If writing for a book publisher in the US the appropriate reference would be the Chicago Manual of Style and monkeys would be preferred. (You might find an online manual under that name.)

If writing for a US newpaper, the Associated Press Style Manual might give the appropriate rule, but since nobody ever proof reads the newspapers any longer you could do just about anything you care to. It being rather immaterial in that case I won't bother to check that reference unless I'm a little more awake once I've had my evening nap.

There are numerous other standard references, and most people who do much writing will have their own favorites. The pedants will arrive shortly, I'm sure, to offer their opinions.

John