In other species, sexual activity is a matter of the right smell and unlimited access. Pheromones are what make a dog in heat irresistible to potential mates across long distances. Pheromones are also what cause a single female palm sawfly in a cage to draw more than 2,000 mates a day from miles away. And, in swine, male pheromones cause females to instantly assume a posture for mating.
Pheromones too are a critical factor in sexual behavior and reproduction in humans, even though the chain of events and stimuli that lead to sexual activity is far more complex. Men naturally produce pheromones that send important chemical messages to prospective mates. Women receive and process these messages through either their olfactory system (which processes smells), an accessory system called the vomeronasal organ (VNO), or both of these. Scientists aren’t yet sure how active the VNO is in humans, although it is highly active in other species.