Chemical communication plays an important role in mammalian behaviour and reproductive processes. Pheromones is one means of transmiting such information. Pheromones are air-borne chemical substances (“signals”) released in the urine or feces of animals or secreted from cutaneous glands that are perceived by the olfactory system and that elicit both behavioural and endocrine responses in conspecifics. In mammals, signalling and priming pheromones are thought to act either singly or in combination through olfaction, auditory, visual (sight) or tactile stimuli. Extensive studies in livestock have established the importance of pheromones in influencing the reproductive performance both in males and females. Pheromones and other allelomimetic cues can exert profound effects on reproductive activity via the hypothalamic system that generates pulses of gonadotropin-releasing hormone. Manipulations of these factors and other pathways linking environmental inputs to reproductive output can lead to developing the concept of “control systems technologies”, aimed at controlling reproductive performance. The knowledge acquired on the effectiveness of biostimulation; the factor which conditions it and the biological mechanism which produces it in livestock species, allows its use as a breeding management tool. The understanding of the role of pheromones could be of potential economic importance in addressing some of the problems associated with livestock production in the tropics. The biostimulation technique offers a potentially useful and practical way to improve reproductive efficiency in livestock species in the tropics. The exact nature of the cues and the role of biostimulation in livestock species especially swine, sheep, goats and cattle in developing countries require more attention.
Livestock productivity in the tropical region is generally low relative to temperate climatic condition, mainly due to nutritional and environmental stresses, which leads to low reproductive efficiency (Fitzpatrick, 1996). Various attempts have been made on the regulation and control of bovine reproduction through the use of hormones and its analogues (Peters and Ball, 1995). Although hormonal treatments are highly effective (Diskin et al., 2002; Patterson et al., 2003), but the growing concern about the clean, green and ethical environment and reduced usage of drugs and exogenous hormones (Martin et al., 2004; Martin, 2009), simultaneously lead to adoption of other managemental tools like socio sexual stimuli for improving reproductive efficiency.
Therefore, harnessing the reproductive potential of livestock species in India may require development of management strategies such as biostimulation to effectively improve reproduction. The knowledge acquired on the effectiveness of biostimulation; the factor which conditions it and the biological mechanism which produces it in livestock species, allows its use as a breeding management tool. The biostimulation technique offers a potentially useful and practical way to improve reproductive efficiency in livestock species in the tropics. Biostimulation proved to be a promising tool in hastening puberty and sexual maturity. Also, in domestic animals, through priming pheromones is helpful on the termination of seasonal anoestrus and shortening of postpartum anestrus. Along with effects on female reproduction this technique is found to enhance male sexual performance by stimulating sexual behavior and endocrine functions of males. Thus understanding the role of pheromones could be of potential economic importance in addressing some of the problems associated with livestock production in the tropics.
What is biostimulation?
Biostimulation is a pheromonal communication which plays an important role in mammalian behavior and reproductive processes. Biostimulation (male or bull effect) is the term coined to describe the stimulatory effect of a male on oestrus and ovulation through genital stimulation, priming pheromones, or other less well-defined external cues (Chenoweth, 1983). It was Heape in year 1901 that performed various researches to define term biostimulation. Biostimulation affects reproductive efficiency by hasetening sexual maturity, ovulation induction, reduction of postpartum anoestrus and also in mating (Burns and Spitzer, 1992).
To study about biostimulation it is utmost important to know about the pheromones and its types. Pheromone is derived from greek word, Pherin- to transfer, Hormones- to stimulate. Pheromones are air-borne chemical substances that are secreted externally by various secretions and cause a specific reaction involving either the release of a specific behaviour or physiological change in the recipient’s endocrine or reproductive system in a receiving individual of the same species (Doty, 1976; Izard, 1983).
In mammals, based on function pheromones are of two type’s i.e, signalling and priming pheromones which are thought to act either singly or in combination through olfaction, auditory, visual (sight) or tactile stimuli.
