Aggressive Behavior

Characteristics of Dogs That Bite: Age and Sex

The etiology of aggressive behavior presents considerable variation from dog to dog. Aggressive behavior is most frequently exhibited by socially mature and intact male dogs (), but young puppies can have serious precocious aggression problems, as well. Mugford (1984) reported that among 50 English cocker spaniels the mean average age of dogs with dominance-related aggression was 7.4 months (range, 3 to 24 months). In another group of golden retrievers treated by Mugford (1987), 24 with aggression problems averaged 2.9 years of age (range, 0.7 to 8.0 years). Of the 24 dogs treated by Mugford, 19 were males, two of which had been castrated. Beaver (1983) found that of 120 dogs with aggression problems (various diagnoses) the mean age was 3 years (range, 9 weeks to 11 years). She reported that 60.1% of the dogs were intact males (14% castrated), with 15.4% intact females (10.5% spayed). Wright (1985) found that the average age of dogs involved in severe attacks was 3 years (range, 0.67 to 10.5 years). All 16 dogs were males. These statistics suggest that considerable variation exists with respect to the time of onset associated with aggression problems. Although most dogs are presented for treatment at 1 to 3 years of age, incipient signs of a developing problem are frequently observed in young puppies, often prior to 4 months of age.

Incidence and Targets of Aggression

Although a number of studies indicate that dog bites against people represent a serious problem, perhaps even having reached epidemic proportions (), the available statistics are incomplete and inadequate. A notable problem is the dog population sampled. Many of the statistics discussed below were obtained from urban populations that may be skewed by a disproportionate number of aggressive, guard-type dogs. Harris and colleagues (1974) note that urban dwellers frequently keep and socialize aggressive dogs to enhance home security in high-crime areas. Also, the number of social contacts in which bites might occur are probably substantially more numerous in the city than in the suburbs or the country. Consequently, it is difficult to draw any hard and fast generalizations, outside of those directly related to the particular populations sampled. In contrast, the statistical information concerning fatal dog attacks is considerably more reliable and complete. What is extraordinary about fatal attacks is the relative rarity of such incidents when considered in the context of the many millions of intimate contacts occurring between dogs and people every day. Statistically, a child’s life is far safer in the presence of its family dog than in the hands of human caretakers or parents.

Overall Situation: Total Number of Bites and Implications

The overall number of dog bites occurring in the United States is widely disputed among reporting authorities. These differences of opinion are attributable (in part) to statistical errors stemming from erroneous population estimates [see Mathews and Lattal (1994)], inconsistent definitions of what constitutes a dog bite, the absence of a consistent and reliable method for tallying dog-bite incidents, and widespread underreporting of dog-bite incidents. A task force on aggression, organized by the American Veterinary Medical Association (AVMA) (), found that there is a need to standardize the ways in which dog bites are reported. The task force has suggested that standardized forms be produced for collecting information about the age of the bite victim, the circumstances of the incident, the extent of the injuries, and the signalment of the dog. In addition, the task force hopes to better define legal requirements for reporting dog bites and to develop better means for collecting and keeping dog-bite statistics. Unfortunately, the AVMA task force did not include a professional dog trainer — a significant oversight, since most owners with dog-aggression problems turn to such people for advice and guidance.

