...and, it appears, they'll just go and build better rats.
The current score is Rat 5, Amy and Brian 0. For the past two nights, the Tomcat rat trap has been armed and set out in the garden, and it's been brought in the next morning still unsprung, yet with the bait gone. Brian, examining it, has concluded that the trigger simply isn't sensitive enough; if the rat doesn't actually put weight directly on it, it won't spring. This makes it safer for us, of course, but also easier for him to circumvent; if he sticks his little furry head in there and just licks the peanut butter off the dowel, without actually touching the trigger, the trap doesn't go off. It would appear that the good old-fashioned wooden trap really is the better system; with those, the bait sits directly on the trigger, so there's really no way to get to it without setting off the trap. (Of course, a truly clever rat could figure out some way to spring the trap first and then eat the bait in perfect safety, but we're still holding out hope that we're not dealing with some kind of mutant genius rodent here.)
Brian still has one more idea for a possible modification to the plastic trap that might work. He's removed the dowel that currently holds the bait, instead leaving just a hole in the wood that the peanut butter will get stuffed into. This will force the rat to push farther into the trap to get at it, and possibly get him to step on the trigger in the process. But just in case, we also picked up one of the traditional wooden snap traps. We can bait this one with a bit of sausage rind, which can be actually pushed onto the trigger switch, so Mr. Rat can't possibly get at it without touching the trigger. We plan to set out both traps tonight, and see which—if either—succeeds in snaring the rat. I just hope that if one of them does catch him, it actually finishes the job; I don't much fancy having to go out and put a wounded rat out of its misery.
UPDATE: As of this morning, both traps were still unsprung. The peanut butter from the first trap was gone, and the sausage was still there...with a bunch of ants crawling on it. Which raises the possibility that perhaps it's been ants taking the peanut butter for the past two nights, and the rat has actually removed himself from the scene. This would explain why the traps weren't triggered, and also why we have seen neither the rat himself nor any sign of his presence for the past two days. Maybe he somehow set off that trap the first night we set it and thought, "Whoa, this thing looks dangerous" and scarpered (after first removing and stealing the cup with the peanut butter in it).
If this is indeed the case, it's probably the best possible result; it means we no longer have a live rat in our yard, and we no longer have to worry about disposing of a dead one. The only unfortunate part is that we can't be totally sure whether he's actually gone—or whether he might be back. Brian did think he'd seen a rat fleeing our compost bin a few months back, the last time he went to turn the pile (yeah, we make our compost the lazy way), but he never saw any sign of it again, so we decided to forget about it. If he did, in fact, see a rat at that time, and if it was this same one, it means that the rat is a creature of nomadic and unpredictable habits. So he may be like one of those old movie monsters; gone for now, but you never know when you might see him again. (Fortunately, rats only live about three years, but I think having him periodically pop up over a three year period and then disappear again would be more than enough hassle.)