Question

1.- Why can ruminant herbivores use ammonia or urea (non-protein nitrogen) as a valuable nutrient? What happens to post-gastric fermenting herbivores, such as horses, when they eat a diet containing high levels ammonia? Why?

2.- Hummingbirds are very small birds (about 10 grams) that are extremely active in the wild and often spend winters in relatively cool places like Davis. Humans like you stay in Davis during the winter too.

A) The maintenance energy requirements of adult hummingbirds (kcal per gram body weight per day) are very high compared to humans. Why is this?

B) Why can hummingbirds that live in Davis meet their amino acid requirements eating foods that have very low levels (5) of protein?

Answer 1

Answer 1. Nonprotein nitrogen (NPN) compounds are utilized by bacteria in the rumen of ruminant herbivores. Studies show that during the normal fermentation process in the rumen, these compounds are broken down into ammonia. Microorganisms residing in the rumen combine the products of carbohydrate metabolism with this ammonia to form amino acids and hence, proteins.

In post gastric fermenting herbivores utilization of NPN as a nitrogen source is less efficient mainly because the site of fermentation is at the end rather than at the beginning of the digestive tract. Dietary urea is utilized inefficiently by horses because it is hydrolyzed to ammonia in the small intestine. Much of the absorbed ammonia is excreted in the urine and wasted.

Answer 2.

A. The maintenance energy requirements of adult hummingbirds are high because:

• Hummingbirds have one of the highest metabolic rates compared to the metabolic body size of many animals on earth.
• Their heart rate is up to 1260 beats per minute and breathing rate is about 250 breaths per minute even at rest.
• At rest, oxygen consumption is about 4 ml O2/g/h. And, during the flight, hummingbird oxygen consumption per gram of hummingbird muscle tissue is approximately 10 times higher than that seen for human athletes.
• B. Most birds are uricotelic, however, nectar-feeding birds are an exception to this which excrete significant amounts of ammonia. Even though ammonia is toxic, its excretion may be facilitated in animals with high intake and putout of water (because it is highly water-soluble ). Floral nectars contain very little protein but are rich in carbohydrates and water. Thus, nectar-feeding birds like hummingbirds are faced with the dual challenge of meeting nitrogen requirements while disposing of large amounts of water. This peculiar diet of hummingbirds suggests two hypotheses:
  • these birds must have low protein requirements, and
  • when they ingest large quantities of water their primary nitrogen excretion product may be ammonia.