I have never seen a male dog change due to sterilization (I'm not saying it doesn't happen, I'm just saying in my experience, I have never seen it happen). If the dog is used to wandering, sterilization will not break the habit but it will save unwanted puppies and I strongly recommend it in both males and females and believe it to part of being a responsible pet owner.
As I understand it, when a male is sterilized, the amount of testosterone is reduced marginally and it's smell is less offensive to full males and consequently a full male will not pick a fight as readily with a sterilized dog.
For males, it also reduces that chance of testicular cancer and prostate problems associated with testosterone. For females, they are unlikely to develop breast cancers and, if speyed before their first season, do not develop pyometron (infections of the uterus). (K. Seksel & G Perry - "Anxiety & Aggression" 2005)
As an aside, a male Chihuahua has as much testosterone as a male Great Dane - never underestimate a dog!
If you do not want to breed from your dog, it is advisable to discuss sterilization with your Vet.
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