Unseen Agression..

I rescued my 1 1/2 yearold border collie Archie when he was just 10 weeks old, since then he has been the most sensitive, alert, intellegent, and friendly dog I have ever met! I began socializing him very well since the day he came into my family.. and he adores all people and got along very well with most dogs, up until now. He is neutered and has a very friendly nature, but I've noticed that recently when Archie is at the dog park playing, if a strange, larger dog comes up to sniff him he'll spin around and snap at the dog. Also, his herding instinct is medium-strong and he'll chase the dogs if they run off, herd around them growling and nipping. This only happens, though, with dogs that are larger than him and usually males. Once, at the dog park, he began full-out herding these two male standard poodles as if they were sheep! I really do not understand this huge temperment change in him. He is such a sweet boy with his buddies, is so so gentle with all of my friends' puppies - I can't imagine why he is suddenly so tense around large dogs now. He grew up with a GSD! I'd really appreciate any advice on how this behavior came out, and tips on how to change the behavior. I have told him a stern "No" and left the dog park with him whenever he has shown agression to another dog.. but I don't think thats doing a lot.

Welcome jigsawyouth. :)

I personally would stop taking him to the leashless dog park.
Not all dogs enjoy a rump with a bunch of strange dogs.
I use to take my female Akita to the Dog Park till she was 3
years old. She loved to play with all the dogs.
I started taking her less, and I noticed
the last time I took her she started guarding me
from the other dogs approaching me.
(She did not attack or anything but her fur would go up.)

Might have been medical, or maybe because I stopped
taking her as often.
Either way since she felt uncomfortable at the dog park,
I stopped taking her.
Now I walk her to the public park or other places.

It's possible that your boy had a bad experience
and no longer feels comfortable/ or at 1.5 years old
he has matured and is no longer is in that puppy
phase where he likes everybody including other dogs.
(instinct takes over)
I would just take him for walks in other public places
where dogs are leashed.
JMHO :)

I have read that it can take a full 2 years before any dog aggression starts to emerge. Out of my four dogs, two are dog aggressive and the other two love other dogs.

It does not sound like "aggression" to me, but just the
herding instinct taking over. Has your dog ever "full out"
attacked another, bitten, drew blood?
I do agree with KYS in that I wouldn't take him to off leash
parks with other dogs. I doubt the other dogs think he's
being aggressive, just weird. Besides, they might not like
being herded.:D

As others have already said... you have a border collie, border collies are natural herders. It is hard to take the natural instinct out of the dog and personally why would you? If you don't like the way the dog acts at the dog park then find another outlet for exercise. I also agree it doesn't sound like agression but more natural instinct which does occur more as the dog matures. At 1.5 years you can expect the maturity of your dog and it's natural instincts to kick in full force. Most border collies that I know do not like other animals coming up from behind since all their herding insticts want the dogs in front.

MHO, appreciate the traits of your breed and learn to adapt to them.

Thanks a lot for all your advice. I know Archie isn't quite dog agressive, but it is becoming a problem when he growls at or nips at even one or two dogs that he doesn't like, not because I believe he would ever attack or draw blood from any other dog, but because I'm afraid he might someday attack a not-so-friendly dog and get hurt. The main reason I take him to the dog park is because I meet my friends and their dogs there, and Archie is buddies with most of their dogs. I guess they'll have to come over from now on. I'm definately going to cut down, or just stop taking him to the park. He gets a LOT of exercise and activity besides that (flyball, agility training, obedience, daily runs, playing with the neighbors dogs, etc) anyway. I think youre absolutely right about quitting the dog park visits. Thank you!