Why does a mammal go tothe energetic expense of making urea from
ammonia rather than simply excreting ammonia, as do bacteria?

Hi casey17

Larger land animals need to convert the ammonia as it is too toxic.
Ther is information on the process, known as the Urea Cycle, here;

http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/U/UreaCycle.html