A common reason can be undiagnosed pain so it is always important to rule out any overt medical conditions with the vet. You can read about pain and problem behaviour here. Pain can exacerbate stress and discomfort and can lead to aggressive displays. It’s sensible to allow a dog to meet each other when there is a physical barrier between them so you can see their body language reactions and the barrier maintains safety. Aggression is often motivated by fear. It is not a desire to dominate and be the ruler/top dog.

Trigger stacking

This is when stress builds up over time. A dog’s reaction to a trigger/stressor can all depend on the age, health and experience of that dog. When a dog is not able to have time to decompress and relax then this can result in them reacting in a way that they may not normally (ie move to a snap/bite). Stressful experiences to a dog may not be noticed by humans but they may include a painful trip to the groomers, then a trip to the vet then an unknown visitor, then building works in the home.

Being able to recognise situations that are a trigger (i.e sharing a sofa) is imperative so we can use management.

Feeding

Feed your dogs separately as often issues can happen around the food bowl. Food is a source of survival for dogs so growling when other dogs approach their bowl is common. Pick up the bowl when both dogs have moved away from it

Going outside and narrow spaces

Doors can be narrow and dogs can have issues with each other in this area as they feel constricted and trapped. Therefore, teaching a wait and letting each dog go out of the door separately is sensible.

Playing

Sometimes dog play can get out of hand and lead to issues if dogs go over threshold therefore rambunctious over stimulating play should be avoided. Sniffing activities can be a good way of lowering excitement. Sprinkle some treats into tiny pieces and let the dog seek them out with their nose.

Toys and beds

Multiple toys is important to ensure that all dogs have adequate resources.  The same thing goes for beds.

Baby gates

These can be a helpful tool to separate dogs, especially during feeding times.

Hot spots

If cuddles on the sofa with you and both dogs results in aggressive displays then making a note of these is imperative so you can use management to avoid them.

Become fluent in canine body language

This ladder of aggression is not necessarily linear and some dogs can skip steps in the ladder if they are stressed or previous attempts to gain space have been egnored.

Listening to the whispers (the green levels) before they turn into shouts is really important.

Chronic stress can result in elevated cortisol. Cortisol can affect learning that happens in a part of the brain called the hippocampus. Stress can also cause atrophy (shrinking) of the hippocampus, and enables fear conditioning to occur in the amygdala (Overall, 2013).