Despite the inherent limitations involved and the risk for erroneous generalizations, a careful study of relevant statistics is revealing and useful. According to the AVMA (1997), approximately 52.9 million dogs live in the United States. The AVMA figure is somewhat lower than the Pet Food Institute’s (PFI) (1999) estimate of 57.6 million dogs, with approximately 37.6% of all American households keeping at least one dog. Calculating the number of dog bites is a much harder statistical task, with the current best guesses ranging from 2 to 5 million dog bites occurring each year. Pinckney and Kennedy (1982) estimated that approximately 2 million people are bitten each year in the United States, with a tenth of these victims requiring sutures, a third missing time away from school or work, and half receiving permanent scarring as the result of their injuries. Since many minor bites and bites delivered by familiar dogs are not reported, the actual number of dog bites is probably higher than this conservative estimate. In 1996, Sacks and colleagues (1996) at the National Center for Injury Prevention and Control estimated that approximately 4.7 million people are bitten in America each year. Of these victims, 899,700 persons required medical attention. They estimate that children were 1.5 times more likely to be bitten, and over 3 times more likely to require medical treatment than adults. Besides the emotional and physical pain of dog attacks to the victims, dog bites represent a serious legal and monetary liability to dog owners. The Insurance Information Institute (1999) estimates that dog bites cost the public approximately 1 billion dollars per year in losses, with insurance companies paying out $250 million to resolve dog-bite claims in 1996. State Farm Insurance (1999) alone reported paying nearly $80 million in dog-related liability claims in 1997. According to State Farm Insurance, one in three homeowner claims involving personal injury pertain to a dog bite, with an average payout of $12,000 per bite incident.

Vital Characteristics: Age, Sex, Risk-taking Propensity, Location of Attacks, Time of Day/Season, and Bodily Target of Attack

Children are bitten at a disproportionate rate when compared to other population groups (). It should be noted in this regard, however, that children are also most commonly associated with homes that keep dogs as pets (). Approximately 1% of all children brought for emergency treatment are victims of dog bites (). Adams and Clark (1989) found that 38% of 105 dog owners interviewed reported that their dog had “nipped” at children or had bitten someone — 62% of these bites were directed toward family members. The majority of dog bites are directed toward children 5 to 14 years of age ().

Boys are bitten nearly twice as often as girls (). Boys also receive the majority of severe bites (60% to 78%) (). Sacks and coworkers (1989) found that, among 29 children between the ages of 5 and 9 who suffered a fatal dog attack, 23 (79.3%) of them were boys. The first clear sign of a sexual differentiation of victims is evident in the 1- to 4-year-old group, with 64.2% of the them being boys. A possible explanation for this difference may be due to the amount of time spent by boys versus girls interacting with dogs. Lehman (1928) performed a large statistical study involving 5000 respondents to determine how children spent their time playing. Children of various ages were asked to respond to a series of questions regarding their daily play activities. He found that boys tended to spend more time interacting with dogs than girls did, with both groups showing a steady decline in the amount of time spent playing with pets (both dogs and cats) as they matured. Another possible cause for the uneven distribution of dog bites between boys and girls may be attributable to a boy’s greater inclination to engage in risk-taking behavior ().

Most bites occur during the summer months (peaking in June) and weekends. On the average day, they are most frequent from 1:00 to 9:00 PM, peaking between 3:00 and 7:00 PM (). Wright (1990) has also reported seasonal and time-of-attack trends. Among 1724 dog bites reported in Dallas, the incidence of attacks peaked between March and May, with 34.6% of the bites occurring during those 3 months. The majority of dog bites (55.8%) took place from 2:00 to 8:00 PM, peaking between 5:00 and 6:00 in the late afternoon. Sacks and colleagues (1989) were unable to detect a similar seasonal trend in the case of fatal dog attacks. Fatal attacks involving pet dogs were actually more common in the winter, whereas stray-dog attacks occurred more often in the fall and least often in the summer.

The majority of bites involving young children are directed toward the face and head, with children under 4 years of age being bitten in the face, head, or neck 63% of the time (). Beck and colleagues (1975) found that 35% of the bites involving children younger than 4 years of age were directed toward the face. In children between 5 and 9 years of age, this pattern shifts dramatically, with 84% of bites being directed toward the extremities or torso and 18.5% toward the face or neck (). Among a population of children receiving severe injuries, 82% of the bites were directed toward the victim’s head or neck.

Dog-bite reports analyzed by Wright (1991) indicate that 87.5% (range, 85.5% to 89.4%) of the dogs involved are owned, with attacks being directed toward family members in 10.5% (range, 5.9% to 15%) of the cases. These estimates probably underrepresent the actual number of persons bitten by their own dog. In a large study involving 3200 children between the ages of 4 and 18 surveyed, 45% had been bitten by a dog at some point in their life. About half of these children were bitten by a neighbor’s dog, whereas nearly a third reported being bitten by the family dog ().

