Materials and Methods

Animals—All cats evaluated for problems with aggression from 1998 through 2006 at the Animal Behavior Clinic of the Barcelona School of Veterinary Medicine were considered for inclusion as case cats. Control cats were randomly selected from from the general database of the same hospital over the same period. All cats included in the study underwent a thorough physical examination.

For information on case cats, medical records were reviewed to obtain data on signalment, living conditions, and age at and source of acquisition as well as a detailed description of aggressive behaviors and other relevant aspects of behavior. A presumptive diagnosis of redirected aggression was established whenever a primary inciting stimulus and an alternative target were clearly identified via the clinical history. Follow-up interviews with owners of all cats with recorded incidents of redirected aggression were conducted via telephone by use of a standardized questionnaire. Clients were specifically asked about the evolution of the problem and whether more episodes of redirected aggression occurred since the last visit to the clinic. Owners of control cats were interviewed via telephone by use of a standardized questionnaire to obtain information on signalment, physical and social environment, and other general characteristics of the cats.

Statistical analysis—A F² test was used to assess associations between redirected aggression and certain cat characteristics, including sex, neuter status, age at acquisition, origin (stray, private owner, pet shop, animal shelter, or professional breeder), history of noise phobia, access to the outdoors, and size of household (≤ 2 people or > 2 people). A value of P < 0.05 was considered significant for all analyses. Data were analyzed by use of statistical software.^a

Results

Of the 336 cats evaluated at the hospital for behavioral problems, 171 (47%) had problems related to aggression specifically. The main complaint of owners was aggression toward other cats (110/171 [64%]), followed by aggression toward people (59/171 [34%]). The remaining 2 (1%) cats were evaluated for aggression toward the family dog. Nineteen (11%) cats had displayed ≥ 1 episode that met the aforementioned criterion for redirected aggression. One episode of redirected aggression was reported in the records of 16 cats, whereas 2 episodes were reported in the records of the remaining 3 cats, for a total of 22 episodes of redirected aggression.

Seventeen of the 19 case cats were domestic shorthair cats, 1 was a Persian cat, and 1 was a Siamese cat. Eleven cats were male (10 neutered and 1 sexually intact), and 8 were female (5 spayed and 3 sexually intact). Mean age of the cats was 3.6 years (range, 1 to 11 years). Nine cats were acquired as strays, 5 (29%) were obtained from a private owner, and 3 were purchased from a pet store. The source of 2 cats was unknown. Mean age at acquisition was 2.5 months (range, 1 week to 18 months). Seventeen cats lived indoors, and 11 of these lived with other cats in the same household.

Information was also collected on 64 control cats. Breeds included domestic shorthair (50/64 [78%]), Persian (7/64 [11%]), Siamese (5/64 [8%]), Bengali (1/64 [2%]), and Carthusian (1/64 [2%]). Thirty-five (55%) cats were male (28/35 [80%] neutered and 7/35 [20%] sexually intact), and 29 (45%) were female (24/29 [83%] spayed and 5/29 [17%] sexually intact). Mean age of the cats was 6 years (range, 6 months to 14 years). Twenty-six (41%) cats were obtained from a private owner, 25 (39%) were obtained as strays, 5 (8%) were purchased from a pet store, 4 (6%) were obtained from a professional breeder, and 4 (6%) were acquired from an animal shelter. Mean age of cats at acquisition was 2.6 months (range, birth to 12 months). Twenty-four (38%) cats were indoor cats, and 40 (62%) had access to the outdoors. None of the control cats were reported to have displayed behavior compatible with redirected aggression.

