Table 1.

Brief descriptions of imprinting or exposure learning, of classical conditioning and operant conditioning as examples of associative learning, and of habituation or desensitisation as a form of non-associative learning.

Imprinting or Exposure Learning	Classical Conditioning	Operant Conditioning	Habituation or Desensitisation
This is any kind of phase-sensitive learning that occurs at a particular age or life stage, is rapidly acquired, and is apparently independent of the consequences of behaviour. Expressed generically, an animal or person learns the characteristics of some stimulus which is then said to be imprinted on the subject. Once imprinted, exposure to the stimulus elicits particular behaviours that are likely associated with consciously perceived subjective experiences. A well-known form is “filial imprinting”. This occurs where a young animal narrows its social preferences to an object (typically a parent) as a result of exposure to that object. This is obvious in the hatchlings of birds such as waders, water fowl, and gamebirds, which imprint on their parents, then follow them around [170] and when separated make “distress calls” [177,178]. Similar behaviours are observable in newborn mammals such as lambs, kids, calves, fawns, and foals, which need to remain near their mobile mothers [14,18,107,179]. Thus, imprinting is a learning mechanism that likely contributes to rapid mother–young bonding as occurs in neurologically mature hatchlings (not discussed here) and newborn mammals (Section 3).	Also known as Pavlovian conditioning, this form of associative learning typically involves repeatedly pairing an unconditioned stimulus, which evokes a specific response, with another previous neutral stimulus, which does not evoke that response [171,180]. When the stimuli are presented separately after conditioning has taken place, the response occurs both to the unconditioned stimulus and to the other unrelated (now conditioned) stimulus. In Pavlov’s historic study, initially a dog presented with meat salivated but did not do so when a bell rang. After conditioning, which involved repeatedly inducing the dog to salivate by presenting meat and then ringing a bell, both stimuli, presented separately, caused the dog to salivate. Classical conditioning involves automatic or reflexive responses, and thus the underlying mechanisms operate below the level of consciousness [180]. Although the operation and linking of these mechanisms is involuntary, the subjective elements of the resulting behavioural responses are experienced consciously. For example, Pavlovian conditioning has been implicated as contributing to the establishment and maintenance of a range of social behaviours, including sexual behaviour, mother–young bonding, maternal suckling and lactation, and social grooming, where their attendant subjective elements are undoubtedly experienced consciously [171].	This method of learning occurs through animals consciously associating “rewarding” (i.e., “reinforcing”) or “punishing” outcomes with particular behaviours. Thus, pleasant or unpleasant consequences inform choices about which behaviours are preferred or avoided, the former usually being retained and the latter rejected or modified [172,173]. Accordingly, investigations of such learning focus on behaviours that can be changed because of their specific experiential outcomes. Application of the principle that “reinforcement” and “punishment” can influence conscious behavioural choices extends well beyond the laboratory setting as such learning often also plays a powerful role in everyday learning [176]. Detailed accounts of various operant conditioning methodologies, their wide applications, and observed neurobiological correlates are available elsewhere [172,173,176].	This is an example of non-associative learning in which there is a progressive diminution of a behavioural response with repetition of a stimulus, such that, following an initial response to the stimulus, the frequency and/or strength of the subsequent responses diminish with repeated stimulation [170,174]. As noted above, the response may be detected by overt behavioural changes and/or internal physiological reactions. For example, pregnant ewes brought from outdoor field conditions into unfamiliar housing and placed individually in adjacent pens, when handled initially, invariably exhibited aggressive, escape, and/or excitable behaviour [181]. However, after 6–8 weeks of daily gentle handling for 5–10 minutes this behaviour was replaced by calm, relaxed, and even positive responses when staff approached. These overt signs of habituation were paralleled by a progressive decline in the cortisol stress and heartrate responses to handling from high initial values to no detectable change [181]. Subsequently, in another laboratory, gentling of wethers was also shown to reduce their heart rate, flight distance, and aversion responses to handling [182]. Regarding the first case, undertaken as a pre-study taming and training protocol for pregnant ewes [181,183], initially the hands-on close proximity of the staff was obviously aversiveness, but as habituation progressed the animals often came forward to greet staff when they entered the pen and then nuzzled them during the physical contact. This suggests that habituation learning predominated at first [174]. However, elements of operant conditioning might also have contributed in those animals that eventually elicited further gentle stroking by nuzzling the staff, stroking which arguably then represented a form of positive reinforcement [173].