Emotional Trauma of Dog Attacks on Children

Surprisingly Jones and Beck found that the experience of being bitten had little effect on the person’s later preference for the dog as a pet. This finding has significant implications for the study of cynophobia, since one would expect from the classical conditioning model of fear that a dog bite should have a lasting negative impact on a child’s attitude toward dogs. In fact, some recent and better-controlled research appears to indicate that there exists a significant independence between having experienced a dog bite as a child and the later development of cynophobia or fear of dogs. Two studies are of particular interest in this regard. First, DiNardo and coworkers (1988), utilizing heart-rate changes as a physiological measure of anxiety, were unable to detect a relationship between a previous dog bite and increased physiological arousal when people were tested in the presence of a friendly dog. Second, Doogan and Thomas (1992) found that most cynophobic adults report that their fear of dogs began in childhood, but there is no clear correlation between the frequency of attacks in childhood and the subsequent development of fear toward dogs. The most important factor in the etiology of such fear is the amount of contact that a person had before the onset of fear. People having minimal contact with dogs as children are more prone to exhibit fearfulness as adults. The researchers suggest that prior “noneventful” exposure to dogs may impede the development of phobic reactions in response to dog bites and other sources of fear (e.g., inimical warnings about dogs):

The role of conditioning events in producing fear of dogs must be considered as nonproven. If such conditioning events do play a causal role then it is only in conjunction with some other factor such as lack of prior uneventful exposure to dogs or in especially susceptible individuals. The present results from children suggest that information transmission may be more important in engendering fear of dogs than studies of adults might suggest. Although most fearful adults report that their fear of dogs began in childhood, it is clear that not all dog-fearful children grow up to become dog-fearful adults, which raises the question of why some children, but not others, eventually lose their fear of dogs. ()

Dogs That Kill

Of particular concern for parents is the possibility of a fatal attack being directed toward an infant or toddler. Although such attacks occasionally occur, most serious attacks are directed toward older children, especially boys. Voith (1984) believes that the majority of fatal or serious attacks directed toward infants are probably instigated by aberrant predatory motivations rather than by sibling rivalry or other commonly cited motivations such as jealousy. Most fatal dog attacks are delivered by dogs known to the victim or the victim’s family, with the majority of them being delivered by the family dog or a neighbor’s dog. Most of the dogs involved had no prior history of aggressive behavior and attacked without known provocation by the victim ().

It should be emphasized that fatal dog attacks on babies are extremely rare. From 1979 to 1988, the total number of infants (birth to 11 months old) killed as the result of a dog attack in the United States was 25. Children at the greatest risk for exposure to a fatal dog attack belong to the 1- to 4-year group, with 56 toddlers dying from dog attacks over that same period (). A more recent study by Sacks and coworkers (1996), covering the years from 1989 to 1994, reported a total of 109 bite-related fatalities, with 57% of the deaths involving children under 10 years of age. Another age group at a higher risk is the elderly, with 18% of the fatal attacks involving persons over 70 years of age (). The researchers found that 77% of the fatalities involved attacks occurring on the owners property, with 18% of the dogs restrained and 59% of them unrestrained. Overall, the death rate involving fatal dog attacks has remained relatively constant over the past 16 years, with approximately 15 to 18 fatal dog attacks in the United States each year.

