**4.1 The boar effect**

The effect of boar exposure on gilts and sow reproduction has been extensively study across the years. Direct contact with the boar significantly reduces puberty onset in gilts [18–22] and reduce sows weaning to estrus interval. The boar provides sows and gilts with olfactory, tactile, visual, and auditory stimuli that together create a maximum response. Below we discuss the effect of each individual boar stimuli on sow and gilt reproduction.

#### **4.2 Visual, auditory and tactile systems**

Pigs have well developed olfactory, tactile, auditory and visual systems. Most of the work on the pig focuses on the pig olfactory sense. Pigs have been used in biomedical research to study the auditory system. The auditory and somatosensory (touch) parts of the brain have been mapped in the pig [23]. The pig auditory system is understudied. In one paper where the auditory, visual and somatosensory regions were mapped in pig reared indoors or outdoors, the authors showed different neuron structures in the outdoor pig in both auditory, visual and somatosensory regions [24]. The auditory neocortex was especially different with diverse housing systems.

Surprisingly few recent studies have been done on the pig visual system. Dudley Klopfer was a psychologist at Washington State University in the 1950s to about 1980. He studied the pig visual system using operant conditioning methods. He found that pigs could see colors. His work was published in a detailed proceedings paper in 1966 [25]. Ewbank [26] and his group put black contact lenses on pigs which made them temporarily blind. Pigs that could not see, had normal fights and formed a dominance hierarchy. In the world, pig do not need their sight to function, even though their eyesight is about the same as humans.

**45**

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

and somatosensory systems are at least as developed as humans.

What we can conclude from the limited work on pig senses is that their olfactory system is much more developed than humans (see below) and their auditory, visual

Meese and Baldwin [27] removed the olfactory bulbs in pigs and this did not change their establishment of a dominance order. When pairs of pigs were tested, they fought the same with or without their olfactory bulbs. However, when the group size increased to 3 or 4 pigs, the bulbectomized pigs were at a disadvantage. For reproduction, removal of the olfactory system had large negative effects on

The boar olfactory stimulus has been widely studied of known mammalian pheromones. During the 1960s, androstenol (5alpha-Androst-16-en-3alpha-ol) and androstenone (5alpha-Androst-16-en-3-one), two steroids secreted by boars' submaxillary salivary glands, were thought to be the boar pheromone. Multiple studies have found that these two steroids have a major role on gilts puberty onset. For instance, puberty age was significantly greater for gilts with their olfactory system inhibited by chemical or mechanical means [29, 30] and for gilts exposed to a sialectomized boar [31]. It is thought that direct contact between boar and sow is necessary to transfer the boar pheromone from boar saliva to the female snout [30]. Although these two steroids are responsible for a significant part of the boar effect on puberty onset, when applied as an individual olfactory stimulus, they were not as effective as the boar [18, 21]. Thus, it was suggested that boar saliva must contain additional analytes that together with androstenol and androstenone are acting as a multicomponent primer pheromone or that other boar stimuli are necessary for the boar pheromone to have a full effect. Recently, it was found that, quinoline was another boar specific salivary molecule (**Figure 4**) [5, 32]. The mixture of androstenone, androstenol, and quinoline induced more sexual behaviors in weaned sows than the mixture of androstenone and androstenol [5]. This finding might explain the lack of response of sows and prepubertal gilts when exposed to androstenone

**4.4 Early work on sow and boar preferences and sensory systems**

Boars, being a non-discriminating breeder, will investigate sows independent if they are in estrus or not but sows will only be interested in a boar when they are in estrus. When sows are in Proestrus, they will seek a boar. This seeking behavior

Early works showed that sows would only seek the boar when they are in estrus [33] and that the boar could not detect a sow in estrus. This turned out to be only partially correct. Boars can learn the smell of an estrus sow. It was reported that the boar could not tell an estrus sow from a non-estrus sow [32, 34]. Some boars were found to be able to find an estrus sow while others could not [32, 34]. This could be a learned behavior, or some boars may have better olfactory acuity than other boars.

