**1. Introduction**

In 2018 the world had over 700 million pigs with over half of them in China [1]. With the recent spread of African Swine Fever (ASF) in China and other parts of Asia, the pig population has rapidly declined. At the same time, movement of breeding animals is restricted in the most pig-dense continent and so rebuilding pig numbers is a challenge. When diseases like ASF break out, and breeding animal movement is restricted, then some sows must be bred without the use of adult males. Successful pig breeding is the key to maintaining and restoring pig numbers and the world's supply of pork.

In cattle herds and poultry flocks, successful breeding often takes place without any adult males. In contrast, most commercial pig farms have adult male pigs (boars) on site to maintain optimum breeding success. Thus, the pig is unique among common food animals in requiring the presence of adult males in

#### **Figure 1.**

*Outdoor systems with natural mating (left) are less common today. Sow in a breeding crate or stall (right). The breeding crate is the most common indoor breeding system. Note that in the outdoor, natural mating system, the sow and boar can fully interact. However, in the breeding crate, a person applies back pressure in the presence of a boar to induce sexual behaviors when a sow is in estrus. If the sow is not in estrus, she will not show sexual behaviors when the person applies pressure to her back.*

commercial production. While some sows will express sexual behaviors without a boar, to get the majority of sows bred, in current commercial production, the male is thought to be required.

In less developed countries, pigs may roam free and are harvested as desired, but these represent a smaller percentage of the world pig inventory over time. Some commercial pigs are kept outdoors in managed systems. The outdoor production system (**Figure 1**, left) represents a small part of the world's pig herd. Most pigs in the world used for pork production are kept on commercial farms using an indoor sow housing system. The most common method of housing the breeding sow is in a crate or pen (**Figure 1**, right) [2]. The breeding crate is large enough (often 0.6 m × 2.1 m) to accommodate the body of the sow but the breeding crate does not allow the sow to turn around or to express her full repertoire of behaviors. The method of keeping breeding sows (outdoor, indoor in pens or crates) clearly impacts their ability to express natural sexual behaviors and the breeding crate reduces the likelihood of successful mating. Sexual behaviors are best observed in freely-moving sows and boars, but the reality of commercial pork production is that sows are in a breeding crate in which they may have limited fence line contact with an adult male—and this makes training of workers challenging. A better understanding by farm workers of sow and boar sexual behaviors will meaningfully improve reproductive success.

The objectives of this chapter are first to review the basic behavioral biology of sexual behavior and reproductive success in the domestic pig. Secondly, this chapter will summarize classic literature on sow and boar sexual behaviors and will review both applications of pheromone technologies and mechanisms by which pheromones can improve reproductive performance in the pig herd. To have a better understanding of pig sexual behaviors and of the impact of the boar sexual pheromone on female reproduction, we will also review pig olfactory system anatomy and physiology.
