My older dog is suddenly very stressed, nervous and going for my younger dog.

[TheKitchenWitch]

Ddog2 has been with us a year. They've always got on really well, we've had no issues or problems whatsoever.

Until now.

Ddog1 has suddenly, over the last 2 weeks, become stressed and very nervous, especially whenever any of us goes near ddog2, and she is attacking her.
Everything seems to set her off - us coming home and both dogs greeting us, sitting on the couch, everything. Ddog2 is fighting back, so we've had a few cuts and bruises on both sides. No proper biting - so the injuries are nasty cuts and scratches caused by sharp teeth - but it's horrible and I need to stop it.

The problem doesn't seem to be about resources (toys or food), but I can't figure out what exactly it is about.

Any ideas on what to do gratefully received.

First port of call would be a vet check. Any sudden change in behaviour should always be checked out by a vet. He/she could be in pain or feeling poorly and therefore have less patience than usual.

You say it's not resource related, but I would say both situations you mentioned - greeting owners on their return and selecting a place on the couch to sleep are both potentially resource guarding areas.

If the vet check is clear, I'd advise getting a qualified behaviourist involved to observe the situation and give you a clearer picture of what's going on. Have a look on the APBC and COAPE/CAPBT sites to see if you can find one in your area and bear in mind that if you dogs are insured, some policies will cover a certain number of consultations.

How old are they and have one or both been spayed?

Just a quick update to say they appear to have settled the matter: it looks like the younger was making a bid for top position (or whatever you want to call it) and older dog was having none of it.
They are now back to being completely normal again, though Ddog2 is being more "respectful" of ddog1, very much like she was at the beginning when she first came.
I'll keep an eye on them, though, of course.

The reason I asked about ages and spaying is because it is not unusual for bitches to fight when one is coming into season.

How old are they?

Sounds like your elder dog was putting a teenager in her place, rather than anything to do 'position'. Older dogs will put up with puppy shenanigans and give them dispensation for all sorts of things, but when they reach a certain age they lose their puppy licence so-to-speak, all bets are off and they get roundly told off and reminded of appropriate adult behaviour etc.

My pup is a large breed, so still not mature, but was 1 on Sunday and my older boy definitely puts him in his place these days, where until recently he'd let him get away with all sorts of naughtiness.

Do keep a close eye on them though as bitch fighting can get very nasty and can't always be easily resolved. If it starts up again please take professional behavioural advice before it escalates.

Neither is spayed, one is 8 and the other is 1.5. The younger one has had 2 seasons up to now. They synchronised almost immediately but there didn't seem to be any problems while they were actually on heat. Lots of mutual licking and cleaning.

More then likely the older dog is in some sort of pain, and he is protecting himself. Animals hide pain well especially around the owners as that is what they are trained to do. Hope you find out what's going on

I agree what moosemama has said is most likely given younger dog's age - older dog is setting down some boundaries for the adolescent now she's becoming an adult. Absolutely also look out for any signs of pain in your other dog though. And do separate them when you can't supervise them if you still have any concerns at all as bitch fighting can get very nasty.

Can I ask how did your older dog put the younger one in his place moosemama? And how often does it occur?

I'm keeping a close eye on them, and have a vet's appointment next week anyway for shots, so will get her to give ddog1 a proper checkup.

They are acting normally again now, although as I said, I notice that ddog2 is slightly more eager to show she's no threat - she's turning sideways, looking away, going down in her play-with-me position. But she doesn't seem fearful or upset.
Ddog1 seems much calmer and more relaxed.

Slightly different situation TKW, given that mine are both males and the older one would rather hide in his bed than stand up for himself generally speaking.

In his case it's a sudden very sharp loud bark, with an air snap if his bark isn't heeded and occasionally a lunge with the air snap, but no contact - although he does have to wrestle himself free from my pup's hooliganistic Lurcher leaping sometimes.

If pup ignored both the bark and air snap, with some dogs this might lead to a scuffle, although actually injury from this - unless accidental, rather than deliberate (think scratches, rather than punctures) - would be unlikely if it is just an adult putting a juvenile in their place.

As it is, pup is always so shocked that our older boy has even barked at him that he immediately steps back and looks sheepish.

Think an adult using their 'voice of doom' on a child that really has gone past the boundaries - the child wouldn't be in danger, it's all about correction rather than intimidation or actual fighting.

Have a look on YouTube, there are plenty of videos on there showing different types of canine interaction. You should be able to find something showing adult/juvenile correction and compare it to genuine aggression.

A really useful book, if you want to learn more about canine body language, is Turid Rugaas' On Talking Terms with Dogs. It will really help you start to read their behaviour and intent a little better and is a really easy, quick read.

I've read that book, which is why I noticed ddog2's head turning and looking away etc. since they sorted it.
Unfortunately when they got going, it was all happening too fast for me to be able to tell what was going on.

We've had no intentional injuries - so no bites, no punctures, but lots of scratches (and they both get covered in dog saliva, which makes their fur stand up like it's been gelled really badly!). Ddog1 has always done that sharp bark thing if ddog2 has taken things too far, but this time it really did seem that ddog2 wasn't listening and the situation was stressing ddog1 a lot.

I'm off to YouTube canine interaction now.

There's a great Facebook group called "observation skills in dog training". They have strict rules, so do read them before getting stuck in, but it's a fantastic resource with people uploading video and photographs of dogs and members trying their hands at interpreting them.