Which excretory product is NOT found in mammals?

Analyze the question: We need the excretory product that mammals do NOT excrete.

Concept: Excretion is the removal of metabolic waste. Mammals excrete:

• Carbon dioxide through the lungs (from respiration)
• Urea through the kidneys, in urine (from deamination of excess amino acids, converted in the liver)
• Water through the skin (sweat), lungs (breath) and kidneys
• Mineral salts through sweat and urine

Ammonia is highly toxic. It is the main nitrogenous waste of many aquatic animals, such as bony fishes and tadpoles, which can flush it out quickly with plenty of water. Mammals do not excrete it as a major product. The liver converts it to the much less toxic urea through the ornithine cycle.

Step by step:

• B. Carbon dioxide is excreted by mammals via the lungs, so it is found.
• C. Mineral salts are lost in sweat and urine, so they are found.
• D. Urea is the main nitrogenous waste of mammals, so it is found.
• E. Water is lost in urine, sweat and breath, so it is found.
• A. Ammonia is not a normal excretory product of mammals, because it is converted to urea first.

Answer: A. ammonia

Tip: Remember how nitrogenous waste is linked to habitat:

• Ammonia: aquatic animals (fishes)
• Urea: mammals and amphibians (adults)
• Uric acid: birds, insects and reptiles

Strictly, trace amounts of ammonia do appear in mammalian urine, but in exams "ammonia" is the expected answer for a product not normally excreted by mammals.