Insights of pheromones in domestic animal reproduction

Introduction

Communication can be understood as the process of transmitting information from a sender to a receiver. Many species rely on chemical signals produced by bodily glands for such communication (Dharani et al., 2024; Górski et al., 2026) [13] . Communication in animal is the process by which one animal transmits information that influences the behaviour of another. (Bradbury and Vehrencamp, 1998) [7] . Mammals communicate with one another using many different signals such as facial expressions, body posture, sounds, touch, smells etc. (Górski et al., 2026). Chemical communication is the most ancient way for exchange of information between organisms. These chemicals for animal communication are known as semiochemicals or info-chemicals. Pheromones are volatile chemical signals that are crucial in social communication among animals (Dharani et al., 2024) [13] . The term 'Pheromone' was introduced by Karlson and Luscher in 1959. It derives from the Greek words 'pherein' (to carry) and 'hormon' (to excite). Pheromones are also referred as 'ecto-hormones' as they are chemical messengers emitted from the body of a sender and received by a receiver of the same species to which they release a specific reaction (Grammer et al., 2005; Brennan and Zufall, 2006) [9,20] .

History

The study of pheromones and chemosensory communication has evolved through several important historical milestones. Some of them are listed here. The vomeronasal organ (VNO) was first described by Ludvig Jacobson in 1813, and is therefore commonly referred to as the Organ of Jacobson (Brennan, 2001) [8] . Meredith Halpern (1976) clarified function of VNO as it processes pheromonal signals. The term Flehmen was introduced by Karl Max Schneider in 1934. It originates from a German word meaning 'to show the upper teeth', describing a characteristic behaviour seen in many mammals during pheromone detection. In 1954, Peter G. Schinckel described the 'Ram effect'. Hilda Bruce (1959) discovered the Bruce effect in female mice. J. G. Vandenbergh (1975) described the Vandenbergh effect in prepubertal female mice. Wesley K. Whitten (1962) reported the Whitten effect in female mice. D. W. Patterson (1968) isolated the first mammalian sex pheromones in pigs. In 1979, M. Goodwin isolated hydroxybenzoate from vaginal secretions of estrus bitches and showed its role in stimulating mounting behaviour in male dogs. David Izard (1982) [24] identified acetic acid and propionic acid in feces and urine of estrus cattle, which trigger sexual excitations in bulls. In 2003, Pageat and Gaultier identified Bovine Appeasing Pheromone (BAP) and Dog Appeasing Pheromone (DAP) from mammary gland and sebaceous gland secretions, which are known to have calming effects and reduce anxiety in young animals. Rajnarayan and Archunan in 2011 [2] identified 4-methylphenol (strong male attractant) and 9octadecenoic acid (induces mounting behaviour) as pheromonal compound in buffalo. Būda et al. (2012) identified p-cresol in the urine of estrus mares as a sex pheromone responsible for triggering the Flehmen response in stallions.

Source of pheromones

Pheromones are secreted in animals by various bodily fluids and excreta (urine, faeces, cervical and vaginal secretions) as well as exocrine gland secretions. Urine is a prime medium that is useful in mammalian chemical communication (Rivard and Klemm, 1989) [44] . Some pheromones are secreted by specialized glands or through particular body regions viz, flank, special scent glands, peri ocular region, perineal region, areas around the external genitalia, anal glands (Sankar and Archunan, 2004) [46] . [18] Cat Sebaceous gland of cheek and perioral region Pageat and Gaultier (2003) [15,36]

Vomeronasal organ

The vomeronasal organ (VNO) is an important part of the accessory olfactory system which helps to detect pheromones and chemical signals involved in social and reproductive communications. Anatomically, it is located between the vomer bone and hard palate. It is a blind-ended tubular structure located bilaterally at the base of nasal septum which connects nasal cavity via narrow vomeronasal duct. In crosssection the VNO has a typical crescent shape with a thick pseudostratified sensory epithelium medially and a thinner and ciliated non-sensory epithelium laterally. Vomero nasal receptors are continually being replaced from basal stem cells because of its limited life span (Brennan, 2001) [8] .

