perhaps your dogs are different than mine ..........my dogs are all born here , they've seen, been around, the turtles here their entire lives, they know they belong here , they won't break into a pen to get a turtle , but I've had them break into a pen to get a squirrel , chipmunk , mole , mouse , ect... that had gotten in there .... only to end up with a turtle instead , the results were devastating to the turtle and me ....... my dogs do keep my turtles safe from raccoons , possums , cat's , anything that eats turtles , but I've found out the hard way that my turtles needed protected from the dogs , my pens while uncovered became more and more dog proof with every bad experience .........possibly your dogs are less prey driven , mine have always been partial to little animals , sadly even little animals they knew ...... don't know about your bulldogs , but I can tell you from experience a turtles shell is not protection from a big mouthed dog . a Bordeaux dogue can crunch up an 8-9 inch turtle as quick as they can get a good grip on it ..........