It is true that a dog that has undergone only a tubal ligation will still have normal heat cycles and attract males during these brief periods. Matings could occur without any fear of a pregnancy resulting. However, in the real world, this is rarely what happens. Today, most of us live inside homes or apartments with our pets. This greatly limits any contact between male and female unless they happen to have the same owner.
Dogs find themselves outside only briefly for exercise or elimination.
Even if the pets are allowed to roam freely in a back yard, these areas are typically fenced or the animal is tied or restrained. The period of a heat cycle in which the female is receptive to the male and will allow him to breed is referred to as estrus. Estrus typically lasts only 4 to 13 days, therefore mating could only occur 8 to 26 days a year. A additional problem with this scenario is that while the female is in estrus for the 4 to 13 days, her desires during that time are nearly constant. Physiological and behavioral patterns press upon her to mate.