These are substances that cause an immediate short term behavioural response, invoking a classical stimulus — response paradigm mediated by the central nervous system (Izard, 1983). Usually, this involves an olfactory cue that transfers specific information and consequently elicits a specific behaviour. A very good example of this is flehmen reaction shown by male on receiving various secretion of female like urine or vaginal secretion. These odours are present at oestrus and proestrus but not at any other time (Johns, 1980). Hence prove to be helpful in detection of oestrus in females. These signalling pheromones may have more than one behavioural function in that they can serve as attractants and or inducers of sexual activity (Izard, 1983).
In this context lot of research has been done for improving reproductive efficiency in animals. It has been reported that urine and vaginal mucus of oestrus cows, when rubbed on the vaginal membranes of non-oestrus cows, attracts and stimulates males through olfaction and in addition a volatile odour in the cervico-vaginal mucus of oestrus cows is a source of sexual attraction for males (Hart et al., 1946). Odours probably play a significant role in signalling the reproduction of cows and that the male can use a combination of factors associated with the female to determine if she is receptive, including visual, tactile, auditory and olfactory stimuli (Izard, 1983). The behavioural pattern of males and a possible involvement of the olfactory cues, or pheromonal substances, in a group of oestrus synchronised cows has been established (Masaki and Ohta, 1990). Such mating behaviour of males occurred at about the same time of LH and FSH surge in the partner cows; explaining how males choose their optimum partners from the rest of the oestrus cows. It also appears that males especially boars, rams, bucks and bulls emit a sex attractant which oestrus females can detect (Izard, 1983).
These are substances that cause physiological events either through inhibition or stimulation of endocrine, reproductive and other systems (Izard, 1983). Usually, it involves an olfactory cue that elicits a measurable physiological response. In domestic mammals, priming pheromones from the male have an influence on the induction of puberty, the termination of seasonal anoestrus and shortening of postpartum anoestrus. Recently, priming pheromones from females have been shown to affect ovarian activity in other females of the same species (Izard, 1983).
How it Works?
Pheromones and other allelomimetic cues can exert profound effects on reproductive activity via the hypothalamic system that generates pulses of gonadotropin-releasing hormone. The pheromones are perceived by vomeronasal organ (VNO) located in the nasal cavity in almost all terrestrial vertebrate species. The VNO contains specific neuron receptors which are activated by specific ligand pheromones that stimulate a cascade of neuroendocrine responses. The neuroendocrine response is unique to the specific subpopulation of neuron stimulated in VNO. The effect of pheromone on reproduction is mainly due to the release of luteinizing hormone releasing hormone (LHRH) in the hypothalamus. LHRH acts on pituitary and regulate the pulsatile release of luteinizing hormone (LH). In sheep, a pulse of LH is observed within minutes of the presentation of the anestrus female to the male and this pulsatile secretion of LH remains high for several hours and a preovulatory surge takes place within 36 hours.
FSH & LH
Positive or negative feed back
Positive feed back
The increase in LH secretion is thought to probably stimulate the ovarian secretion of estradiol that via positive feed back effect at the hypothalamic level induces the preovulatory LH surge thus overriding the inhibitory effects of low concentration of estrogen on the hypothalamus (Rekwot et al.,2001) and hence ovulation occur (Signoret et al., 1990).
