Estrus refers to the period when most female mammals are “in heat” which occurs during the breeding season. During this part of the sexual cycle, the female is very much ready to accept the male to mate. Females who are in the estrus period may be easily recognized by their male counterparts due to the released pheromones (airborne compounds that stimulate sexual interest) and other physical cues. For instance, a female dogs’ female genitalia swell and release pheromones when they are ready to mate.