Production, transport, excretion and perception of pheromones

Pheromones are synthesized from various body tissues and secreted into bloodstream. In circulation, they bind with specific carrier proteins known as lipocalins. Function of lipocalins is to bind and transport small hydrophobic molecules such as pheromones, odorants and steroids. The binding of pheromones with lipocalins protects it from degradation in the bloodstream and facilitates their transport to different body fluids. The perception of pheromones in animals begins at the mucosal epithelium of the VNO, where the odorant binding proteins (OBPs) are present. OBPs are a carrier protein which are present in both the main olfactory system and the vomeronasal organ. This dual presence enables the animals to detect a wide range of pheromonal signals that influence social and reproductive behaviours. OBPs possess multiple binding sites where pheromones are bind. These OBPs delivers the pheromone molecules to the chemosensory neuron receptors. These neurons encode the type and strength of pheromone signal. The signal is transmitted as a strong electrochemical impulse to brain and stimulates hypothalamus and ultimately results in a neuroendocrine response (Pevsner et al., 1985) [37] . Active pump mechanism is an alternative method used by domestic animals to detect pheromones. This mechanism is mainly associated with a behavioural response called Flehmen reaction. During this reaction, animal stands rigidly with extended neck and curling of upper lip to expose teeth. This specific movement alters the pressure in the nasal cavity, and helps vomeronasal organ to detect pheromones.

Classification of pheromones 1) Territorial Pheromones:

Mammals rely on pheromones to define their home range or territory. In addition to marking territory, animals use urine and faeces to mark pathways, resting grounds, feeding areas and sleeping sites. Dogs clearly scent mark and overmark urinary signals, which may relay information like age, sex, general health and when the animal was present at that place (McGlone and Hendersen, 2022; Dharani et al., 2024) [13,29] .

2) Alarm Pheromones

In response to attack by natural enemies, aphids secret droplets of fluid from cornicles, (located on the upper, posterior surface of the abdomen). These secretions contain an alarm pheromone that induces behavioural responses in other receiving insects. β-farnesene and germacrene A are primary component of this pheromone in aphids (Vandermoten et al., 2012) [51] .

3) Aggregation Pheromones

Aggregation pheromone is defined as a pheromone which helps to form an aggregation that serves for protection, reproduction or feeding. Through attraction; aggregation pheromones help to form a group of individuals in a particular location. Aggregation pheromones are beneficial because they help to find a food source, protection or reproduction (Wertheim et al., 2005; Dharani et al., 2024) [13,53] .

4) Calming Pheromones

Calming pheromones play a significant role in reducing stress and promoting comfort among animals. Pig Appeasing Pheromone (PAP) is secreted by sow and it helps in calming and comforting the young piglets by reducing anxiety and distress during weaning or transport. Fecal semiochemical also contain Myristic acid as calming compound. When weaned piglets are exposed to sow feces containing myristic acid, they display a noticeable calming effect and reduced aggressive behaviour. Additionally, this compound has been associated with an increased feed intake, which ultimately contributes to improved growth rates among piglets. Thus, both Pig Appeasing Pheromone and Myristic acid play a crucial role in maintaining the well-being of piglets by promoting calmness (Borpujari et al., 2020) [6] . Likewise, Bovine Appeasing Pheromone (BAP) (Pageat, 2001) [35] and Equine Appeasing Pheromone (EAP) (Riley et al., 2002) [43] also act as calming agent and growth promoter to young one.

5) Sex Pheromones

Sex pheromones are volatile chemical compound which signal that a female animal is physically ready to mate. These signals are usually produced by females; however, males can also release some pheromones that give information about their species and genetic makeup. When a female is in estrus, she releases pheromones into the environment that act on male animals, causing an immediate change in the behaviour of than male animal (Dzięcioł et al., 2012; Kekan et al., 2017) [27] .

5.1) Releaser Pheromones

Releaser pheromones are those chemical signals that cause a short-term alteration in the behaviour of the recipient (within seconds or minutes) and act as attractants (Gokuldas et al., 2023) [16] . Ex. Boar sex pheromone. This Boar sex pheromones contain androstenone & androstanol which are secreted through saliva. Which induces immobilization reflex in estrus sow (Sankarganesh et al., 2022) [48] .

5.2) Priming Pheromones

Priming pheromones are substances that cause physiological events and comparatively long-term effects either through inhibition or stimulation of hormone. In domestic mammals, priming pheromones from the male have potential influence on induction of puberty, shortening of seasonal anoestrus and resumption of ovarian activity and estrus (Gokuldas et al., 2023) [16] .

Roles of pheromones in different domestic animals

Chemical communication is very important in mammalian reproduction. The sense of smell plays a major role in sexual and mating behaviours in mammals. It helps animals to find animal of the opposite sex. In addition, scents help animals choose the most suitable partner (Coombes et al., 2018) [12] .

