It is estimated that dogs are the first animal species domesticated by humans. As a result of selective breeding, about 400 different breeds of dogs are recognized around the world, representing a large variety in terms of size, use, or weight ^[1][2][3][1,2,3]. In addition, keeping dogs was associated with the need to learn about their behavior in order to maintain proper relations between the dog and the owner, as well as to eliminate the undesirable behavior, such as aggression ^[1][2][3][4][1,2,3,4].

To aim to generalize the dogs’ behavior and understand their behavioral needs, ethograms have been developed. Communication of any kind, including aggression, is a natural part of a dog’s ethogram. An ethogram is a species-specific list of natural behavior. Dog ethograms include affiliate, agonistic, defensive, sedative, sexual, demonstrative, warning, stressful, playful, grooming, exploratory, related to hunting, or related to the reduction of stress ^[5][6][5,6]. Additionally, in recent years, new methods have been developed to help understand how a dog’s brain works: neurocognition. The combination of the dietary aspect, physiology (endocrine system), and neurocognition may allow for a thorough understanding of the impact of aggression on the dog’s body and to obtain more detailed information on the basis of its occurrence ^{[7][8][9][10]}[7,8,9,10].

Understanding the causes of the occurrence of aggression in dogs is crucial to both animal welfare and the safety of their caregivers. In the circumstances in which a dog shows undesirable aggression, they can inform the competent caregivers about the disposition of the animal, as well as take appropriate measures to eliminate aggressive behavior [4]. Additionally, in North America, aggressive behavior is strongly contributed toward the abandonment and/or euthanasia of animals, especially dogs [8]. Many types of aggression can be distinguished in dog behavior depending on motivation (defensive/offensive), target (other animals, owner, unfamiliar people), etc. In addition, the definition of the underlying cause of aggression has not been fully explained, as it can be both behavioral (related to unmet needs, especially in the early time of life, anxiety or stress, poor socialization, etc.), diseases (such as cancer, brain disorders or diet, etc.) or genetics ^{[9][10][11][12][13][14][15]}[9,10,11,12,13,14,15]. Recently, apart from the use of appropriate training for behavioral disorders, the diet and physiology (including the endocrine system) of dogs have also come to be seen as an aspect of the disorders [14]. In particular, the focus was placed on nutritional additives such as fatty acids, amino acids, and probiotics.

