Part of any assessment of pain should include the animal's response to standard stimuli; a commonly used practical example is the response of sheep to an approaching shepherd or dog. Healthy farm animals, particularly with youngsters at foot, normally respond to the presence of a dog; they become alert, may stand up and face the dog, stamp their front feet, attack the dog or they run away.
Animals which are ill or in severe pain e.g. lambs, during the first two hours after rubber ring castration and tail docking show inert lateral lying and may fail to respond to such an approach. Some ewes (maternally motivated) paw vigorously at their lambs if they see them lying immobile in this lateral posture and this appears to be the ewe's approach to evoking activity to help alleviate her concern for the "state" of her lamb.
Standardised noxious stimulation by palpating, pinching or probing is difficult to achieve but is frequently used during physical examinations. The presence of pain makes some animals less tolerant to any approach, handling or manipulation of particular parts of the body. The response may be a change in posture, vocalisation, struggling or even attack. Other individuals in pain may not move when approached, as to move is painful.
Under experimental conditions, quantitatively controlled mechanical, chemical and thermal stimulation may be used for the assessment of changes in the sensitivity of tissues to pain (hyperalgesia or allodynia) (Welsh & Nolan 1995).