*Understanding Sow Sexual Behavior and the Application of the Boar Pheromone to Stimulate… DOI: http://dx.doi.org/10.5772/intechopen.90774*

behavior was developing in the USA. Altmann studied female pig sexual behavior because she used pigs in her conditioning studies, and she wanted to be sure if sows were or were not in heat when she trained them on an operant task. In 1941, she reported several aspects of sow sexual behavior that we know to be true. She said there was a 18–23 day cycle among adult females. She indicated that domestic sows (unlike wild boar) bred year-round, although they often had a "silent heat" in warm weather. She found external signs of estrus to be not reliable indicators of estrus; these included, vaginal mucous, swelling of the vulva and rectal temperature changes. We recently confirmed her observations with quantification of anatomical changes (see below). She indicated behavior and activity were the best methods to determine heat, but a combination of methods increased accuracy.

The next scientist to publish studies in pig sexual behavior was Jean Pierre Signoret from France. His research on sow sexual behavior in the 1960s and early 1970s were summarized in a chapter Signoret co-wrote in Hafez's 1964/1968/1975 editions of the book "The Behaviour of Domestic animals" [4]. The picture of sowboar sexual behavior in that chapter has been widely used to describe pig's sexual behavior. In that picture, he lists sequences of boar-sow behaviors that are shown in **Figure 2**. The sequence of sexual behaviors between a sow and boar include mutual head sniffing, and then the boar sniffs the sow's rear, he then pushes and may lift the sow from the side, then he sniffs and licks and pushes on the sow's rear. These olfactory and tactile behaviors are accompanied by grunting by the boar and, if the sow is in estrus, she will be silence or she will make soft rhythmic grunts in response (she will squeal if she is not in estrus as a form of objecting to the boar behavior). After touching, smelling and licking her rear, he will mount her and if she is fully in estrus and showing "standing reflex or locked up" behavior, he will copulate with her.

**Figure 2.** *Drawings of sow-boar sexual behaviors from Signoret [4].*

*Animal Reproduction in Veterinary Medicine*

is thought to be required.

*behaviors when the person applies pressure to her back.*

**Figure 1.**

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

*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* 

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.

**2. Sexual behavior in the domestic pig: early studies and preferred** 

While pig farmers have observed sexual behavior for millennia, the earliest scientific description of sow sexual behavior in the scientific literature was in 1941 by Altmann [3]. Altmann was a psychologist at the University of Chicago when animal

**38**

**terminology**

We present in **Table 1** our definitions of sow sexual behaviors that are observed while a sow is in a breeding crate. Note that because her movements and boar interaction are restricted while in a breeding crate, she will not express the same number of behaviors as in the wild. When the sow expresses intense standing still, we say she is "locked up." A sow can stand still for a few seconds at any time and not be locked up or in estrus. Being locked up is the primary and only reliable sign that a sow is in estrus.

The boar is non-discriminating when deciding which animal or object to mount. The boar will attempt to mate sows not in estrus and they will mate an object shaped roughly like a sow (e.g., a boar semen collection dummy)—anything shaped like a cylinder that stands still will be mounted by an adult boar.

If the sow is not in full estrus, she will avoid the boar. If she experiences an adult boar while not in heat, then she will vocalize (squeal) and be aggressive towards him to express her objection. In the author's personal observations of feral pigs and outdoor pigs allowed to mate naturally, a sow not in estrus is very aggressive towards adult males. With the matriarchal social structure in nature, the sow is clearly dominant to the boar, except when she is in estrus.

In the period of proestrus, the sow is becoming interested in the boar, but she will still not be willing to stand still for mounting. In this period, she will seek a boar, interact with him and allow for mutual sensory exploration. When these behaviors are observed and the BPT is administered, the sow will not stand still. This period is referred to on the farm as a sow that is coming into estrus but not fully in estrus (proestrus).


**41**

onset [6].

pastured pigs.

**3. Overview of the estrus cycle**

**3.1 Hormonal changes**

into metestrus and diestrus.

*Understanding Sow Sexual Behavior and the Application of the Boar Pheromone to Stimulate…*

In a breeding crate, the most noticeable sexual behaviors expressed by a sow are locked up and pricked ears (see **Table 1** for definition). Stock people that breed sows will be familiar with these sexual behaviors. However, we recognize that there is considerable variation in sexual behaviors within genetic lines and among genetic lines (and breeds). Some individual sows show more extreme sexual behaviors and some show only mild signs of being in estrus. Video in the following link (https:// youtu.be/DdgxK1U8ZUo) shows a sow with a strong sexual behavioral response after boar exposure. Video in the following link (https://youtu.be/tspB7RkviBo) shows the same sow that expresses sexual behavior after application of the new boar pheromone [5]. Note the locked up behavior, sniffing and chomping by the sow.