Signoret's classic early research [4] (**Table 3**) on boar induction of sow sexual behavior is often cited in textbooks and seminars. In his work, he found that the boar odor was the best single stimulus to induce sexual behaviors in estrus sows. He applied back pressure to estrus sows with no odor stimulation and found that 59% of the sows showed standing reflex. If farms found only 59% of the sows that are in heat, they would not be profitable. The goal is to find 100% of the sows in estrus. With a live boar across a fence, Signoret found that 97% of the sows were detected

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

**4.3 Olfactory systems**

reproduction [28].

and androstenol alone.

intensifies when sows are fully in estrus.

This remains to be determined.

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

What we can conclude from the limited work on pig senses is that their olfactory system is much more developed than humans (see below) and their auditory, visual and somatosensory systems are at least as developed as humans.

### **4.3 Olfactory systems**

*Animal Reproduction in Veterinary Medicine*

detection by farm workers [17].

**4.1 The boar effect**

Altrenogest to synchronize their estrus cycles.

stimuli on sow and gilt reproduction.

**4.2 Visual, auditory and tactile systems**

even though their eyesight is about the same as humans.

**4. Sensory system impacts on the estrus cycle: the boar effect**

Replacement gilts are selected based on their growth rate, body composition, and their mother's reproductive success [17]. In general, gilts selected as replacement are moved from the growing facility to the gilt development unit (GDU) within the breeding farm when they are around 150 days old. To accelerate the onset of puberty, gilts in the GDU are often exposed to live boars. Gilts can have direct contact with a boar or indirect contact through pen fencing. Usually, groups of vasectomized boars are introduced into gilt's home pen for at least 20 min per day. Boar should not be housed in the GDU unit since gilts will be habituated the boar olfactory, visual, tactile, and auditory stimuli and this could decrease effective heat

When boars are introduced into the gilt pen, farm personnel will check gilts for estrus behavior and vulva changes described above. Daily boar exposure will induce estrus in most gilt within 10–20 days [17]. Gilts in heat are then moved to breeding stalls so they can habituate to the new environment and are breed in their second estrus. The term heat-no service (HNS) is commonly used to identify gilts that had their first estrus but were not breed. At the time of first service gilts should weight 135–150 kg and have a back fat of 12–18 mm [16, 17]. After 23 days of boar exposure, gilts that have shown no sign of estrus, can be hormonally treated to induce estrus by use of PG600. Gilts that did not come into heat after 28 days of boar exposure are usually culled from the breeding herd [16]. Gilts can be treated with

The effect of boar exposure on gilts and sow reproduction has been extensively study across the years. Direct contact with the boar significantly reduces puberty onset in gilts [18–22] and reduce sows weaning to estrus interval. The boar provides sows and gilts with olfactory, tactile, visual, and auditory stimuli that together create a maximum response. Below we discuss the effect of each individual boar

Pigs have well developed olfactory, tactile, auditory and visual systems. Most of the work on the pig focuses on the pig olfactory sense. Pigs have been used in biomedical research to study the auditory system. The auditory and somatosensory (touch) parts of the brain have been mapped in the pig [23]. The pig auditory system is understudied. In one paper where the auditory, visual and somatosensory regions were mapped in pig reared indoors or outdoors, the authors showed different neuron structures in the outdoor pig in both auditory, visual and somatosensory regions [24]. The auditory neocortex was especially different with diverse housing

Surprisingly few recent studies have been done on the pig visual system. Dudley Klopfer was a psychologist at Washington State University in the 1950s to about 1980. He studied the pig visual system using operant conditioning methods. He found that pigs could see colors. His work was published in a detailed proceedings paper in 1966 [25]. Ewbank [26] and his group put black contact lenses on pigs which made them temporarily blind. Pigs that could not see, had normal fights and formed a dominance hierarchy. In the world, pig do not need their sight to function,

**44**

systems.