Dog Attacks versus Human Fatal Assaults on Children

Despite the tragic occurrence of dog attack fatalities, the average child is at a far greater risk of being seriously hurt or killed by a parent or relative than by the family dog. A recent report compiled by the U.S. Department of Health and Human Services (1999) found that 1196 children were killed in the United States as the result of maltreatment in 1997. An earlier government study placed that number closer to 2000; that is, approximately 5 children every day lose their lives to maltreatment and child abuse homicide (U.S. Advisory Board on Child Abuse and Neglect, 1995). Over 85% of the perpetrators are either parents (75%) or relatives (10%) of the victim. In addition to deaths, nearly 1 million children experience substantiated or indicated abuse and neglect annually. According to the USDHHS study, children 3 years of age or younger accounted for 77% of the reported fatalities. By way of comparison with dog attack fatalities, according to the aforementioned study performed by Sacks and colleagues (1996), during the 5 years between 1989 and 1994, 45 children (from birth to 4 years of age) were killed by dogs. During a similar length of time, extrapolating from the foregoing statistics for 1997, among children 3 years or younger, an estimated 4605 were killed by humans. Given that approximately nine children of this age group are killed by dogs each year, these sobering statistics of child-abuse homicide reveal that it would take dogs over 100 years to kill as many children as are killed by their own parents, relatives, and other guardians on an annual basis. In other words, in any given year, children at greatest risk of abuse are 100 times more likely to be killed by a parent or relative than by the family dog.

Voith and Borchelt (1985) state the threat of serious dog attacks on children in a fair and balanced way:

Few infants are severely injured by dogs, and the number of infants killed by dogs is very small, probably no more than 10 per year throughout the entire United States. In contrast, many thousands of infants in the U.S. are victims of automobile accidents, burns, drowning, choking, suffocation, and poisoning. It has also been estimated that each day in the U.S. one child under 10 years of age is killed in a handgun accident. Despite the small risk, there is still cause for concern about a dog’s reaction to your infant and precautions are well worthwhile.

When fatal or severe dog attacks occur, the situation is often exploited by “expert” media pundits who frequently fail to emphasize the statistical rarity of such anomalies, while pandering to the publics morbid interest in the gruesome details. Overall, the effect is to produce terror and media hysteria over a widespread threat that does not exist. Such incidents predictably spawn demands from dog-hating politicians and other busybodies for immediate action, including stronger animal control regulations and unfair legislation restricting dog breeding and ownership. Obviously, efforts must be made to educate the public about the risk of dog bites and how they can be prevented, but this can be accomplished without resorting to alarmist, unfair, and divisive breed-specific legislation punishing innocent dogs and owners for the actions of a few culpable and irresponsible offenders.

Despite the gloomy appearance of the foregoing statistics, most epidemiological studies have found that the majority of dog bites result in minor physical injury (). In a major study by Parrish and colleagues (1959), 88% of the bite injuries treated were judged to be minor, with 2% producing no evidence of injury. This is not to say that serious attacks do not occur — they do and all too frequently — but the majority of dog bites are neither life-threatening nor disabling for the victim. Although dog bites result in relatively minor injuries, it is important that efforts be taken to prevent such attacks. These efforts should include appropriate education for both children and parents (). Other key pre-ventative measures include early training and socialization of dogs, responsible breeding and selection of dogs that are destined for homes with children, and early behavioral intervention when problems first appear. In addition, children should be taught how to interact more safely with dogs, and parents should become better informed about how to control their children around dogs.

Basic Categories

Aggressive behavior is expressed in one of three general ways: threat, defense, or attack. The sort of aggression that a dog exhibits depends on its motivational state and the presence of significant triggers. Konorski (1967) divides aggressive behavior into two general types, depending on the behavioral traits of the organism and the environmental or motivational circumstances present at the moment of arousal (see Preparatory and Consummatory Reflexes). He notes that the same trigger stimulus may elicit either fear or anger in the stimulated animal and consequently result in either defensive or offensive actions. For example, painful stimulation may evoke a massive fear-and-escape reaction in a solitary animal, whereas if the animal is in the presence of a companion, the same stimulation could result in an angry offensive attack (). Konorski also points out that the differential display of defensive or offensive behavior is strongly influenced by environmental circumstances, noting that an animal threatened on its own territory is more likely to become angry and engage in offensive aggression, whereas the same animal may show fear and react defensively if threatened while in an unfamiliar place. Fear- and anger-elicited attacks are forms of affective aggression, both involving the presence of a high degree of emotional arousal. As is discussed in greater detail below, affective aggression is distinguished from predatory or quiet-attack behavior.