Role of Biostimulation in Reproduction
Hastening Puberty and Sexual Maturity
In domestic animals, priming pheromones from the male have an influence on the induction of puberty in female (Izard, 1983). Positive effects of biostimulation in cyclic activity of cows have been demonstrated with bulls (Fernandez et al., 1993; Berardinelli and Tauck, 2007), androgenized steer (Ungerfeld, 2009; Fiol et al., 2010b) and androgenized cows (Burns and Spitzer, 1992). Before artificial insemination vasectomised bulls for estrus detection may be an effective management tool to decrease age at puberty. Exposing heifers to vasectomised bulls resulted in puberty at significantly lower age (23.1 months) than non-exposed heifers (26.4 months) (Rekwot et al., 2000a). It has also been reported that 67% of heifers exposed to bull urine and 32% of heifers exposed to only water attained puberty, supporting the hypothesis that bull urine contains a priming pheromone that hastens the onset of puberty in heifers (Izard and Vandenbergh, 1982b). In small ruminants the “ram effect” or “buck effect” (priming pheromone) reported to hasten the onset of oestrus activity and helpful in oestrus synchronization (Schinckel, 1954 a, b). During normal breeding season the introduction of males enhance the onset of oestrous cycles in seasonally anoestrus ewes (Knight et al., 1978; Pearce and Oldham, 1984) and goats (Shelton, 1960). In swine also the “boar effect” has been well documented. The presence of a boar accelerate puberty in gilts by about 30 days and also introduction of boars to gilts at about 190 days of age (i.e. mean age at puberty) can lead to a marked synchrony in attainment of the pubertal oestrus (Brooks and Cole, 1970). Age of male is also important, gilts exposed to mature boar of 11-24 months attained puberty 24 days earlier than gilts exposed to boars of 6.5 months of age (Kirkwood et al., 1981).
However, several other factors also affect the onset of puberty in heifers includes breed (Arije and Wiltbank, 1971), nutrition and growth rate (Grass et al., 1982) photoperiod, environmental temperature and season (Grass et al., 1982) apart from bull and biostimulation (Izard and Vandenberg, 1982, Roberson et al., 1991).
Intensity of Estrus and Reduction of Postpartum Anoestrus
In domestic animals, priming pheromones from the male have an influence on the termination of seasonal anoestrus and shortening of postpartum anestrus (Izard, 1983). Along with this it is also helpful in increasing the intensity and duration of estrus in animals. Primiparous cow showed longer duration of estrus (23.65 h vs 13.50 h) and more intense heat and also shows more mounting behavior (100% vs 52%) when exposed to male continuously (Khanh et al., 2012). 95% gilts displayed a standing response in the presence of boar and in absence of boar 60%. This effect of the bull on resumption of cyclic ovarian activity is mediated via the central nervous system by stimulating LH release immediately following exposure (Custer et al., 1990). The first increase in progesterone, which indicated onset of oestrous cycles, occurred at 43 versus 63 days postpartum in bull-exposure and no bull-exposure cows, respectively. The same study reported that a higher percentage (89 versus 18%) of cows were oestrual in the bull exposure than no exposure groups, respectively. The effect of a teaser bull on ovarian and behavioural activity of suckling beef cows indicated that in the group with the teaser bull, the proportion of cows in heat and ovulation was higher than in the control group (heat: 67.9 versus 32.7%; ovulation: 73.6 versus 40.4%) (Alberio et al., 1987).
In one study it has been proved that it is not the bulls or cows that produce any effect (Berardinelli et al., 2004), but it is their excretory products responsible for biostimulation (Joshi, 2002). Oro-nasally administered bull urine to cows on day 7 postpartum increased mean LH and FSH serum concentrations within 80 min after exposure (Baruah and Kanchev, 1993). Joshi (2002) showed that cows exposed to the excretory products of bulls resumed ovarian cycling activity earlier after calving than cows not exposed to excretory products of bulls. Tauck et al.(2006) evaluated resumption of cyclic activity in postpartum, anovular, primiparous cows exposed to bulls or to excretory products of bulls. Anestrus postpartum length did not differ between females exposed to males or to excretory products of males, but it was shorter than in non-exposed females. On the other hand, continuous exposure (24 h/d) of primiparous cows to male urine using a urine delivery system during 64 d did not affect postpartum anestrus interval compared to cows not exposed to urine (Tauck and Berardinelli, 2007; Tauck et al., 2006). Tauck et al. (2010b) proposed that the alternation of “stimulation” and “relaxation” periods may be necessary to obtain a positive response to biostimulation. Exposure during 6 or 12 h reduced anestrus postpartum compared to isolated females, but the cumulative proportion of cyclic females at 10 days intervals was greater for females exposed during 12 (h) than during 6 h.