Cattle

Acetic acid, propionic acid and 1-iodo-undecane are chemical compounds found in milk, urine and feces. These substances act as indicators of estrus and also help in attracting males during the reproductive period. Similarly, trimethylamine, 4propyl phenol and pentanoic acid are present in saliva. These compounds mainly play a role in attracting males and influencing mating behaviour (Sankar et al., 2007; Archunan and Rameshkumar, 2012) [39,47] . Buffalo 4-methylphenol, 9-octadecenoic acid, 1-chlorooctane and pcresol are chemical compounds present in urine of estrus buffalo. 9-octadecenoic acid is also found in vaginal secretions. These compounds play an important role in attracting males and stimulating mounting behaviour in buffalo bull (Rajanarayanan and Archunan, 2011; Karthikeyan and Archunan, 2013) [2,26,38] .

Goat

The pheromonal compounds of bucks are octanoic acid and 2,6-di-tert-butyl-4-methyl phenol. These compounds are mainly secreted from the fleece of the buck which influence the reproductive cycle of does by stimulating release of LH from anterior pituitary, which plays an important role in inducing ovulation (Iwata et al., 2003) [23] .

Sheep

In rams, certain compounds such as Fatty Acid I, Fatty Acid II and long-chain alcohols like hexadecane-1,2-diol and octadecane-1,2-diol have been identified as pheromonal substances. These compounds are secreted from the fleece of the ram. They play an important role in stimulating the release of LH in ewes, which is a key hormone requires for ovulation (Over et al., 1990) [34] .

Pig

In boars, the salivary glands produce saliva that contains steroid pheromones such as 5α-androstenone, 3α-androstanol and 3β-androstanol. These compounds are mainly produced in the testes, transported through bloodstream and stored in adipose tissues. They are known for standing reflex in sow. In addition, non-steroid pheromones known as pig appeasing pheromones (PAP) have been identified in the mammary skin secretions of lactating sows, which modifies the behaviour of weaned piglets by promoting calmness and reducing stress or aggressive behaviours (Sankarganesh et al., 2022) [48] .

Horse

In horses, the main compounds identified are p-cresol and mcresol, which are present in urine of estrus mare. These compounds act as indicators of ovulation in mare and helping stallion to detect right time for mating. This improves the timing and efficiency of reproduction in horse. In addition, pcresol has been shown to enhance penile erection in stallion, indicating its role in stimulating sexual arousal and readiness for mating (Būda et al., 2012; Mozūraitis et al., 2012).

Dog

In female dogs, methyl-p-hydroxybenzoate has been identified as a sex pheromone that is released in urine during estrus. This compound helps to communicate and identity reproductive status to male dogs (Goodwin et al., 1997; McGlone and Hendersen, 2022) [18,29] . Another important pheromone is the Dog Appeasing Pheromone (DAP), which is secreted from the mammary glands of lactating females. DAP plays a key role in promoting calmness, strengthening social bonding and reducing anxiety in puppies as well as adult dogs (Şafak and Sulu, 2026).

Cat

Pheromones in cats are very important for communication and marking their territory. Chemicals involved are oleic acid, pimelic acid and palmitic acid. These are produced by sebaceous glands located on the cat's cheeks. When cat rubs its face on objects, these chemicals are deposited and helping the cat to recognize familiar places and feel comfortable (Goodwin et al., 1979) [18] . Another group of chemicals, such as 5β-cholestan acid is also released from the cheek glands. These are mainly involved in marking territory and creating a sense of calmness for the cat (Pageat and Gaultier, 2003) [15,36] .

Effects of pheromones 1) Puberty acceleration:

Puberty acceleration plays a crucial role in reproductive management of livestock. Early onset of puberty improves breeding efficiency and overall productivity. Research has shown that a male primer pheromone, 1-iodo-undecane can accelerate puberty in females of various species. This suggests that pheromones play a significant role in regulating reproductive maturity (Rekwot et al., 2001) [41] .

Species Material Method Onset of Puberty Reference

Cattle Heifers exposed to vasectomized bull

months earlier

Rekwot et al., 2000 [42] Goat Pre-pubertal goat exposed to sexually mature buck

weeks earlier

Augusto et al., 2020 [3] 2) Reduction in postpartum anoestrus: Reduction in postpartum anoestrus is an important factor for improving reproductive efficiency in livestock. In ewes, introducing rams during the postpartum period reduces the interval between parturition and conception, particularly during the autumn and spring seasons (Gokuldas et al., 2010) [17] .