Commercial farms must train workers who perform artificial insemination (AI). Pig breeders must understand sow sexual behavior to achieve success. With training and experience, the AI technician can achieve very high breeding and farrowing rates, but rarely 100% of sows are bred and remain pregnant. Training workers is challenging because sows vary widely in their sexual behaviors. AI workers must be trained with the specific genetic line of pig they are expected to breed. They must understand what normal sexual behavior is for that genetic line and then how to modulate that behavior to achieve high levels of reproductive success. Training is challenging because sows in breeding crates are not able to express the full repertoire of sexual behaviors that they express in an open area and boars are not able to stimulate crated sows through olfactory and tactile senses as they can for penned or

The sow estrus cycle is of 18–24 days long [6], with the median and mode of 21 days. Sows are polyestrous animals. This means that with the appropriate nutrition, good health, and the proper environmental conditions, sows will cycle through the year. The sow estrus cycle is only stopped by pregnancy or lactation, and possibly old age. In sows, lactational anestrus is due to the inhibition of the GnRH pulse by the suckling stimulus [7]. Gilts reach puberty at 5 or 6 months old. For sows, the first estrus post-weaning takes place 4–7 days after weaning for most, but not all

During the estrous cycle, sow's hormones change. The hormonal changes mark the different stages of the estrus cycle. The sow estrus cycle is divided in two main phases, follicular and luteal. These phases are further divided into four stages. The follicular phase is divided into proestrus and estrus, while the luteal phase is divided

For our discussion, we will assume that the first day of estrus is day 0 of the cycle (**Figure 3**). The onset of estrus is mainly caused by an increase in estrogens. Sow estrus usually last 40–60 h (from day 0 to day 1–2 of the cycle) [6] but some sows can be on estrus for longer. Sow will only show sexual behaviors and accept the boar during the estrus stage of the cycle. During estrus, estrogen, Follicle stimulating hormone (FSH) and Luteinizing hormone (LH) secretion peaks. During estrus, ovulation is caused by the LH surge. Ovulation occurs 30–40 h after estrus

Almost immediately after ovulation, sow will enter the luteal phase. During the metestrus stage of the luteal phase (days 2–5 of the cycle), the follicle tissue will start its development into a corpus luteum. This process is called luteinization [6]. The end of metestrus and the beginning of diestrus is marked by the end of the

sows (some have a longer or shorter wean to estrus interval).

*DOI: http://dx.doi.org/10.5772/intechopen.90774*

#### **Table 1.**

*Definitions of sow sexual behavior when she is in estrus.*

*Understanding Sow Sexual Behavior and the Application of the Boar Pheromone to Stimulate… DOI: http://dx.doi.org/10.5772/intechopen.90774*

In a breeding crate, the most noticeable sexual behaviors expressed by a sow are locked up and pricked ears (see **Table 1** for definition). Stock people that breed sows will be familiar with these sexual behaviors. However, we recognize that there is considerable variation in sexual behaviors within genetic lines and among genetic lines (and breeds). Some individual sows show more extreme sexual behaviors and some show only mild signs of being in estrus. Video in the following link (https:// youtu.be/DdgxK1U8ZUo) shows a sow with a strong sexual behavioral response after boar exposure. Video in the following link (https://youtu.be/tspB7RkviBo) shows the same sow that expresses sexual behavior after application of the new boar pheromone [5]. Note the locked up behavior, sniffing and chomping by the sow.

Commercial farms must train workers who perform artificial insemination (AI). Pig breeders must understand sow sexual behavior to achieve success. With training and experience, the AI technician can achieve very high breeding and farrowing rates, but rarely 100% of sows are bred and remain pregnant. Training workers is challenging because sows vary widely in their sexual behaviors. AI workers must be trained with the specific genetic line of pig they are expected to breed. They must understand what normal sexual behavior is for that genetic line and then how to modulate that behavior to achieve high levels of reproductive success. Training is challenging because sows in breeding crates are not able to express the full repertoire of sexual behaviors that they express in an open area and boars are not able to stimulate crated sows through olfactory and tactile senses as they can for penned or pastured pigs.