Krushinskii (1960) argues that defensive behavior in dogs presents in two characteristic ways: passive and active defensive reactions. Passive defensive reactions take the form of fear and include all types of freeze and flight responses elicited, for example, by loud-unfamiliar sounds or the close presence of strangers. An active defensive reaction, or what Pavlov refers to as a “watch reflex,” is expressed in two forms: (1) defensive barking without an effort to bite or (2) defensive behavior that includes an effort to bite. According to Krushinskii, the dog’s active and passive defensive behaviors are the result of a combination of various unitary reactions that are coordinated to produce complex behavioral patterns having biological significance for the dog as a species. Unitary reactions are variably composed of both conditioned and unconditioned reflexes. However, unlike individual conditioned and unconditioned reflexes, whose pattern of expression is apparent from the beginning (stimulus) to end (response), the unitary reaction is only fully recognized during the final stages of its expression. Unitary reactions are functionally integrated and organized into species-typical behavior patterns and epigenetic routines in order to perform various social and biological functions efficiently by means of interacting with the environment. Consequently, although conditioned and unconditioned reflexes variably influence behavioral thresholds (e.g., fear and anger), the functional significance of defensive unitary reactions only become evident as they are organized into integrated species-typical patterns of active and passive defensive behavior.

In general, two broad categories of aggressive behavior exist, intraspecific and interspecific, depending on whether the aggression is directed toward conspecifics or toward other animals not belonging to the aggressors species, respectively.

Intraspecific Aggression

Intraspecific aggression consists of both ritualized and overt forms of aggressive behavior directed toward conspecifics, that is, individuals belonging to the same species. Most intraspecific aggression is highly ritualized and serves some biologically significant function (e.g., social organization, population dispersion, or sexual selection). In general intraspecific aggression provides a countervailing and distance-increasing function over place and social attachment processes but without breaking down affiliative contact altogether. As such, ritualized intraspecific aggression imposes social order (e.g., the formation of a dominance hierarchy) and territorial limits on the interaction between individuals belonging to the same species. This ordering and distancing function of aggression is especially evident among familiar individuals belonging to the same social group. Whereas aggression directed toward conspecifics belonging to the same group is often highly ritualized and inhibited, aggression toward conspecific outsiders is usually not so well inhibited and may, as among wolves, result in an overt attack and the intruder’s death if it cannot put up an adequate defense or escape by running away.

Interspecific Aggression

Interspecific aggression refers to aggressive behavior directed against another species and includes both offensive and defensive elements. Although intraspecific aggression is most often associated with competition between closely socialized animals belonging to the same species, interspecific aggression is most frequently associated with self-protective goals, as, for example, occur when a prey animal defends itself against the attack of a predator. The dog’s relationship with humans is complex in this regard, with both competitive and self-protective aggression being exhibited under different situations. Many ritualized elements of intraspecific aggression are shown toward people with whom dogs are closely socialized. On the other hand, dogs may also exhibit defensive behavior aimed at self-protection and having nothing to do with the establishment of dominance and territory, as appears to be the case in most forms of intraspecific aggression. In the absence of adequate socialization, interspecific aggression predominantly consists of defensive behavior, lacking ritualization and inhibition, and performed with the intention of doing damage.

Many forms of aggression classified as dominance related () may be more defensive than offensive. For example, Line and Voith (1986) report that the majority of attacks by dogs diagnosed with dominance aggression occurred while the dogs were being disciplined. Some dogs may interpret human disciplinary actions (hitting, slapping, kicking) as physical threats and react aggressively in an effort to defend themselves. Predatory behavior is often viewed as a form of interspecific aggressive behavior, but, as will be discussed momentarily, predation is not influenced by the same motivational substrates mediating the expression of competitive and self-protective (affective) aggression. In fact, affective and predatory aggression appear to have evolved under independent pressures and are regulated by relatively distinct and segregated neural circuits and hormonal systems. In many ways, predation is more appropriately interpreted as a form of food-getting behavior motivated by hunger and mediated by the seeking system ().

