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I have enjoyed reading and learning from your UCSB science line web site. I work with Jean-Michel Cousteau’s Ocean Futures Society (OFS) located in Santa Barbara. I teach tropical biology programs through OFS and the subject came up with a student a while back about the advantage of uric acid excretion in birds. Here is the question: since bird embryos must live with their waste in the egg, isn’t a big evolutionary advantage of excreting uric acid a reduction in self pollution of nitrogenous waste in the egg. Since water is limited and there is no way to get rid of ammonia or urea, uric acid is logically the best and only safe way to deal with such waste in the egg.

I’ve looked on line and in some text books and not been able to find an answer to this question. Your help would be greatly appreciated.

Question Date: 2016-10-31
Answer 1:

So you are correct that birds primarily produce their nitrogenous waste in the form of uric acid, rather than urea or ammonia. Ammonia is toxic in part because it raises the pH of the bodily fluids. Urea actually requires far less energy to produce than uric acid, but uric acid has the advantage of reducing water loss and is less toxic. The advantages of reducing water loss and lower toxicity are probably some of the reasons birds produce uric acid rather than urea.

However, self-pollution of nitrogenous waste is not an issue for birds because they store their nitrogenous waste in a reservoir called the allantois. The actual embryo is in a separate compartment called the amnion and the uric acid takes a one-way trip to collect in the allantois. Therefore, there is no waste accumulation is direct contact with the embryo.

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