The American Naturalist, a University of Chicago Press - Journals Division journal, specifies that AASTeX should be used to typeset submissions; however, it does not provide any BibTeX BST bibliography style file. That means that authors still have to manually manage their reference list rather than using BibTeX and a central BIB bibliography database.
The BST: I've tried to remedy this by creating amnatnat.bst, a natbib-compatible BibTeX Bibliography STyle (BST) file for the American Naturalist.
As far as I can tell, this BST will generate a properly formatted BBL file that should not require modifications. Note that American Naturalist requires authors to upload their BBL files and NOT their BIB files (i.e., they don't have the ability to run BibTeX on the server).
ONE CAVEAT: If you have titles that contain colons in them, you will have to manually (either in the BIB or the BBL) change the case of the letter following the colon. For example, if "Some Title: Subtitle" is the title, change the "S" in "Subtitle" to "s". A feature/bug of BibTeX is that its sentence case keeps letters following colons capitalized. This does not match the sentence case definition used by many journals, which considers the words after a colon being at the middle of a sentence rather than the start of a new sentence. As far as I know right now, there is no easy way to change the BST file to do this automatically for you.
FYI: This BST file was generated using amnatnat.dbj (for more details, see amnatnat_full.dbj) and amnatnat.patch.