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

different olfactory organs of the pig. Only two of the five olfactory organs described in mammals have been described in the pig. **Figure 5**. Shows the five olfactory organs described in rats. Of these five organs, only the main olfactory epithelium (MOE) and the vomeronasal organ (VNO) have been described in the pig (**Figure 6**). The Grueneberg ganglion (GG) is the sensory organ that senses alarm pheromones in mice. Scientist believe that pigs may also have alarm pheromones [39], but they have not been isolated, nor has the GG been found in the pig. Little is known about the septal organ (SO) or the chemical sensory cells of the Trigeminal Nerve in the pig (or in other species). We do believe that the MOE receives molecules in an aerosol, while the VNO receives molecules in liquid form. The GG and SO may also need an aerosol because they are in the nasal airway where aerosols pass as the animal breathers or sniffs.

#### **Figure 5.**

*Animal Reproduction in Veterinary Medicine*

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).

*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* 

**Source Odor source Sows showing estrus**

Boar odor in pen 81% Fence-line contact with live boar 97%

Signoret, 1975 [4] No boar odor 59%

Melrose [35] Androstenone 78%\*

*increased sow sexual behavior by over 63%. The N for this study was 947 sows [5].*

*Note that Androstenone (\*) was not as effective as fence-line contact with a boar.*

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

The pig is a species with one of the highest numbers of functional olfactory genes [36]. To understand pig pheromone biology, one must understand the

**46**

**Figure 4.**

**Table 3.**

**5. The pig olfactory system**

*Chemosensory epithelia in the rat nose. GG, Grueneberg ganglion; MOE, main olfactory epithelium; SO, septal organ of Masera; TG, trigeminal system/nerve; NPal, nasopalatine duct; NPhyr, nasopharyngeal duct. Arrows represent the direction of air flow. From Dauner et al. [37].*

#### **Figure 6.**

*Ignacio Salazar pictures of the pig VNO (right) and the main olfactory bulb (left) with the AOB shown. VNO pictures are from Salazar et al. [38]; olfactory bulb histology is from Salazar, personal communication.*

In addition to the olfactory organs, the nasal mucosa contains several olfactory binding proteins (OBPs). Patricia Nagnan-Le Meillour [40] has done the most recent work in pig's OBPs. When the nasal epithelium receives a chemical signal, that signal can bind an olfactory receptor directly, or more commonly for bioactive chemicals, it binds an OBP and it is the OBP odorant complex that activates the olfactory receptors. It is likely that, before pheromone exposure, a small amount of OBPs are present in the olfactory epithelium mucosa. Pheromone exposure increases OBPs synthesis. Thus, we speculate that a second pheromone exposure 30**–**60 min after the first exposure could have a large effect on olfactory perception because more OBPs will be present to carry the odorants to the olfactory receptors. However, this still needs to be experimentally demonstrated.

In **Figure 6**, we show excellent anatomical histological pictures of the pig MOE and the VNO by Salazar [38] (personal communication). He and his laboratory they showed that the MOE and VNO are fully present at birth in piglets.

The VNO is thought to be the olfactory organ in which pheromones are perceived. But we know now that this is not always the case. One of the boar sexual pheromone molecules is sensed by the MOE [41]. The other three molecules may be sensed by the VNO or the MOE or any of the other olfactory organs not yet described in the pig. Our behavioral observations of the sow when she experiences a liquid containing a pheromone show that they chomp (see **Table 1**). We believe this behavior is analog to the flehmen behavior in other animals and that, by doing this, sows expose the VNO as well as the MOE to the pheromone.

The VNO receives chemical signals from liquids. Some mammals (except humans and some primates) show flehmen (lip curl) behavior when they are drawing liquid chemical signals into the VNO. An example is when the bull licks cow urine and draws it into the VNO. It is likely that more than one olfactory organ is needed to sense complex pheromones that are mixtures of molecules (like the boar pheromone). The pig is not well-known to show Flehman, but they may Flehman when they receive a chemical signal in liquid form.
