Male Versus Female
Many dog owners tend to anthropomorphize, or attribute human characteristics to their dogs. For instance, it is a commonly held belief that girl dogs are sweet, gentle, nurturing, and loving, while boy dogs aren't very affectionate, are rough and tumble, and are more likely to be protective and/or act aggressively. Nothing could be further from the truth.
Male dogs are often the more loving and tolerant of the two sexes, particularly if they are neutered, or altered. Altering a male dog accomplishes many things. First, the dog is not on the search for females in season, which makes it less likely that he will want to escape and roam the neighborhood. Second, the dog is not as likely to mark in your home. Third, if a dog has any propensity toward being aggressive, neutering can often significantly lessen this undesirable trait. And finally, neutering eliminates the risk of certain cancers.
The majority of stray dogs picked up by animal-control officers are unneutered males. Additionally, the majority of dogs hit by cars have not been neutered.
Female canines are less likely to be tolerant of any human action they find annoying or threatening. In other words, a female may be quicker to snap for smaller infractions. (Think of it in terms of a mother dog feeling it's her obligation to correct her puppies.) Females can also be less tolerant than neutered males of other dogs. Though Chihuahuas love to be pack animals (particularly when the other dogs are Chis), when a female dog decides she needs to be the ruler of the pack, she can be quite obstinate about it.
Altering or spaying a female prevents many reproductive cancers, unwanted pregnancies, and the mess of a dog in season. Altering does not mellow a female in the same way that neutering an aggressive male can.
Which to Choose?
So, which sex is for you? Chihuahua breeders often recommend looking at the temperament and characteristics of each individual puppy rather than focusing on one sex. If, however, you already own a Chi and want to add another, the best choice (the one with the least likelihood of starting a long-term argument) is a Chi of the opposite sex — both altered, of course!