There are three main groups of nouns. Those ending with -o are almost always masculine, those ending with -a feminine. The third group ends with -e and can have any gender (il pesce, il latte, la automobile, la posizione, etc.) Aside of these three groups there are irregullar words, especially those coming from foreign languages (il sofà).
"Nel" goes in front of nouns that use the article "il", like "nel mondo"; "nello/a" in front of those who uso the article "lo/la" like "nello zoo, nella scuola". Also, "nello" becomes " nell' " in front of nouns like "uomo", that use the article "lo/la" but starts with a vowel: nell'uomo, nell'aria :) hope i was helpful!
"The tigress eats at the zoo." is not only correct but more accurate, as the Italian la tigre is femenine. My answer is thus correct.