**6. Pheromone concepts**

The field of sexual behavior and pheromonal modulation of sexual behaviors have unique terminology. **Table 4** provides definitions of key terms used in pheromone biology in its widest sense. This may help the reader navigate this area. The broadest term is Semiochemical in which all chemical communication falls. Sexual pheromones are species specific (the cattle sex pheromone is different than the pig sex pheromone). Certainly, the pig uses chemical communication within and between species, for example, when a pig finds a buried truffle or when a plant drives away insects—these between-species forms of chemical communication are not discussed here. We are mostly concerned with the species-specific boar pheromone that stimulates sow reproductive behavior and performance.

The pheromone concept was first described in insects by Karlson and Luscher in 1959 [42] and at the time, they were primarily referring to insect pheromones. Certainly, semiochemicals are found in most plants and animals. Pheromones were previously referred to as ectohormones. This means a hormone that works outside the animal. But pheromones do not meet the definition of a hormone, although the definitions are similar. Karlson and Luscher [42] used the term pheromone to describe a species-specific molecule that is secreted or excreted

**49**

animal scientists.

**Table 4.**

effects in the highly-olfactory domestic pig.

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

Pheromone • Substances that are excreted to the outside by an individual and received by

more behavioral responses [43]

species [45]

warning [47]

organisms [48]

species) [50]

*Definitions of common words in mammalian olfactory communication.*

tion to members of the same species [44] Kairomone • A chemical substance emitted by one species and especially an insect or plant

Allomone • A chemical substance secreted externally by certain animals affecting the

Synomone • An interspecific semiochemical that is beneficial to both interacting

Semiochemical • A chemical substance or mixture released by an organism that

Interomone • An interomone is defined as a semiochemical that acts as pheromone of one

pheromone molecules have not yet been identified [49]

a second individual of the same species, in which they release a specific reaction, for example, a definite behavior or a developmental process [42] • A chemical substance that is usually produced by an animal and serves especially as a stimulus to other individuals of the same species for one or

• A pheromone is an externally secreted signal that sends meaningful informa-

that has an adaptive benefit (such as a stimulus for oviposition) to another

• Kairomones are ligands emitted from one species that generate behavior in another species (such as aversion upon detection by a prey species) [45]

species but elicits physiological responses in a different species where the

affects the behaviors of other individuals (could be between or within

behavior or physiology of another species detrimentally [46] • A chemical that is released by one species that influences the behavior or physiology of a different species. The organism releasing the substance usually benefits. Allomones are a type of semiochemical used in

from one animal that changes the physiology or behavior of another animal of the same species. The concept was adopted rapidly among invertebrate and vertebrate

Most of the work on mammalian pheromones has been done with rodents. The pig has a highly developed olfactory system (see above), but little work has been done recently on pig pheromones including sexual pheromones. We can learn from work on other species so that we can predict possible pheromones and pheromone

An early concept was the bifurcation between priming and releasing pheromones. This dichotomy will be familiar to people who work in pig breeding with gilts and sows. The gilt develops from pre-puberty to post-puberty at around 120– 200 days of age. A pheromone that stimulates the onset of puberty, for example, would be a priming pheromone. Boars certainly cause gilts to have an earlier onset of puberty (see Boar effect above). The molecule(s) that are responsible for priming gilts have not been described, but are likely to be the same as the boar pheromone. Most people might believe at this time that the boar sexual pheromone that causes sexual behavior and stimulates gilt puberty are likely to be the same molecules. A releasing pheromone is one or more molecules that cause a rapid onset of behavior; sexual behavior in this case. Another releasing pheromone might be one that causes pigs to eat, or piglets to nurse, or pigs to fight, or pigs to stop

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

**Word Definition**

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


#### **Table 4.**

*Animal Reproduction in Veterinary Medicine*

demonstrated.

