new dog aggresstion

Question
I recently got married and moved my 2 yr old male Black Lab (Vader) into my husband's home with his 10 yr old female Black Lab mix (Grace) & 14 yr old female German Shephard (Janice).  The dogs have lived together now for 6 months and get along fine.  

Our roommate's boyfriend began bringing his 12 yr old female heinz 57 mix - she looks like a dingo! - (Zoey) over.  She had been staying w/ us for over 2 weeks and was doing fine, she is very subdued and kept mostly to herself.  

Yesterday, Zoey, Vader, & Grace (Janice doesn't get around as much as the 'younger' ones) were outside.  My roommate's boyfriend heard what was not a normal bark and looked outside to see Zoey on her back, Vader with his bite on her back side, and Grace on her throat.  The dogs backed away when he ran outside to help, and Zoey ended up needing one stitch in her backside, and was very shaken up.  

We had been jokingly commenting over the previous several days that Grace was getting mad that "another woman" was taking Vaders attentions away from her.  Could this be what actually happened?  

Grace has no history of aggression, nor does Vader.

This evening, my sister broght her 2 yr old chocolate lab (Eli) over to play with Vader.  Due to the cold, they were inside some of the evening and Grace barked and growled and got up close to them when they played, which is mildly rough (but never aggressive) play.  I wouldn't have normally thought too much about this since Grace will often bark with excitement, but it got me worried that she really is jealous or just getting crotchety in her old age?

Can we still have our dog.guests over?  We love doggy play time, but don't want to put anyone (man or beast) in danger.

Any insight would be greatly appreciated!

thank you,
Rho

Answer
Hi Rho

social hierarchies between dogs are very complex and also fluid - the situation changes all the time depending on resources, personalities and arousal levels.  arousal levels (basically levels of excitement but not necessarily good) are important as they determine the speed and force of response.  Rough play between dogs will always cause elevated arousal levels in all dogs present and others will often join in impulsively and things can quickly turn aggressive.  Think of two young (human) boys playing a game, one accidentally hurts the other, he retaliates and very quickly they are having a scrap!

Dogs tend not to get jealous of one another - that is a human trait.  However, they do experience competition over resources and also have a strong drive to establish a hierarchy with dogs they are with a lot.  I imagine this is what happened in the yard with poor Zoey.

My advice is that you can continue to have other dogs round but be sensitive to their needs - don't just assume they all want to socialize, feel comfortable or should play together.  Being indoors is difficult as the space is limited and there are also resources to guard (people, beds, toys, even water bowls) which will put them on edge, especially as two dogs live there already.

The fact that Zoey is subdued indicates that she would rather be shut in a separate room (or remain at home), and perhaps grace should be shut away when Eli comes over.  See what works for you but you certainly need to think about how much fun each individual dog is having.  Ideally take them out for a run together where they can play and burn of energy with lots of space.

All the best

Lucy