Species Exposure First post-partum estrus Reference

Cattle Not exposed 62 days Berardinelli et al., 2005 [4] Teaser bull 43 days Excretory product of bulls 45 days Buffalo Not exposed 61 days Gokuldas et al., 2010 [17] Vasectomized bull 55 days

Cattle

Normal saline 92 days Kamal, 2023 [25] Bull urine 60 days Synthetic pheromone compound 85 days

3) Influencing the standing posture by male pheromones in sow: The steroid chemo signals identified in boar saliva have been demonstrated to evoke the immobilization reflex to sows in heat and characterize their readiness to mate (Brooks and Cole, 1970) [10] .

4) Estrus detection by female pheromones:

Effective estrus detection would greatly help cattle reproduction for artificial insemination. Silent heat is common issue in buffalo.

Successful recognition of estrus prior to the actual time of ovulation may improve conception rate in cattle and buffaloes (Rao et al., 2013) [40] . Hence, successful identification of urinary sex pheromones in the farm animals provides a tool to overcome this problem (Rameshkumar et al., 2002) [39] .

Species Sample

Measured chemical compound

Result Reference

Cattle Faeces Acetic acid Estrus -36 ± 8 ng Mozuraitis et al., 2017 [32] Diestrus -19 ± 5 ng Propionic acid Estrus -10 ± 3 ng Diestrus -2.3 ± 1 ng

Buffalo Urine p-cresol

Estrus -No colour Muthukumar et al., 2018 [33] Pro/Diestrus -Pink colour 6) Penis erection and sperm motility: Female urinary sex pheromones have been shown to enhance penile erection and improve semen quality in buffalo. Similarly, urinary pheromones from estrus mare are reported to stimulate penile erection in stallion. These findings suggest that female sex pheromones can be utilized in assisted reproductive technologies under specific conditions in certain species (Kekan et al., 2017) [17] . [52] Exposure to acetic acid and [15,21,28] .

Rabbit exposed to female rabbit urine and vaginal swab

Advances in pheromone based estrus detection

Reproductive performance is one of the most important factors that determines the economic success of a farm. Reproduction related problems are a major reason to cull animals. To enhance fertility of farm, detecting estrus at the right time and accurately is very important (Ali et al., 2022) [1] . To overcome silent heat and for assisted reproductive techniques, a need for simple, non-invasive and reliable methods for heat detection.

Portable E-Nose device can detect estrus by sensing changes in volatile compound from perineal region of cattle and buffalo. It was developed by GmBH, Germany. A prototype called MENT-EGAS (Milking Machine and Electronic Nose Technology-Egypt, Asmaa Shaaban prototype) was used to detect odour changes from the perineal region of cows. 2022) [1,54] . [33] developed a simple urine-based kit to detect estrus in buffaloes. This kit works by identifying a specific chemical called p-cresol (estrus specific compound) in urine. In this method, a drop of the reagent is added to a urine sample collected from the animal. If no colour develops, it indicates a positive result, meaning the animal is in estrus.

In contrast, a pale pink colour shows the proestrus stage, while a dark pink colour indicates the diestrus. This method is simple, quick and useful for identifying the correct time for breeding.

Synthetic maternal pheromones:

A synthetic mixture of androstenone, androstanol and quinoline mimics function of boar saliva, which can stimulate estrus behaviour in sows ex. Boar Mate ® , Hog Mate ® etc. This mixture also helps to reduce the time between weaning and the onset of estrus (Sankarganesh et al., 2022) [48] . In addition, a synthetic product known as porcine appeasing pheromone (PAP) is commercially available and widely used in the pig industry, such as FerAppease ® (Brooks and Cole, 1970; Pageat, 2001) [10,35] . [43] .

Conclusions

Pheromones are powerful natural chemical signals that play a crucial role in regulating reproduction, behaviour and social bonding in domestic animals. They help in estrus detection and to optimize the best timing of artificial insemination to improve fertility. Pheromones can also help to accelerate puberty, early resumption of ovarian cyclicity, overcome postpartum anestrus and improve semen quality. In addition, maternal appeasing pheromones reduce stress and promote growth in young animals, which ultimately leads to improved reproductive efficiency and productivity. In nut shell, pheromone therapy is a safe, natural and effective approach to enhance production, reproduction and welfare in animals, making it an important tool in modern livestock management system.

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