Rucete ✏ Campbell Biology In a Nutshell
Unit 7 ANIMAL FORM AND FUNCTION — Concept 44.2 An Animal’s Nitrogenous Wastes Reflect Its Phylogeny and Habitat
Animals must eliminate nitrogenous waste generated from protein and nucleic acid breakdown. The form of nitrogen waste—ammonia, urea, or uric acid—varies by species and reflects evolutionary history, habitat, and water availability.
1. Ammonia
- Highly toxic and must be excreted in large volumes of water
- Common in aquatic animals (e.g., most bony fishes and invertebrates)
- Diffuses readily across membranes and body surfaces into water
- Requires minimal energy to produce but impractical for terrestrial life
2. Urea
- Less toxic and highly soluble in water
- Produced in the liver by combining ammonia and CO₂ (urea cycle)
- Excreted by mammals, amphibians, and sharks
- Requires moderate water to excrete and more energy than ammonia
- Amphibians often switch from ammonia (as tadpoles) to urea (as adults)
3. Uric Acid
- Least toxic and excreted as a semisolid paste with minimal water loss
- Primary waste in insects, land snails, birds, and reptiles
- Most energetically expensive to synthesize
- Ideal for arid environments and shelled eggs, where waste must be stored safely
- Can cause issues like gout in humans and uric acid stones in Dalmatians
4. Evolution and Environment
- Waste type correlates with water availability and egg structure
- Reptile and bird eggs use uric acid to avoid toxic buildup inside the shell
- Mammals and amphibians excrete soluble wastes that can be diluted or transported by the mother’s body
- Endotherms produce more nitrogenous waste due to higher metabolism
- Carnivores excrete more nitrogen than herbivores or omnivores due to high protein diets
In a Nutshell
Nitrogenous waste type reflects an animal’s evolutionary lineage and ecological context. Ammonia is cheap to produce but requires water. Urea offers low toxicity with moderate energy and water cost. Uric acid conserves water but demands high energy. Each form represents a trade-off shaped by habitat, diet, and physiology.