My current fort is almost completely cut off from the rest of the world, with the only two methods of getting in (when the drawbridge is up) being flying and "jumping" around ten z-levels or actually jumping over a one square gap (not currently implemented as far as I know). However, that did not prevent an alligator from getting inside and killing one (maybe two) of my fifteen dwarves. Although I do believe I know how the gator got inside, I thought I might want to get some second opinions. Also, if there is sufficient demand I will (try) to upload the fortress onto the map archive for observation. Do note that the gator managed to make it into a high traffic construction zone without being noticed where it killed one dwarf and caused another to jump into the river (also the gator is currently mildly injured).
I have a pretty good idea as to how the gator made it inside the fortress, but would like to make this a "competition" of sorts, with the winner receiving this +cat skull totem+ I was waiting to be created when this little incident occured.
To be fair I shall post some of the possibilities:
A: Alligators can jump across small gaps.
B: Alligators are suicidal/kamikaze dive bombing relatives of the dreaded carp.
C: It is possible for alligators to pull themselves out of the river (without a stairway of any sort) through an unlocked hatch on the z-level above the water.
D: It's raining gators! While the weather is clear...
E: The alligators tag teamed in a wrestling match in order to throw one of their own into the fortress.
F: Anything else you come up with.
So, have fun!