Classifying Aggression: Motivational Considerations

A Nomenclature of Aggressive Behavior

Predatory Behavior

Moyer’s inclusion of predatory attack as a form of aggression alongside fear-induced or irritable attack is questionable and potentially misleading. As previously mentioned, predatory behavior might best be treated under some independent category such as “killing for food.” This seems appropriate, since predatory behavior is not primarily motivated by affective arousal (anger). In addition, predatory aggression or quiet attack typically occurs without signs of sympathetic arousal. In contrast, affective attack is distinguished by the presence of strong sympathetic arousal and anger. Predatory behavior appears to belong to a distinct behavioral and neurological system operating independently of affective aggression, perhaps involving the appetitive seeking system (). An interesting neu-robiological finding in this regard is the observation that the neurotransmitter norepinephrine inhibits predatory aggression while facilitating affective aggression like fighting (). These findings (and many others like them) support the assumption that predatory aggression and affective aggression are mediated by very different biological and behavioral systems (see Neurobiology of Aggression [Hypothalamus]). Although predation belongs to an independent motivational system, predatory behavior may be influenced by coactive anxious and frustrative influences that may ultimately lead to the expression of affective aggression, a possibility emphasized by Panksepp:

Of course, this does not mean that the whole predatory attack sequence or any other real-life emotional pattern ever remains under the control of a single emotional system. A predator surely experiences irritability or frustration if the prey struggles so vigorously that it seems liable to escape. Thus, in real life, there are sudden shifts in emotions depending upon the success or failure of specific behavioral acts, as well as in the changing cognitive expectations and appraisals of each situation. ()

None of the foregoing should be construed to imply that predatory aggression is innocuous or in any sense less dangerous than other forms of canine aggression. Predatory motivations have been implicated in several cases involving vicious maulings and deaths of humans by dog packs (). In one of these cases, a large pack of eight dogs, with a known history of predatory behavior, attacked and killed a 14-year-old boy who was riding a motorcycle. Reportedly, the pack had been observed earlier attacking a deer that they had brought down but that managed to escape. This incident occurred approximately 1 hour prior to the attack on the boy. In another incident, a pack of dogs attacked an 11-year-old boy who survived severe injuries to report hearing the dogs “baying, as if chasing something” approximately 15 minutes before the attack. In both cases, there appears to have been a frustrated or redirected predatory motivation involved in the attacks, suggesting that some forms of “predation” are motivated by more than simple hunger and nonaffective neural circuitry. Another case involved a pack of several dogs that was kept by an elderly couple in rural Indiana (). The dogs, which were permitted to run free, attacked a 10-year-old girl riding her bicycle near the couples property. In an effort to escape the attack, the child ran into a nearby wooded area, where she was later found dead. The child received numerous wounds and parts of her flesh had been torn away and apparently eaten by the dogs (). Winkler (1977) reviews the case histories of 11 fatal dog attacks and cites “threatening behavior or territorial invasion” as the most common causes, without mentioning the possible role of predation or a history of predatory behavior in the dogs involved. Although not mentioned specifically, several of the cases he describes are not entirely inconsistent with a predatory interpretation. Incidentally, of the nine cases where the sex of the dog was known, males accounted for seven of the attacks, with the remaining incidents involving a female and a male and female pair. These data suggest that male dogs may be at a significantly greater risk of delivering a fatal attack than are female dogs.