Meese and Baldwin [27] removed the olfactory bulbs in pigs and this did not change their establishment of a dominance order. When pairs of pigs were tested, they fought the same with or without their olfactory bulbs. However, when the group size increased to 3 or 4 pigs, the bulbectomized pigs were at a disadvantage. For reproduction, removal of the olfactory system had large negative effects on reproduction [28].

The boar olfactory stimulus has been widely studied of known mammalian pheromones. During the 1960s, androstenol (5alpha-Androst-16-en-3alpha-ol) and androstenone (5alpha-Androst-16-en-3-one), two steroids secreted by boars' submaxillary salivary glands, were thought to be the boar pheromone. Multiple studies have found that these two steroids have a major role on gilts puberty onset. For instance, puberty age was significantly greater for gilts with their olfactory system inhibited by chemical or mechanical means [29, 30] and for gilts exposed to a sialectomized boar [31]. It is thought that direct contact between boar and sow is necessary to transfer the boar pheromone from boar saliva to the female snout [30]. Although these two steroids are responsible for a significant part of the boar effect on puberty onset, when applied as an individual olfactory stimulus, they were not as effective as the boar [18, 21]. Thus, it was suggested that boar saliva must contain additional analytes that together with androstenol and androstenone are acting as a multicomponent primer pheromone or that other boar stimuli are necessary for the boar pheromone to have a full effect. Recently, it was found that, quinoline was another boar specific salivary molecule (**Figure 4**) [5, 32]. The mixture of androstenone, androstenol, and quinoline induced more sexual behaviors in weaned sows than the mixture of androstenone and androstenol [5]. This finding might explain the lack of response of sows and prepubertal gilts when exposed to androstenone and androstenol alone.

#### **4.4 Early work on sow and boar preferences and sensory systems**

Boars, being a non-discriminating breeder, will investigate sows independent if they are in estrus or not but sows will only be interested in a boar when they are in estrus. When sows are in Proestrus, they will seek a boar. This seeking behavior intensifies when sows are fully in estrus.

Early works showed that sows would only seek the boar when they are in estrus [33] and that the boar could not detect a sow in estrus. This turned out to be only partially correct. Boars can learn the smell of an estrus sow. It was reported that the boar could not tell an estrus sow from a non-estrus sow [32, 34]. Some boars were found to be able to find an estrus sow while others could not [32, 34]. This could be a learned behavior, or some boars may have better olfactory acuity than other boars. This remains to be determined.

Signoret's classic early research [4] (**Table 3**) on boar induction of sow sexual behavior is often cited in textbooks and seminars. In his work, he found that the boar odor was the best single stimulus to induce sexual behaviors in estrus sows. He applied back pressure to estrus sows with no odor stimulation and found that 59% of the sows showed standing reflex. If farms found only 59% of the sows that are in heat, they would not be profitable. The goal is to find 100% of the sows in estrus. With a live boar across a fence, Signoret found that 97% of the sows were detected

#### **Figure 4.**

*Sow sexual behavioral response to Androstenone (An), Androstenol (AL) and Quinoline (Q ) alone or in combination. Note that androstenone increased estrus sow sexual behavior by 13.5% while all three molecules increased sow sexual behavior by over 63%. The N for this study was 947 sows [5].*


#### **Table 3.**

*Early research on sensory system impacts on sows showing estrus.*

in estrus. Further, he found that 81% of sows were detected in estrus when they were moved and heat checked in a pen containing the boar odor. This is better than 59%, but not as good as the live boar. Later, when Androstenone was used, Melrose [35] found 78% of the sows in heat—similar to Signoret's finding with the boar odor. Scientists and producers thought at that time that Androstenone was the boar pheromone. However, why would the fence line contact be better than the odor of the boar or Androstenone alone? This is because more than Androstenone (e.g., other molecules) is needed to induce sexual behavior in the sow (see details below).
