Answer:
In the excerpt, the signal word "however" lets the reader know that alligators and crocodiles are being contrasted.
Explanation:
Comparing means looking for similarities between two things. Contrasting, on the other hand, means looking for differences between them. As we can see, the sentence is showing a difference that exists between alligators and crocodiles. Even though those animals are, generally speaking, quite similar, the former are only found in the US, the latter are found all over the world. The adverb "however" is used to make that contrasting idea even clearer. There would be no reason to add a "however" to the sentence if we were presenting similarities between the animals.
