They must be wild, look at those leashes

"Real"working JR's are pretty tough.(most you see around have been bred for pets). If they feel like it they can chew through a normal lead in seconds. I think you may know these terriers are still used to hunt underground for Fox and Badgers ,both in this country and more so in Europe. In Germany they use packs of JR's to hunt wild Boar.In Africa packs have been used to keep lions off properties. They have also been know on their own to tree black bears, a friend of mine in NH had a very small JR that in its day treed three bears !. So I have to keep them on double chains as several times they have cut loose and dispatched several Mt Lions and tried to run off and breed with some very large coyotes.... most embarrassing when you are half way up a climb.(Tonge in Cheek)........PS The chain is also used to link two terriers together about 9" apart allowing them to walk free from a lead but prevents them going down when not wanted an animals den hole.