Genetics and Aggression

There appears to exist a strong heritable factor affecting the predisposition of dogs to behave aggressively. Numerous studies have identified a genetic influence affecting animal behavior in the opposing directions of increased fearful behavior, on the one hand, and increased aggression, on the other (see Genetic Predisposition and Temperament). In general, domestication has exerted selective pressure toward behavioral thresholds conducive to reduced fear and aggression, thereby making dogs more socially responsive and tamable by humans (). Although the general trend has been toward a reduction of fear and aggression, significant variations of excitability exist between breeds and individuals within these different breeds. With respect to aggression, some dog breeds appear, on the whole, to be more aggressive and reactive than others to emotionally provocative stimulation. Scott and Fuller (1965), for example, found clear differences in the aggressive behavior of different breeds emerging at an early age. Of the five breeds observed and tested, they found wirehaired fox terriers to be the most aggressive, basenjis and shelties somewhat less aggressive, and beagles and cocker spaniels much less aggressive. Hart and Hart (1985) analyzed the cumulative opinions of 48 veterinarians and 48 obedience judges with respect to the ranking of 56 breeds according to 13 behavioral traits. They found that the surveyed professionals shared significant uniformity in their assessment of various traits, enabling the authors to perform a cluster analysis for the various breeds represented in terms of such things as their relative aggressiveness, trainability, and reactivity. Their results show some conformity with Scott and Fullers earlier findings. For example, the fox terrier is included in the cluster characterized by “very high aggression, high reactivity, medium trainability” whereas beagles and cocker spaniels are included together under the cluster “high reactivity, low trainability, medium aggression.” Although such statistical studies as the above represent a good starting point, the results are difficult to generalize because they are limited to personal opinions about behavior — not objective assessments. Even the opinions of professionals are subject to considerable individual and cultural bias. In other words, the study tells us more about how veterinarians and obedience judges feel about the behavior of various breeds than it tells us about the actual behavior of the breeds specified. To make the results reliable with respect to dog breeds, they must be validated by comparison with more objective assessment tests and experimental observations of breed differences, such as provided by Scott and Fullers work.

A putative heritable factor in the expression of dominance-related aggression has been identified in the English springer spaniel (ESS). As the result of a random national survey of ESS owners, Reisner (1997) found what appears to be a significant breed disposition toward developing dominance-related aggression. She reported that 26% of the ESS had bitten someone, with 65% of those persons bitten being family members or people with whom the dog was familiar. In addition, 48% of the dogs had growled at, snapped at, or bitten family members in a dominance-related context. Finally, the tendency to exhibit dominance aggression was associated with dogs coming from one particular kennel, suggesting the possibility of a popular sire effect. The influence of breed predisposition is apparent in some epidemiological studies of reported dog bites (Wright, 1991). Although mixed breeds are most often implicated in biting incidents, representing between 41.1% and 47.4% of bites reported, some specific purebred dogs appear to represent a greater risk than others. For example, Gershman and colleagues reported that German shepherds and chow chows were most likely to bite non-household members, victims who were often children. It should be emphasized, however, that interpreting breed-related bite statistics is fraught with difficulties (), not the least of which is breed identification. Many dogs may be misidentified and lumped together under a particular breed. Also, as Wright points out, statistical bite rates relative to breed must be carefully weighted against the numbers of a particular breed living in the geographical area from which the sample is derived — a requirement that is not usually satisfied by statistical analyses comparing dog bite rates by breed.

Hormones and Aggressive Behavior

Nutrition and Aggression

A great deal of speculation exists concerning the effects of nutrition on behavior, but little scientific knowledge is known about these effects. Animal behavior consultants commonly cite this or that nutritional imbalance as being responsible for causing or predisposing dogs to a exhibit a particular behavioral problem. Recommendations ranging from supplemental B complex for aggressive behavior to massive doses of calcium and other minerals for destructiveness have never been demonstrated clinically or in the laboratory. Campbell (1992), for example, claims that a positive correlation exists between relative protein/carbohydrate proportions in a dog’s diet and general excitability levels. High protein levels supposedly decrease excitability while at the same time producing various benefits such as increased trainability. In the opposite direction, high carbohydrate levels are believed to increase excitability and promote distractibility. In addition, he recommends supplementing his stress diet with B complex as nutritional “insurance,” even though the dog appears healthy without it. Unfortunately, no experimental data are presented to support these various recommendations or the hypotheses on which they are founded.