With respect to the 22 episodes of redirected aggression reported for the 19 cats, the most common inciting stimuli were loud noises (11/22 [50%]), followed by interactions with other cats (10/22 [45%]) and visiting people (1/22 [4%]). Specific inciting auditory stimuli included loud noises from falling objects, televisions, cellular phones, and an electric drill. Eight episodes of redirected aggression triggered by loud noises occurred in cats with a concomitant noise phobia; 5 of those 8 cats displayed a clearly defensive body posture before, during, and soon after the attacks. Half (n = 5) of the 10 episodes of redirected aggression triggered by other cats involved 2 cats living in the same household. In those situations, redirected aggression occurred during a fight or after high-pitch vocalization from one of the cats.

Owners were able to remember elements of the body posture of the cats in 18 of 22 (82%) episodes of redirected aggression. When recalling 14 (78%) of these episodes, owners described a defensive body posture, including an arched lateral display, flattened ears, inverted U-shaped tail, piloerection, hissing, and highpitched vocalization. Physical characteristics of cats during the other 4 (22%) episodes of redirected aggression were characterized as direct eye contact, constricted pupils, a straightforward view, and tail lashing.

The most commonly reported alternative target was the owner (14/22 [58%]), followed by another cat living in the same household (7/22 [29%]), an unfamiliar person (2/22 [8%]), and the family dog (1/22 [4%]). In 2 aggressive episodes, there were 2 targets: the owner and another cat living in the same household. When only multicat households were considered, aggression appeared to be equally directed toward people and other cats; however, this apparent association was not statistically evaluated because of the limited number of cats in multicat homes.

Twelve of 19 owners were contacted for long-term follow-up at least 6 months after the last behavioral consultation on record. Eight of 12 cats were reported to have displayed episodes of redirected aggression after the consultation, and 2 of these had been euthanatized for that reason. The remaining 4 cats reportedly did not display redirected aggression again.

Statistical analyses revealed 3 factors that were unevenly distributed between cats with redirected aggression and control cats. Cats with redirected aggression were significantly (F² = 8.49; P = 0.004) more likely to have a sound phobia than were control cats. On the other hand, control cats were significantly (F² = 5.96; P = 0.015) more likely to be outdoor cats than were cats that displayed redirected aggression. In addition, cats living with ≤ 2 owners were significantly (F² = 6.53; P = 0.011) more likely to exhibit redirected aggression than were those living with > 2 owners. Case and control cats were not significantly different with respect to distributions of other factors such as sex, neuter status, age at acquisition, and origin.

Discussion

Aggression was the most common behavior problem reported by cat owners in the study reported here. This finding agrees with results of other studies^5,14,15 that indicate aggression and housesoiling are the most common behavioral problems in cats evaluated at referral practices. The most common alternative target of aggression in our study was other cats, which has also been reported by other investigators.^2,3,15 A survey¹⁴ performed in the United Kingdom revealed that 23% of cats evaluated for behavior problems in 2005 had reportedly displayed intercat aggression, while 13% had displayed aggression toward people. Another study¹⁶ conducted in the United States revealed that 25.1% of cats evaluated at an animal behavior clinic had displayed intercat aggression and 13.6% had displayed aggression toward people.

The proportion of cats that had displayed redirected aggression in our study (11%) was lower than that reported for another study (33%).² Investigators in that study determined that approximately 50% of all diagnoses of people-directed aggression in cats met the criteria for redirected aggression. This apparent disagreement between results may be explained by the different methodologies used in the studies. First, researchers in the other study² did not require that a primary inciting stimulus be specifically identified when classifying cats as having displayed redirected aggression; we did. In fact, in that other study, 33% of all cats in which feline aggression was diagnosed were considered to have displayed definite redirected aggression, whereas 9% of cats were reported as having displayed probable redirected aggression. Second, in the other study,² only cats that displayed people-directed aggression were included in the denominator when calculating percentages of cats with redirected aggression, whereas in our study, all cats evaluated for feline aggression were included. When the same approach used in the other study was applied to our data, the percentage of cats with redirected aggression increased to 27%.

