal physiology in other animals can be very different, and a sound knowledge of these differences is important for understanding health and disease processes that involve the kidneys, as well as ion and water homeostasis. Many animals rely on multiple organs along with the kidneys to maintain osmotic, ionic, and pH balance. Some animals rely heavily on postrenal modification of urine to conserve water and salt balance; this can influence the interpretation of disease signs and treatment modalities.

Physiological Constraint to Food Ingestion in a New World Nectarivorous Bat

We investigated the intake response of the nectarivorous Pallas's long-tongued bat Glossophaga soricina to different nectar concentrations to test the hypothesis that bats show compensatory feeding. Bats were offered sucrose solutions between 146 and 1,168 mmol L(-1). Contrary to our expectations, long-tongued bats did not show compensatory feeding, suggesting that volumetric food intake is physiologically constrained. Energy intake was lower at the most dilute solutions (146-584 mmol L(-1)) and then remained relatively constant at more concentrated diets (876 and 1,168 mmol L(-1)). The shape of the observed intake response was very similar to the one predicted by a model dependent on intestinal morphology and in vitro sucrase activity. However, the model predicted higher volumetric food intake at the lower concentrations tested, which suggests that the intestines of the bats were not functioning to their full capacity. Rates of sucrose hydrolysis and water processing probably constrain food intake in long-tongued bats as diets get more dilute.

The effect of cold-induced increased metabolic rate on the rate of 13C and 15N incorporation in house sparrows (Passer domesticus)

Animals with high metabolic rates are believed to have high rates of carbon and nitrogen isotopic incorporation. We hypothesized that (1) chronic exposure to cold, and hence an increase in metabolic rate, would increase the rate of isotopic incorporation of both 13C and 15N into red blood cells; and (2) that the rate of isotopic incorporation into red blood cells would be allometrically related to body mass. Two groups of sparrows were chronically exposed to either 5 or 22 degrees C and switched from a 13C-depleted C3-plant diet to a more 13C-enriched C4-plant one. We used respirometry to estimate the resting metabolic rate (VO2) of birds exposed chronically to our two experimental temperatures. The allometric relationship between the rate of 13C incorporation into blood and body mass was determined from published data. The (VO2) of birds at 5 degrees C was 1.9 times higher than that of birds at 22 degrees C. Chronic exposure to a low temperature did not have an effect on the rate of isotopic incorporation of 15N save for a very small effect on the incorporation of 13C. The isotopic incorporation rate of 13C was 1.5 times faster than that of 15N. The fractional rate of 13C incorporation into avian blood was allometrically related to body mass with an exponent similar to -1/4. We conclude that the relationship between metabolic rate and the rate of isotopic incorporation into an animal's tissues is indirect. It is probably mediated by protein turnover and thus more complex than previous studies have assumed.

Can birds be ammonotelic? Nitrogen balance and excretion in two frugivores

We measured minimal nitrogen requirements (MNR), total endogenous nitrogen loss (TENL) and the effect of protein and water intake on the nitrogenous waste composition in two frugivorous bird species: yellow-vented bulbuls Pycnonotus xanthopygos and Tristram's grackles Onychognathus tristrami. The nitrogen requirements of both species were much lower than expected for their body mass. The two species differed in the composition of the nitrogenous waste that they produced. The grackles were uricotelic, and the chemical composition of their nitrogenous waste products was relatively independent of water and protein intake. In contrast, the bulbuls were`apparently ammonotelic'. Their ammonotely was related to low protein intake and high water flux, and was the result of post-renal urine modification. We suggest two non-exclusive mechanisms for the post-renal modification of urine in these birds: bacterial catabolism of uric acid and reabsorption of uric acid in the hindgut. As uric acid functions both as a nitrogenous waste product and as an antioxidant, birds might benefit from its reabsorption.

Are the low protein requirements of nectarivorous birds the consequence of their sugary and watery diet? A test with an omnivore

Nectar-feeding birds have remarkably low nitrogen requirements. These may be due either to adaptation to a low-protein diet or simply to feeding on a fluid diet that minimizes metabolic fecal nitrogen losses. We measured minimal nitrogen requirements (MNR) and total endogenous nitrogen loss (TENL) in the omnivorous European starling Sturnus vulgaris, fed on an artificial nectar-like fluid diet of varying concentrations of sugar and protein. The MNR and TENL of the birds were similar and even slightly higher than allometrically expected values for birds of the starlings' mass (140% and 103%, respectively). This suggests that the low measured nitrogen requirements of nectar-feeding birds are not simply the result of their sugary and watery diets but a physiological adaptation to the low nitrogen input. We also measured the effect of water and protein intake on the nitrogenous waste form in the excreta and ureteral urine in European starlings. Neither high water intake nor low protein intake increased the fraction of nitrogen excreted as ammonia. Ammonia was excreted at consistently low levels by the starlings, and its concentration was significantly higher in ureteral urine than in excreta. We hypothesize that ureteral ammonia was reabsorbed in the lower intestine, indicating a postrenal modification of the urine.

Sugar and protein digestion in flowerpiercers and hummingbirds: a comparative test of adaptive convergence

Flowerpiercers are the most specialized nectar-feeding passerines in the Neotropics. They are nectar robbers that feed on the sucrose-rich diet of hummingbirds. To test the hypothesis that flowerpiercers have converged with hummingbirds in digestive traits, we compared the activity of intestinal enzymes and the gut nominal area of cinnamon-bellied flowerpiercers (Diglossa baritula) with those of eleven hummingbird species. We measured sucrase, maltase, and aminopeptidase-N activities. To provide a comparative context, we also compared flowerpiercers and hummingbirds with 29 species of passerines. We analyzed enzyme activity using both standard allometric analyses and phylogenetically independent contrasts. Both approaches revealed the same patterns. With the exception of sucrase activity, hummingbirds' digestive traits were indistinguishable from those of passerines. Sucrase activity was ten times higher in hummingbirds than in passerines. Hummingbirds and passerines also differed in the relationship between intestinal maltase and sucrase activities. Maltase activity was two times higher per unit of sucrase activity in passerines than in hummingbirds. The sucrase activity of D. baritula was much lower than that of hummingbirds, and not unlike that expected for a passerine of its body mass. With the exception of aminopeptidase-N activity, the digestive traits of D. baritula were not different from those of other passerines.