Recently in one study found association of biostimulation and temporary weaning which is helpful in increasing follicular development ovulation synchronisation and, consequently, the pregnancy rate in beef cows (Filho et al., 2015).
Enhance Male Sexual Performance
Pheromones are released by females during estrus, thus signalling the stage of the cycle, and stimulating sexual behavior and endocrine functions of males (Dehnhard and Claus, 1988). Male sexual performance can be improved by various sexual stimulation technique (Mader and Price, 1984; Gonzalez et al., 1991; Rosa et al., 2000; Bailey et al., 2005). Sexual stimulation techniques that improve male sexual performance have been reported in several farm animal species (Mader and Price, 1984; Gonzalez et al., 1991; Rosa et al., 2000; Bailey et al., 2005). Studies have demonstrated that the sudden introduction of oestrous ewes in a group of rams induces certain changes in the behavioural and endocrinological state of the rams, a phenomenon which can be referred to as the ‘female effect’. Sexual interaction of males with females tend to increase the plasma levels of testosterone which further associated with rapid rise in LH level and related to a rapid elevation of LH concentration in a variety of species (Hemsworth et al., 1981; Gonzalez et al., 1991; Rosa et al., 2000) which is responsible for libido in males. In one study it was found that the bulls show improved sexual behaviour like sexual aggressiveness (SA) after application of cervical mucus (3.44±0.063) and pherobull (3.45±0.060) as compared to control (2.56±0.061) in adult Sahiwal bulls. Reaction time decreased significantly (p<0.01) in cervical mucus group (207.23±23.33 sec.) and pherobull group (212.45±25.85 sec.) as compared to control group (332.19±32.12 sec.).Further libido score improved significantly (P<0.01) in cervical mucus group (36.51±1.67%) and pherobull group (37.94±1.85%) as compared to control group (25.77±1.20%) (Singh , 2014).The duration of flehmen behaviour and number of mounts of bulls was significantly higher when exposed to the mixture of three synthetic compounds i.e, acetic acid, propionic acid, and 1-iodo undecanethane found in faeces of Bos Taurus than that of individual compounds. Behavioural investigations demonstrated that 1-iodo undecane is primarily involved in attracting the bull whereas acetic acid and propionic acid acts as mounting pheromone in cow (Sankar and Archunan, 2008). Age of male is also one of the important factors for producing effects in an individual. Lunstra (1986) suggested that young, inexperienced bulls must undergo a learning process and (or) acclimation with females and behavior testing paradigms before they exhibit stable reproductive behaviors. Couttie and Hunter (1956) suggested that when young bulls were reared in isolation, many failed to respond or showed less interest in mounting when first exposed to estrual females compared to bulls raised with females. Exposure of ram lambs to ewes during the post-weaning prepubertal period enhanced their sexual performance (Katz et al.,1988). Yearling rams kept close to ewes in oestrus for 6 months found to have larger testes, higher plasma testosterone levels and greater sexual activity than control rams isolated from ewes (Illius et al., 1976).
Exposure to estrual females, viewing copulating herd mates and periodical change of females have been reported to enhance male sexual performance in several species (Mader and Price, 1984; Rosa et al., 2000; Silvestre et al., 2004).Exposure time of females and change of females is also responsible for producing effect in males. In sheep, exposing sexually satiated males to new, unmated, estrous females restores their sexual performance up to 95% of the original ejaculation rate (Pep-elko and Clegg, 1965). Also, in rams, the reaction time was restored when a new female stimulus was presented (Pepelko and Clegg, 1965). Therefore, changing the stimulus female found to shorten the sexual reaction time of males of several species (Thiery and Signoret, 1978) and increase in number of mounts which is indicative of high libido.