2. Aggression—Definitions, Types, Consequences, and Possible Causes

Dog aggression can be divided depending on the mental state of the dog and the circumstances in which the aggression appears. Acts of aggression can be categorized against dogs, people, other animals, and moving objects, such as bicycles and cars ^[10][13][15][10,13,15] (Table 1). In this paper, however, the definition created by James O’Heare [20] will be used. The author summed up aggression as “an attack, a deliberate attack, or a threat of an attack on another individual”. The types of dog aggression can be categorized according to the sequence of behaviors and the circumstances in which they occur. The following types of aggression can be distinguished; defensive, distancing, territorial, maternal, irritable, fearful, displaced, competitive between dogs, competitive between dogs and people, possessive, possessive, between dogs belonging to different social groups, hunting, excessive, on command, etc. Figure 1 [17]. Aggression is a natural part of dog behavior, and it is part of the ethogram. However, excessive aggression disrupts the relationship between the dog and its environment ^{[17][18][19][20]}[17,18,19,20]. Aggression can be based on motivational basis (territorial-, fear-, possessiveness-related, etc.) or target basis (stranger-, owner-, dog-directed, etc.) [33]. The change of motivation occurs most often when one of the dog’s needs has not been met. For example, in a hungry dog, the change of motivation will manifest itself in looking for food, devoting all the time to it, including the rest period. It will be accompanied by anxiety, distraction, and irritability. The change of motivation also occurs at the time of the onset of the disease. Pain can lead to an avoidance of touch and a closer relationship with the environment and prompts the animal to defend itself, thus showing aggressive behavior [10]. Frustration can change your motivation, which may occur when a reward for expected behavior is missing/delayed or when the dog cannot achieve the goal ^[34][35][34,35]. Frustration-induced behavior can vary depending on the situation and the nature of the dog. As a result, the dog may increase efforts to achieve the desired goal or vocalize ^[36][37][36,37]. Frustration has been found to be an important emotion that influences a number of behavioral problems in dogs. Barrier frustration may arise when the dog is unable to achieve the desired goal by restricting its accessibility, such as with a leash, which may result in diversion and/or aggressive behavior ^[37][38][37,38]. Studies carried out in the years 2008–2016 showed that aggression against unfamiliar dogs (aggression targeted at the dog) had been shown by 22–47% of studied individuals. In many cases of such aggression, the death of one of the dogs involved can be a consequence ^[39][40][39,40]. A study by Van der Borg et al. [41] on Rottweilers showed that as many as a quarter of dogs (mainly without pedigrees) showed behavior related to aggression targeted at both humans and animals. In addition, they were also, in most cases, aggressors in a given interaction. The occurrence of targeted aggression in dogs in such situations may be due to a low level of anxiety (when the aggressor is the dog). Some breeds originally used as protection and guard dogs have a natural predisposition to low levels of anxiety ^[42][43][44][42,43,44]. On the other hand, when a dog with a high level of fear is subjected to a “stressor” such as unfamiliar people, other pets (such as cats or dogs), or the environment, it may result in defensive aggression ^[43][44][43,44]. Aggression based on fear can also be related to defensive aggression. This aggression is related to the animal’s reaction to a factor that it considers to be a threat, causing fear. This type of factor can also cause physiological and behavioral changes. The main physiological changes are increases in blood pressure, changes in cortisol levels, body temperature, increases in heart rate and blood glucose levels. In terms of behavioral changes, activation takes place at this time of the sympathetic adrenal medullary axis and the hypothalamic-pituitary-adrenal cortex axis ^{[30][45][46][47]}[30,45,46,47]. The occurrence of aggression in dogs has consequences related to their abandonment or euthanasia. Additionally, in the case of some breeds, their keeping or breeding has been subject to legal regulations, such as American Pit Bull Terrier or Tosa Inu in most European countries ^{[26][48][49][50]}[26,48,49,50]. The created lists of aggressive dog breeds are controversial and do not provide certainty about the possibility/risk of aggressive dog’s behavior. Therefore, the issues of risk assessment are important, primarily based on two main aspects: the first is the ability to predict dog bite risk factors in the general canine population, and the second is how to assess and predict future risk to society ^[51][52][53][51,52,53]. In a study by Notari et al. [45], it was suggested that aggressive behavior mainly related to early experience, bad socialization process, origins (genetics factor), and welfare of dogs can have play an important role in both intraspecific and interspecific aggression ^{[20][44][54][55][56][57][58]}[20,44,54,55,56,57,58]. However, aggressive reactions may occur because of poor health and well-being. Dog aggression can be caused by behavioral and health problems. Failure to meet the basic needs related to the animal’s welfare (access to water, food, play, the possibility of socialization) may cause due to stress, aggressive behavior, as