My folks have 2 dogs - Jack Russell's. We had the older dog about 10 years ago and he was already fully grown. He'd been abused and we don't know how old he really is or what the real background was. He was always very skittish - any sudden move and he'd jump back. He didn't eat too much either.
Anyway, about 4 years back we got a puppy as we had the idea that the older dog wasn't going to live too long. To be honest, getting the puppy really brought the older dog out of his shell - stopped his nervousness and meant that he really started eating properly for the first time since we had him. Probably caused him to live to now.
Now the "pup" is pretty much fully grown and there's a real dominance problem between them as he is in effect now the "alpha". Most of the time they are fine, but if there's visitors they tend to compete for attention and this can cause them to fight. They have elaborate little performances when they eat - the younger one will tuck in while the older one waits until he's just finished before starting. The younger one will finish his then pester the older one. The older one wont allow the younger one to butt-in, but eats about 80% of his meal then turns around and walks back about 6 foot. The younger one then eats what's left and while he is eating the older dog can pass him to get past. Really weird, but they do the same every single day.