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

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

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,

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

The field of sexual behavior and pheromonal modulation of sexual behaviors

The pheromone concept was first described in insects by Karlson and Luscher in 1959 [42] and at the time, they were primarily referring to insect pheromones. Certainly, semiochemicals are found in most plants and animals. Pheromones were previously referred to as ectohormones. This means a hormone that works outside the animal. But pheromones do not meet the definition of a hormone, although the definitions are similar. Karlson and Luscher [42] used the term pheromone to describe a species-specific molecule that is secreted or excreted

have unique terminology. **Table 4** provides definitions of key terms used in pheromone biology in its widest sense. This may help the reader navigate this area. The broadest term is Semiochemical in which all chemical communication falls. Sexual pheromones are species specific (the cattle sex pheromone is different than the pig sex pheromone). Certainly, the pig uses chemical communication within and between species, for example, when a pig finds a buried truffle or when a plant drives away insects—these between-species forms of chemical communication are not discussed here. We are mostly concerned with the species-specific boar pheromone that stimulates sow reproductive behavior and

showed that the MOE and VNO are fully present at birth in piglets.

sows expose the VNO as well as the MOE to the pheromone.

when they receive a chemical signal in liquid form.

**6. Pheromone concepts**

**48**

performance.

*Definitions of common words in mammalian olfactory communication.*

from one animal that changes the physiology or behavior of another animal of the same species. The concept was adopted rapidly among invertebrate and vertebrate animal scientists.

Most of the work on mammalian pheromones has been done with rodents. The pig has a highly developed olfactory system (see above), but little work has been done recently on pig pheromones including sexual pheromones. We can learn from work on other species so that we can predict possible pheromones and pheromone effects in the highly-olfactory domestic pig.

An early concept was the bifurcation between priming and releasing pheromones. This dichotomy will be familiar to people who work in pig breeding with gilts and sows. The gilt develops from pre-puberty to post-puberty at around 120– 200 days of age. A pheromone that stimulates the onset of puberty, for example, would be a priming pheromone. Boars certainly cause gilts to have an earlier onset of puberty (see Boar effect above). The molecule(s) that are responsible for priming gilts have not been described, but are likely to be the same as the boar pheromone. Most people might believe at this time that the boar sexual pheromone that causes sexual behavior and stimulates gilt puberty are likely to be the same molecules.

A releasing pheromone is one or more molecules that cause a rapid onset of behavior; sexual behavior in this case. Another releasing pheromone might be one that causes pigs to eat, or piglets to nurse, or pigs to fight, or pigs to stop

fighting [39, 51]. None of these releasing pheromone molecule(s), other than the boar pheromone have been described.

What can we learn from other mammalian sexual pheromones? Given that the pig has so many functional receptors, it is likely that if a type of pheromone was described in another mammal that the pig would have a pheromone with a similar effect. Here we summarize the classic reproductive pheromones. Note that each early reproductive pheromone was named after the scientist who first reported the effect.

#### **6.1 The Bruce effect**

Hilda Bruce described what has been called the Bruce effect in 1959 [52]—before the concept of pheromone was established. She showed that when a pregnant mouse was exposed to an adult male, preferably a dominant male, that the pregnant mouse lost her pregnancy. The Bruce effect has been replicated by many investigators and what we know is that each male has a specific major histocompability complex (MHC) class 1 protein that is secreted in its urine. The father of the mouse litter has a given MHC protein. If a new male enters the cage with a different MHC protein, the female is likely to lose her pregnancy (not 100% of the time, but at a significant rate). The male MHC protein binds the VNO in the female mice. It makes one wonder if heat checking with a boar during pregnancy might contribute to a lower farrowing rate.

