**2.1 Types of pheromones**

On the basis of chemical structure, nature of molecules, mechanism of action, interactions and functions pheromones can be classified into various types, as described below:

### *2.1.1 Releaser pheromones*

This type of pheromone elicit an immediate short term behavioral response (degraded quickly) either acting as attractant or repellent [14]. It has been observed that some organisms use powerful attractant molecules to get the attention of their mates as far as two miles or even more. These type of pheromones have been observed to stimulate immobilization reflex in sow, that is caused by the sex pheromones (released in the saliva) synthesized in the testes of boar, chemically identified as the steroids, 5α-androst-16-en-3-one and 5α-androst-16-en-3α-ol [13].

### *2.1.2 Primer pheromones*

These pheromones mediate slow developing and longer-lasting changes to the endocrine state or development through activating the hypothalamic–pituitary–adrenal axis [13]. Priming pheromone resulted early puberty in prepubertal heifers receiving weekly oronasal treatments (7-week experimental period) with bull urine as against water-treated heifers [15]. Similarly, priming pheromones from males have been observed influencing induction of puberty, shortening of postpartum anestrus and the termination of seasonal anestrus in case of mammals, more pronounced effect observed in case of small ruminants [16]. Similar effect has been observed in case of gilts exposed to a mature boar, resulting in early onset of puberty and synchronizing effect on first oestrus [17].

### *2.1.3 Signal pheromones*

These pheromones inform about individual or group identity crucial for parent-offspring recognition and mate choice. These pheromones cause shortterm responses mediated by the central nervous system through neurotransmitter release, such as, gonado tropic releasing hormone (GnRH) in rats elicit a lordosis behavior. Teaser bulls exhibit flehmen behavior upon receiving the estrus specific chemical signal from females [13].

### *2.1.4 Major urinary proteins (MUPs) and odorant binding protein (OBPs)*

First described in mouse and rat, MUPs and α2u proteins are lipocalins synthesized in the liver and excreted in the urine, and have got similarity with OBPs discovered in the nasal tissues of several vertebrates.

The MUPs have several important roles such as, transporting the pheromone in biological fluids, extending period of bioavailability by delaying the pheromone liberation, and modulating the pheromone activity [13].

### **2.2 Characterization of bovine pheromones**

Dispersion of various estrus-specific compounds isolated from different samples such as vulvar swabs, vagina, urine, milk, feces, saliva and blood have been demonstrated in the bovine body [18]. Nine estrus-specific compounds comprising of four amines, one ether, one alcohol, one diol, and two ketones, isolated from samples of estrus cows, tested positively in a bull behavioral assay [19]. Analysis of urine from cows at different stages of cyclicity had revealed presence of estrus specific pheromones such as 1-iodo undecane and di-n-propyl phthalate [20], the 1-iodo undecane have been also detected from feces of cows at different stages of the estrus along with acetic acid and proprionic acid [21]. Analysis of saliva have revealed five estrus-specific compounds, i-e, acetic acid, pentanoic acid, phenol 4-propyl, proprionic acid and trimethylamine, among which the role of trimethylamine in attracting the bull to the estrous cow has been proved in a bioassay [22]. Higher levels of 1-hexadecanol have been found in urine samples collected during estrous against the samples collected during the luteal phase, the compound supposed having a pheromonal effect in different animal species [23]. Various volatile compounds have been identified from bovine estrous urine such as 6-amino undecane, 2-butanone, coumarin, 1,2-dichloroethylene, 9-octadecenoic acid and squalene, found responsible for improving reproductive function of a bull in terms of enhanced libido and semen production [24]. Acetaldehyde an estrus specific pheromone from bovine vaginal secretions, was found helpful in predicting estrus and ovulation by monitoring its levels in blood, breath, milk, saliva, sweat, and its levels were found sharply decreased before, and at onset of estrus [25]. A gradual increase in the concentration of free fatty acids in estrous vaginal discharge before estrus and a sharp decline post estrus was observed, and it was noted that ruminal concentration of fatty acids affect the concentrations in urine but not in the vaginal discharge [26]. Quantification of methyl heptanol from vaginal secretion has been exploited as a method for detecting bovine estrus [27].
