How much ground would a ground hog hog...?

We have in our yard a forty-foot by seventy-foot fenced area in which our dogs go out to play in good weather. Today they were VERY anxious to run out to the pen, so I zoomed them out there and shut the gate.

Then I found out why they were so excited. A groundhog was in the pen. The dogs had a lovely time chasing it, cornering it, and taking turns charging it and scuffling with it, before I got them back out of the pen.

This is the first time I've ever seen a live, wild groundhog up close. I learned that they're fairly quick, but not as quick as my dogs. I learned that they have quite sharp teeth (not surprising in a rodent); this one managed to nick my dog Sarah on the lip. And I learned that a cornered groundhog erects all its hair and chatters its teeth, which is pretty intimidating even in a small generic-looking mammal.

Now that everyone's pulse rate is back to normal, all appears to be well. The groundhog apparently found his or her way back out of the pen, either over or under the fence or through the gate I left wide open for it. And the dogs seem okay; the cut on Sarah's lip, after bleeding freely for a few moments, has closed up and disappeared.

Whew. Now all I need is a groundhog detector I can switch on before I take the dogs out to the pen again.