The Bruce effect has not been clearly documented in the pig. Assuming the Bruce effect is found in pigs, one would change the management of the sow herd. On most farms, pregnant sows experience a live, often dominant, boar walking the aisle to see if any bred sows are now in heat (meaning their pregnancy has failed). That live boar would not have the same MHC as the father of the litter because they are commonly bred by artificial insemination. We know that a small (5–10%) percentage of sows lose their pregnancy from breeding until farrowing. Part of this effect could be due to the Bruce effect. To manage this situation, pregnant sows should never experience a live boar that is not the father of her litter or perhaps is not the boar present during breeding. Keeping in mind that the Bruce effect is mediated by MHC proteins and not the boar pheromone, one can use the boar pheromone to check for return to estrus in pregnant sows without inducing the Bruce effect.

### **6.2 The Vandenbergh effect**

John Vandenbergh first described this pheromone in a paper published in 1975 [53]. He showed that female mice have an accelerated onset of puberty when exposed to an adult male mouse or urine from an adult male mouse. The molecule was thought to be a protein, but the actual molecule had not been described.

Pigs clearly show the Vandenberg effect. Gilts will have a delayed onset of puberty if they do not experience an adult boar. With boar exposure, the onset of puberty is accelerated in gilts [54]. The pheromone molecule(s) that are responsible have not been described. One might predict that the boar pheromone that stimulates sow reproductive behavior and performance [5], is the same pheromone that stimulates the accelerated onset of puberty in gilts. However, if these boar pheromone molecules are responsible for the Vandenberg effect in gilts, then the dose and number of applications required to cause the Effect have not yet been determined.

**51**

*7.2.1 Sows*

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

Whitten described the Whitten Effect in a number of papers from 1956, 1957 and 1966 and 1968 [55–58]. The Whitten Effect states that in a group of postpubertal females, the presence of either other cycling adult males will cause the females to synchronize their estrus (or menstrual) cycles. Likewise, adult females tend to synchronize their cycles over time when they are housed together. The Whitten Effect has no valuable application in modern pork production at this time that we can think of; however, production systems change over time and there could be an application in the future. The Whitten Effect takes weeks or months to have its effect. Therefore, we do believe that when gilts approaching puberty are exposed to a boar, that the number or percentage that come into estrus is not evenly distributed over the 21-day cycle, so this may be happening. Boar exposure may partially

Boars are found on most modern pig farms. They are needed to find sows in estrus when AI is used. Below are reasons to not have boars on the farm. The reasons

The boar costs money to buy and they cost money to maintain. Besides the direct cost of the boar, the boar does not live a good life. They are heat checking sows often and rarely if ever breed. They are often housed in a crate or stall individually for

Boars are dangerous to have on farms. One large farm in the USA reported that they budget \$500,000 per year for boar-induced human injuries. The boar can take a single swipe at a person and damage the person severely. If a boar was very aggressive, they could do great damage to a person. While rare, boars sometimes step on people, or bite people or knock them down if a person stands in the way of the boar

Boars carry disease. While sows move from breeding to gestation to farrowing and back to breeding, the boar resides in the barn for a long time (a year or more). The boar can be a reservoir of disease and continually infect new breeding sows. When a serious disease (foot and mouth, ASF, etc.) is found in a country, they often limit movement of adults in some or all regions. If the farm cannot get live adult boars, and have no access to pheromones, the breeding rates will be very low.

Melrose [35] first suggested Androstenone was the boar pheromone. But we and others have observed that this single molecule was not sufficient to elicit the full sexual behavioral response in estrus sows. This led to the project to seek and discover the complete boar pheromone. This was accomplished by using advanced GC-MS technology to identify three unique molecules that are found in boar saliva and not found in sow saliva [59]. If one examines **Figure 4**, is clear that androstenone alone has only a small effect on sows expressing estrus when they are in fact in heat. But the three molecules together give the largest increase in sow sexual

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

synchronize a group of gilts first estrus.

**7. Benefits to not using a live boar**

include cost, safety and disease control.

**7.2 Pheromone applications in the field**

their own safety and the safety of other sows and boars.

**7.1 General benefits**

and his intended direction.

**6.3 The Whitten effect**

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