Over the past several years, a growing concern has been expressed regarding the effects of food coloring and chemical preservatives on the development of hyperactivity and other behavior problems (see Dietary Factors and Hyperactivity). One result of this concern has been the production of a new generation of diets containing fewer additives — a change in dog food manufacturing that can do no harm, but the potential good of such diets is not clearly known or demonstrated. Research on this topic is scanty and, at present, little scientific evidence exists showing a direct causal relationship between food additives and the incidence of behavior problems in dogs.

However, some evidence does suggest that adjusting dietary protein levels may provide a viable means for influencing the behavioral thresholds of some forms of aggressive behavior (see Diet and Serotonin Activity). For example, Mugford (1987) reported observing a significant decrease in aggressive behavior in a group of golden retrievers after they were placed on a low-protein diet. More recently, a multiclinic study that compared the effects of low-protein versus high-protein diets on aggressive behavior in dogs found that reducing dietary protein levels exerted a beneficial influence in dogs exhibiting territory-related aggression with fear (). The strongest evidence for a linkage between aggression and dietary protein levels has come from basic brain research. Numerous studies have indicated that dietary protein levels significantly affect the amount of tryptophan reaching the brain for the manufacture of serotonin (). Paradoxically, high levels of circulating protein in the blood may deprive the brain of adequate tryptophan. This effect is due to a transport mechanism responsible for the selective transfer of nutrients from the blood into the brain. When the blood contains high levels of protein, other relatively more abundant circulating amino acids compete with tryptophan for a limited number of transport molecules, thereby causing an impediment of tryptophan transport into the brain. This situation can be nutritionally modified by simultaneously lowering dietary protein levels while increasing the intake of carbohydrates. The ingestion of carbohydrate-laden foods stimulates the secretion of insulin. Insulin biochemically alters competing amino acids, causing them to move into surrounding muscle tissue. The net result is that tryptophan obtains a numerical advantage over other amino acids competing for limited transport channels providing passage through the blood-brain barrier.

Serotonin serves many important functions as a neurotransmitter, especially the management of stress, impulse control, and mood regulation. Decreased serotonin activity is associated with depression and increased irritability. Many antidepressant psychotropics are believed to work by increasing serotonergic activity. When serotonin levels are low, dogs may become more impulsive and irritable and exhibit a lowered threshold for aggressive behavior. Diets adjusted toward decreased protein intake (less than 18%) coupled with increased carbohydrate intake appear to exercise a mild threshold-raising influence, perhaps by enhancing serotonin-mediated impulse control and improving the brains ability to manage stress. Recently, DeNapoli and colleagues (2000) have reported evidence suggesting that supplementation of the canine diet with tryptophan may exercise a significant modulatory effect over certain forms of aggressive behavior. In the case of dominance aggression, tryptophan supplementation of high-protein diets yielded a significant decrease in aggression scores. In the case of territorial aggression, scores were most strongly reduced in dogs that were fed a low-protein diet supplemented with tryptophan. Although this research is promising, increasing nutritional tryptophan levels may not necessarily result in an appreciable increase of serotonin production. Above a certain point, the rate-limiting factor, tryptophan hydroxylase, is saturated and unable to support further synthesis of 5-hydroxytryptophan (5-HTP) — the immediate precursor of serotonin (5-HT) (). Given the aforementioned limitation, supplementing the protocol diet with 5-HTP might have proved significantly more efficacious for enhancing serotonin production. In addition to being more directly and efficiently converted into serotonin than tryptophan, 5-HTP moves more freely through the blood-brain barrier (not needing to compete for transport molecules). Furthermore, unlike tryptophan, which remains banned from over-the-counter sale, 5-HTP is readily available and sold at health food stores () — a significant consideration if 5-HTP is ultimately shown to exert a beneficial effect on aggression problems in the dog.

Aggressive Behavior: Role of Integrated Compliance and Obedience Training

Selections from the book: Handbook of Applied Dog Behavior and Training. Volume Two. Etiology and Assessment of Behavior Problems (2001)