Best dog temperament for companion jack russell terrier?

Its not neccessarily the breed that matters (it somewhat does), but more importantly the energy level of the individual dog.

Try to find a dog who has an energy level either similar to (i'm assuming your jack has a pretty high energy level) or a dog with a slightly lower energy level than your current dog.

This is very important since introducing a dog with a higher and more intense energy level than your current dog can create a battle between the two. They will compete to rule out who will be more dominant.

If the new dog has a slightly lower level of energy, this will more likely not be an issue.

(also be aware that larger dogs have generally shorter life spans due to heart and joint issues).

This site has alot of good info on specific breeds.

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