Change in behaviour

Question
Until last week, we had two rottweilers - Ted a 9 year old neutered male and Lulu a four year old neutered female.  They were 'pussycats' and were together 24/7.  Unfortunately we lost Ted to cancer and now Lulu is 'changing'.  She is becoming more aggressive to human visitors - something she never did before - but not in a nasty way - just more dominant than before.  She is also moping about a lot - she used to be lively all the time, but is now very quiet.  We can understand the quietness - she is obviously pining for her friend - but are concerned about the new show of guarding symptons - that presumably was Ted's job before.  Any ideas?

Answer
The Rottweiler is, by nature, a dominant, guarding breed.  The female is, by far, the most aggressive (this varies breed to breed.)  Your female is making up for the loss of her backup, her male companion.  Imagine her confusion at the loss of Ted; she has no idea what happened to him, and this may very well have formed a sudden, conditioned fear response to whatever occurred when he was last taken out of the home.

Her depression will extinguish in a few weeks; however, this is not necessarily merely a reaction to the loss of her male companion!  When a dog becomes more quiet than usual, s/he is also OBSERVING the environment and reestablishing his/her rank.  You need to make Lulu a great deal more emotionally secure by becoming the highest ranking in your household.  This is accomplished by training her to one obedience behavior, such as "sit" (but using another word) using positive reinforcement training.  When you have a 100% reliable response to your command (indoors), which can take several weeks, make Lulu earn everything: eating, treats, going in/out, being petted, etc.  This will elevate you  psychologically, engage her intellect and teach her how to make choices that reward, and make her far more emotionally secure.  I suggest you purchase Paul Owens' book, "The Dog Whisperper" (NOT Cesar Milan!!) which will instruct you on positive reinforcement training as well as give you some insight on dog psychology.

In the meantime, protect Lulu from visitors!  Do not allow her to greet them; she's not psychologically in any position to handle this and the more she learns about dominance and aggression, the worse this will get.  Once she has a firmly established response to one obedience command, train another, and use the first to establish her need to "sit" when visitors are present, rather than attempt to control them.  Any further questions, please repost.