Effect on Semen Biology
There is effect of new female stimulus on semen variables which depends on degree of sexual satiation. It was reported that changing the female stimulus after the first ejaculation, semen volume was notably increased (Prado et al., 2003). But semen volume and sperm number decrease as the numbers of successive collections increase (Amir et al., 1986). It has been reported that rams were stimulated with physical contact of a ewe in estrus; an acute increase in semen volume was evident on day 1. These effects were not evident on subsequent days and thus the overall benefits on ram libido and semen quality of exposing rams to ewes in estrus are minimal (Fahey et al., 2012 ).Concerning other semen characteristics, such as motility and live sperm rate, no references were found in the literature on caprine species.
It is reported that boars in contact with the 92- h sows groups produced semen with the highest semen quality and quantity. Ejaculates collected at the 96-h sow intervals had larger volumes, sperm motility, progressive sperm movement, live sperm, sperm concentration per milliliter, total sperm per ejaculate and normal acrosome morphology than ejaculates collected on the 24-h intervals (Umesiobi, 2010).
Chemical Compound Present in Various Secretory or Excretory Products
The chemical identities of some biostimulatory substances in various secretory and excretory products of different animals have been explored in recent years. The identity of bovine urinary cues is not yet explored fully. Some of the chemical compounds and there function is presented in Table below.
Table: Various chemical compounds in various secretory and excretory products
|Species||Secretory/Excretory Products||Chemical Compounds||Function||Reference|
|Pig||Saliva||5-α-androst-16-en-3-one and 5α-androst-16-en-3α-ol||Estrus detection in sows as it cause estrus sows to take up the mating stance or lordosis reflex||Dorries et al.,1997|
|Cow||Urine||1-iodoundecane and di-n-propyl phthalate||Highly effective in stimulating the bulls sexual behavioral like attraction, salivation, urination, tongue manipulation, flehmen and vocalization||Ramesh and Archunan, 2002; Archunan and Ramesh, 2012|
|Buffaloes||Urine||1-chlorooctane, 4-methylphenol and 9-octadecenoic acid.||Repeated flehmen behaviour towards 4-methyl phenol. When exposed to 9-octadecenoic acid bulls displayed penile erection and mounting.||Rajanarayanan and Archunan, 2011|
|Cow||Faeces||Acetic acid, propionic acid, and 1-iodo undecane||1-iodo undecane attractin bull, Increases flehmen reaction and acetic acid and propionic acid helps in mounting behaviour||Shankar and Archunan, 2008|
|Cow||Vaginal fluid||Trimethylamine, acetic acid, and propionic acid||Precopulatory behaviours and successful coitus||Sankar and Archunan, 2011|
|Cow||Saliva||Trimethylamine, acetic acid, phenol 4-propyl, pentanoic acid and proprionic acid,||Attracting the bull to the oestrous cow||Sankar et al.,2007|
However, further studies are needed on bioassay and behavioural aspect of these pheromonic compounds in farm animals. The specific urinary compounds after knowing the chemical properties can be purified from urine or prepared synthetically for induction of estrus cyclicity in heifers and postpartum cows.
Biostimulation or pheromonal communication plays an important role in mammalian behaviour and reproductive processes. Pheromones in the urine, feces, or from cutaneous glands can be perceived through the olfactory system to elicit both behavioural and endocrine responses in conspecifics. Bio stimulation can exert profound effects on reproductive activity via the hypothalamic system that generates pulses of gonadotropin-releasing hormone. Biostimulation have effect on both male and female. In females it is helpful in hastening puberty and sexual maturity, termination of seasonal anoestrus and shortening of postpartum anoestrus. Effect on male is on improvement of sexual performance and semen characteristics.
The economic benefits of using biostimulation through pheromones to enhance early onset of puberty and the significant reduction of postpartum anoestrus in domestic animals, may serve as a management tool in India, where livestock production has some constraints. The biostimulation technique offers a potentially useful and practical way to improve reproductive efficiency in livestock species. The exact nature of the cues and the role of bio stimulation livestock species especially swine, sheep, goats and cattle require more attention for better productivity. Therefore, studies are needed on bioassay and behavioural aspect of these pheromonic compounds in farm animals.
Conflict of Interest Statement
The authors do not have any conflict of interest.