I have a pair of male Jack Russell terriers. One 5 yoa, and 1 is 1 yoa. Raise them both. They weigh about 200 lbs each, but actually only appear to weigh about 20 lbs each. Perhaps the 200 lbs is in their minds.
I originally bought them because I had a lot of copperheads crawling around my place and field rats and mice when harvest time rolled around. Those problems have long since disappeard. I soon found that they are squirrel hunting machines, will tackle the biggest of our plentiful feral hogs, and are not mouthy around the house. If they bark, you need to go look. They will literally hunt and tackle anything, and are totally fearless. If boared they occasionally fight with each other, and that can get pretty rough sometimes. They are people dogs, and show good common sense in the woods. They will get out of sight, but not for long as they will come back and check in. These dogs were born and bred to hunt. They like people but, I don't recomend them for families that have children that are in the habit of mistreating animals. They will love you and protect you, BUT will take up for themselves if abused.
One other thing. CATS. They will take them apart to see how they work but, aren't good about putting them back together when their finished, and loose some of the parts. This you will most likely NOT train that out of a Jack Russel. To them, cats are varmints. There are exceptions I am sure, but they aren't allowed at my house because I agree with Huff and Tank when it comes to cats.