<P> The phase of competition for teats and of nosing the udder lasts for about one minute, and ends when milk flow begins . In the third phase, the piglets hold the teats in their mouths and suck with slow mouth movements (one per second), and the rate of the sow's grunting increases for approximately 20 seconds . The grunt peak in the third phase of suckling does not coincide with milk ejection but rather the release of oxytocin from the pituitary into the bloodstream . Phase four coincides with the period of main milk flow (10--20 seconds) when the piglets suddenly withdraw slightly from the udder and start sucking with rapid mouth movements of about three per second . The sow grunts rapidly, lower in tone and often in quick runs of three or four, during this phase . Finally, the flow stops and so does the grunting of the sow . The piglets may then dart from teat to teat and recommence suckling with slow movements, or nosing the udder . Piglets massage and suckle the sow's teats after milk flow ceases as a way of letting the sow know their nutritional status . This helps her to regulate the amount of milk released from that teat in future sucklings . The more intense the post-feed massaging of a teat, the greater the future milk release from that teat will be . </P> <P> In pigs, dominance hierarchies can be formed at a very early age . Domestic piglets are highly precocious and within minutes of being born, or sometimes seconds, will attempt to suckle . The piglets are born with sharp teeth and fight to develop a teat order as the anterior teats produce a greater quantity of milk . Once established, this teat order remains stable with each piglet tending to feed from a particular teat or group of teats . Stimulation of the anterior teats appears to be important in causing milk letdown so it might be advantageous to the entire litter to have these teats occupied by healthy piglets . Using an artificial sow to rear groups of piglets, recognition of a teat in a particular area of the udder depended initially on visual orientation by means of reference points on the udder to find the area, and then the olfactory sense for the more accurate search within that area . </P> <P> Pigs have panoramic vision of approximately 310 ° and binocular vision of 35 ° to 50 ° . It is thought they have no eye accommodation . Other animals that have no accommodation, e.g. sheep, lift their heads to see distant objects . The extent to which pigs have colour vision is still a source of some debate; however, the presence of cone cells in the retina with two distinct wavelength sensitivities (blue and green) suggests that at least some colour vision is present . </P> <P> Pigs have a well - developed sense of smell and use is made of this in Europe where they are trained to locate underground truffles . Olfactory rather than visual stimuli are used in the identification of other pigs . Hearing is also well developed, and localisation of sounds is made by moving the head . Pigs use auditory stimuli extensively as a means of communication in all social activities . Alarm or aversive stimuli are transmitted to other pigs not only by auditory cues but also by pheromones . Similarly, recognition between the sow and her piglets is by olfactory and vocal cues . </P>

How long does it take for a domesticated pig to become feral