At around six months of age your puppy will be turning from a juvenile into an adolescent. This can bring about changes in body shape as growth slows down and the body starts to fill out. Long legged gangly puppies will adopt more elegant adult proportions over the next six months.
This is also the time that male puppies start to think about the opposite sex, albeit in a somewhat confused way. For some dogs, cushions, furnishings and even their owner's legs can provide an outlet for their frustrations and mounting behavior is common at this age. While annoying and embarrassing, this behavior will normally settle down again after a couple of months but some dogs will continue to mount objects after this time. Neutering your male dog is a good solution in the vast majority of cases. This routine operation involves removal of the testicles - the main source of the hormones that cause these behaviors.
The other change you might notice as your male dog grows up is that he will start lifting his leg to urinate. This is quite normal but a small number of dogs may also start to mark their territory by lifting their leg and urinating on objects in the home. Aggression towards other male dogs may also occur during these 'teenage' years. Again, the vast majority of these types of problems are solved by neutering and many owners choose to have their male dogs neutered at around six months before any of these potential behaviors can be demonstrated.
Some young male dogs may start to act to defend things that they feel are their own - this may be your home and garden, a treasured toy, or even the members of your family. While it is comforting to know that your dog wants to defend your home and family, such protective behavior can cause problems if your dog also starts to react aggressively. If your dog growls when approached while eating or playing with a toy, these should be taken as warning signs that the territorial instinct has become too well developed. There are a number of behavioral techniques that can be used to help these dogs and your vet can provide advice or may suggest a referral to a pet behaviorist. Territorial aggression is also sometimes helped by neutering.
There is one other type of behavior that you might notice that will only be demonstrated from time to time. If a female dog is 'in season' in the local area you may find that your male dog goes off his food, becomes sad and listless; or restless and anxious to roam. It is not uncommon for male dogs to make active attempts to escape to find the target of their devotion. This can result in dogs becoming involved in accidents or getting lost. Extra vigilance and a little pampering will help him through this troubled time. Of course, like teenagers, some male puppies are more difficult than others but thankfully most will scrape through adolescence with everyone's sanity more or less intact.