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That's a small dog thing - I think they have a complex because they're so little, so they need to prove that they're just as good as the big dogs! My parents have a jack russell & a german shepherd, and watching those two play is hilarious. The German Shepherd will actually go into a drop and let the Jack Russell jump all over him and bite at his ears & mouth and everything. The thing is, the Jack Russell is over 10 years old, and the German Shepherd isn't even 2 yet!
haha. My parents neighbours had a jack russell, mum and dad had a GSD, and they both used to do the same thing. Just goes to show German Shepherds are sooks!