Male dogs are early bloomers, showing signs of sexual behavior - mounting and thrusting both male and female litter-mates - almost as soon as they can walk. While female puppies also mount their litter-mates, this behavior is not as frequent or as common. While it's all perfectly normal, correct your puppy from mounting people, because it's both a nuisance and a dominance challenge.

Females reach sexual maturity at different times, with small breeds attaining it more quickly than the largest ones. A dog's first season, or heat, (and the accompanying vaginal bleeding) can occur anytime from about six months to 24 months, although most dogs come into season for the first time before reaching their first birthday. The point when males become fertile is not so obvious, but they are capable of enthusiastic reproduction from about six months onward.

Every vet always gets many phone calls from puppy owners who are worried because their dog isn't lifting his leg. Male puppies start this scent-marking behavior - which some female dogs also do - as early as four months or so, but there are a lot of variables. A puppy who sees other male dogs lift their legs will likely start this behavior sooner, as may the dog who isn't neutered young.

Eventually, almost all male dogs lift their legs to leave their mark, although some are more enthusiastic about it than others. Some dogs stand on all fours to relieve themselves - leaning forward, but not really squatting as females I do - and then lift their legs in a couple of places. Other dogs seem to believe that every drop of liquid they have is best deposited on a vertical object. All of this behavior is in the realm of normalcy and is nothing to worry about.

Don't panic if your female pup gets to her first birthday without showing signs of being in heat. You may have missed it. Some dogs bleed very little, or they keep themselves especially clean. Chances are if you have a male dog in your house or nearby you know when your dog's in heat, but if that's not the case, you may miss it entirely. Talk to your breeder about the age when most of her females come in season. If nothing has happened by the time you go in to get your pet's annual vaccinations, make a note to discuss the matter with your veterinarian.