The circumstances in which the keeper of an animal is liable for damage caused by his animal depend upon the category to which the animal belongs: animals are either dangerous or non dangerous. Section 2(1) of the Animals Act 1971 imposes upon the keeper of an animal of a 'dangerous species' strict liability for any damage caused by the animal. Non dangerous species do not have a regime of strict liability imposed upon them unless: (a) the damage is of a kind which the animal, unless restrained, was likely to cause or which, if caused by the animal, was likely to be severe; and (b) the likelihood of the damage or of its being severe was due to characteristics of the animal which are not normally found in animals of the same species or are not normally so found except at particular times or in particular circumstances; and (c) those characteristics were known to that keeper. These conditions are expressed in the conjunctive rather than the alternative which means that all three conditions must be met.
A. Type of damage
Subsection 2(2)(a) provides that the damage must be of a kind which the animal, unless restrained, was likely to cause or which, if caused by the animal, was likely to be severe.
In Mirvahedy v Henley  UKHL 16 Lord Nicholls gave the example of a large and heavy domestic animal such as a mature cow where section 2(2)(b) may not be satisfied. He said that: "There is a real risk that if a cow happens to stumble and fall on someone any damage suffered will be severe. This would satisfy requirement (a). But a cow's dangerousness in this regard may not fall within requirement (b). This dangerousness is due to a characteristic normally found in all cows at all times. The dangerousness results from their very size and weight. It is not due to a characteristic not normally found in cows 'except at particular times or in particular circumstances'".
B. Abnormal characteristics
Section 2(2)(b) of the Animals Act has been determined by the House of Lords in Mirvahedy v Henley in a split decision. Section 2(2)(b) relates to the likelihood of the damage or of its being severe was due to characteristics of the animal which are not normally found in animals of the same species or are not normally so found except at particular times or in particular circumstances. Lord Nicholls, giving the leading majority judgment noted that this subsection aimed to create strict liability for abnormal conduct of non dangerous species. The first limb of paragraph (b) identifies one class. The animal must have characteristics 'which are not normally found in animals of the same species'. The second limb of paragraph (b) identifies the other class of qualifying characteristics. The animal must have characteristics which are not normally found in animals of the same species 'except at particular times or in particular circumstances'.
The phraseology of section 2(2)(b) is notable for its capacity to be interpreted in diametrically opposite ways. There is no problem with the first part of section 2(2)(b)-do animals normally or are they prone to, for example, bite or kick? The problem is with the second part: does one cancel the double negative 'not normallyâ¦except' and ask whether what was done in the special circumstances was normal behaviour for the species as a general rule; or is the right approach to ask whether what was done was normal for the species in the particular circumstances even if it will be abnormal in the absence of such circumstances. In Cummings v Granger  QB 397, the first of these approaches was adopted where Lord Denning MR said: "Those characteristics-barking and running around to guard its territory-are not normally found in Alsatian dogs except in circumstances where they are used as guard dogs. Those circumstances are 'particular circumstances' within section 2(2)(b). It was due to those circumstances that the damage was likely to be severe if an intruder did enter on its territory." This approach was followed by the majority in Mirvahedy v Henley (see also Curtis v Betts  1 WLR 459).
How the majority interpretation works in practice is that a bitch with her litter, a guard dog, a cow with her calf, will be covered by section 2(2): in essence normal behaviour in specific circumstances. In Livingstone v Armstrong (11/12/2003)(unreported) it was found that there was no negligence on the part of the cow's keeper in maintaining the fences on his farm. It was further found that the cow had in fact jumped a properly maintained fence. Evidence from the cow's keeper was that it was not normal for cows to jump over fences. There was no evidence that the cow was frightened or that it had bolted. The claim failed on the basis therefore that section 2(2)(b) had not been met because the behaviour in the particular circumstance was not normal. The problem becomes, of course, that every situation becomes a 'particular circumstance' and that animals, being animals, have behaved in a normal way. In litigation of this kind it is extremely important to identify the special circumstance in order to establish the normal behaviour of the animal.
The Animals Act is minefield of potential interpretive errors. Mirvahedy was thought to be a low point for defendants but there is more potential for hope or despair in the judgment (depending on whether you are a claimant or a defendant) than appears on first sight to be the case.