It is necessary to prove the existence of a dangerous propensity under some circumstances. A number of cases have specified what is a dangerous propensity, and what is not. For example, in Barger et al. v. Jimerson et al. (1954) 130 Colo. 459, 276 P.2d 744, the Colorado Supreme Court held that a dog owner who is free of negligence shall nevertheless be liable for injuries and losses resulting from a dog bite, if the owner knew or should have known that the dog was dangerous prior to attack. The original complaint in Barger stated that the dog owner (the defendant) "allowed said dog to run at large and loose." At the beginning of the trial, the victim (the plaintiff) asked that this allegation be stricken from the complaint, but at the end of the trial, asked that the allegation be added again. The Colorado Supreme Court held that it was not necessary to allege or prove that the owner allowed the dog to run at large. The Court stated that there was:
The Court also held that the victim proved that the owner knew the dog was dangerous because, among other things, the owner kept it confined:
In E.P. Swerdfeger v. John L. Krueger, Minor, etc. (1960) 145 Colo. 180, 358 P.2d 479, the Colorado Supreme Court held that a victim who knew that the dog might bite, and had to trespass to encounter the dog, cannot recover for his injuries and losses when he trespasses and is bitten. The Swerdfeger decision adopted the language of the Restatement of the Law -- Torts:
In Reynolds v. Reichwein (1973) 510 P.2d 895, 1973 Colo. App. Lexis 857, the Court of Appeals of Colorado held that the appellate courts will not overturn the trier of fact's determination that that owner knew his dog was vicious, as long as the determination finds some support in the record. In Reynolds, the dog previously bit a child that provoked it. Despite the provocation, the trial court ruled that the dog was vicious and the owner knew or had notice of the dog's vicious tendencies.