Although redirected aggression could be secondary to other diagnoses such as fear or territorial aggression, this form of aggression is included in most classifications of feline aggression as an independent diagnostic category.¹⁷ The lack of consensus regarding the terminology used to categorize behavior problems remains a matter of discussion.¹⁸

A basic medical assessment for cats evaluated for aggression should include a physical examination and laboratory tests, including a CBC, serum biochemical analysis, and urinalysis. Nevertheless, in our study, some diagnostic tests had to be postponed to avoid stressing the cats as well as to prevent new episodes of aggression when cats had to be handled by their owners and the veterinarian. Similar difficulties in conducting complete medical examinations in behavioral medicine have been described in many clinical reports.^7,19–21

Our findings regarding inciting stimuli for redirected aggression in cats correspond with those of another study² : loud noises and other cats are the most common causes. According to the frustration-aggression hypothesis,⁹ when an inciting stimulus cannot be the target of aggression (eg, a loud noise), the aggressive reaction is redirected toward an alternative target, whereas when the inciting stimulus can be targeted, aggression may still be redirected because of the existence of a physical or social barrier.

In the present study, cats in homes with ≤ 2 people were more likely to display redirected aggression than were cats in homes with > 2 people. Although we do not have a clear explanation for this, perhaps the socialization of cats with people was better in households with > 2 people. Inappropriate socialization is one of the main reasons of fear-aggressive behavior.^3,4,22 In addition, our results indicated that indoor cats were more likely to display redirected aggression than were outdoor cats. It may be that indoor cats have fewer opportunities to escape when threatened and are less habituated to loud noises. Moreover, indoor cats may be more likely to be near other alternative targets.

Some behaviorists distinguish between displaced aggression and triggered displaced aggression with respect to alternative targets. Displaced aggression refers to a situation in which a primary stimulus induces a high state of negative arousal that is subsequently redirected to a neutral stimulus. In triggered displaced aggression, the secondary target is perceived as a provocative stimulus on its own. Thus, redirected aggression results from the effect of the alternative target on a previously aroused subject.²³ In some cases of redirected aggression included in the present study, review of the clinical history revealed a prior conflict between the alternative target and the cat, whereas in others, the alternative target had an otherwise neutral and even friendly social relationship with the aggressor. A similarity between the triggering stimulus and alternative target has been suggested.^9,24 In other words, when the triggering stimulus is a cat, the alternative target will also be a cat. Our results do not support that hypothesis because people were the most common target in our study. It could be argued that not all cats in our study lived with other cats and, therefore, other cats were not available as a target. However, even when only multicat households were considered, aggression appeared to be equally directed toward people and other cats, although this comparison was not evaluated statistically. This could suggest that the alternative target is a matter of chance.

Behavior problems in cats can become chronic, depending on the tolerance of the owner. That tolerance is influenced by previous experience, cultural background, and expectations regarding acceptable behavior.²⁵ According to the results of our study, with the exception of 2 cats, owners reported just 1 episode of redirected aggression that occurred prior to the consultation. This finding supports the idea that redirected aggression is an intense, unpredictable, and dangerous form of aggression that impels the owner to quickly seek help.

The function (if any) of redirected aggression in cats is still unclear. Redirected aggression may serve as a coping mechanism when cats are exposed to stressful events. In support of that hypothesis are studies^11,26,27 that have revealed that acts of redirected aggression decrease plasma glucocorticoid concentrations in rats, baboons, and fish. Two findings from our study may give additional support to this hypothesis. First, in most situations of redirected aggression, owners reported that their cats displayed a clearly defensive posture, suggesting the underlying motivation was fear, which is linked to stress.^28–30 Second, cats that displayed redirected aggression were more likely to have noise phobias than were control cats, which suggests that they perceived noises as more stressful. However, additional research is needed to obtain a better understanding of the physiologic and functional bases of redirected aggression. On the basis of the findings of our study, we recommend that veterinarians encourage owners to socialize their kittens via interaction with unfamiliar people and other cats and to habituate kittens to various sounds and novel objects.