Sterilizing your pet is an essential part of responsible pet ownership. Pet euthanasia and overpopulation are an ongoing problem and sterilizing pets helps to save lives down the track. Sterilization also reduces a number of serious health problems in later life that are expensive to treat and potentially fatal.
Mating for a male dog is a basic instinct along with hunting for food and protecting the pack. Unsterilized male dogs can become aggressive and anxious. As an entire male they can become stressed and frustrated that they are unable to mate. They are more likely to run away, fight and display other antisocial behaviour like destruction of property.
If an unsterilized pet, following its natural urges, escapes from a property, it not only unwittingly contributes to the population problem but also runs the risk of seriously injuring itself. The cost of property damage and serious injury will far outweigh the one off cost of sterilization not to mention the heartache of losing a pet.
Sterilizing a male dog lessens their urge to roam, mark territory and become a dominant pack member. It also eliminates their chance of developing testicular cancer.
Sterilizing your dog will not directly change its personality, the fact that it does not have to seek out a mate just gives it one less need to think about and can naturally lead to your pet displaying a calmer and more settled manner. It will not lessen its other natural instincts.
Sterilization is often done at a time when the initial puppy growth and activity stage is slowing down which gives the perception of the sterilization process causing weight gain. Weight gain or loss is exactly the same as it is in all species and is directly related to food intake, food type, genetics and exercise.
Having an unsterilized female dog is messy and attracts unwanted attention from any male dog in the vicinity. (Not just the handsome ones!) The most prolifically spread myth about female dog sterilization is that they should have a litter 1st. This generally comes from people projecting their own feelings of parenthood onto animals. There is no truth in it at all.
Sterilizing your female dog prevents it developing uterine or ovarian cancer and greatly reduces the chance of breast cancer. As with male dogs sterilization does not cause weight gain. Factors such as diminishing growth stage and diet are the most likely contributors to any gradual weight increase (see above).
Sterilization at the recommended age inhibits undesirable behavioural patterns developing. These include aggression, dominance, territory marking (like the furniture) humping and straying. On the whole it makes for a better behaved, more well adjusted domestic pet.