well as health problems ^[52][53][52,53]. In the absence of access to water or food, the dog may search for them in inappropriate places such as toilets or garbage cans. Strengthening inappropriate feeding behaviors (including the speed of food, amount of food given, bowl positioning, etc.) can lead to stomach twists, choking, aggression, fighting for food and defending the bowl, stress, and asking for food ^{[22][57][58][59][60]}[22,57,58,59,60]. When it comes to meeting physiological needs, the dog should be provided with appropriate space. In the studies on dogs staying in the shelter, it was shown that increasing the space twice had a positive effect on the behavior of dogs. Similar results were also obtained by Clark et al. [61] and Hetts et al. [58], which in turn indicates that both genetic and environmental factors can influence a dog’s behavior ^[39][61][62][39,61,62]. Otherwise, stress may occur due to the need to urinate or defecate or due to anticipating the penalty that awaits the dog for urinating or defecating in the wrong place. By praising the dog for inappropriate behavior or the owner not reacting to such behavior, the dog repeats it. This behavior of the owner be the cause of the reinforcement of inappropriate behavior and lead to the formation of undesired habits and situations resulting from them, e.g., barking at the sound of the doorbell. Improper behavior may be strengthened accidentally and unknowingly, also during conscious, as the guardian might think, training with a clicker. If the dog’s praise occurs too long after the desired behavior, the owner may unwillingly reinforce the following one, thus strengthening unwanted behaviors [63]. Research conducted in recent years also suggests that significantly fewer behavioral problems were shown in dogs that were trained solely through rewards, compared to dogs that were trained using only some form of punishment or a combination of both. The potential negative behavioral effects of aversive training techniques have also been identified in other studies such as Schilder and van der Borg [60], Hiby et al. [22], or Blackwell et al. [39]. During the period of socialization, the dog should get to know and establish positive contact with as many things, situations, and living creatures as possible. Socialization should last a dog’s life. However, it is one of the most important stages in a puppy. Properly socialized puppies are less likely to exhibit undesirable behavior and can develop a positive, lifelong relationship with their owner. Socialization practices should be age-appropriate (start within a few days of birth and should continue into adulthood as previously mentioned). They should be designed to expose the dog in a controlled and enjoyable manner to the many types of experiences, people, and objects it may encounter during its lifetime. Individuals that are properly socialized as puppies are less likely to exhibit behavioral problems in adulthood, including aggression and fear. Otherwise, stress may be manifested in certain situations and coped with, for example, through aggression. Dogs that have undergone a socialization program are much more self-confident and can cope with stressful situations more easily than those that have been less socialized ^{[64][65][66][67][68]}[64,65,66,67,68]. The psychosomatic background, on the other hand, is associated with somatic diseases that have an inhibitory and productive effect on behavior. The somatic diseases affect the previous factors, less intense or not manifested behaviors at all, for example, aggression, defecation, and urination in the wrong places, vocalization, food and water intake, or self-mutilation ^[10][69][10,69]. The occurrence of somatic diseases such as thyroid disease, cancer (especially those related to the nervous system) influences behavior by a general change of motivation. Diseases such as brain cancer can direct disturbance of the brain’s work, which may result in, among others, the occurrence of aggressive behavior. Diet can significantly influence the occurrence of aggressive behavior in dogs too ^{[69][70][71][72][73]}[69,70,71,72,73].

3. Selected Aspects of Dogs’ Diet and the Occurrence of Aggression

A properly balanced is the basis for the proper functioning of the animal and its health. There are many specialist diets available on the market to support the treatment of, among others, renal failure, arthritis, or cognitive impairment syndrome. The amount of tryptophan in the diet has a great influence on the dog’s behavior, as this amino acid influences the activity of serotonin ^[14][73][14,73]. Studies in humans have long pointed to the influence of diet on behavior. A study conducted on children by Schoenthaler and Bier [69] found that a diet rich in vitamins and minerals can reduce antisocial behavior in school-aged children. Additionally, studies by Gesch et al. [73] confirmed these findings but also showed that supplementation with both vitamin and mineral supplements and essential fatty acids (SFA) such as docosahexaenoic acid—22: 6n-3 (DHA) reduced the occurrence of violence-related behaviors ^[73][74][73,74]. In the case of dogs, models of their behavior are also considered as a model for humans, and previous research indicates that diet plays an important role in the behavior of these animals [74]. For many years, the relationship between the amount of protein in the diet, including tryptophan metabolism, and aggressive behavior in dogs has been the subject of discussion and interest. Research by DeNapoli et al. [75] and Dodman et al. [76] showed that the level of protein in the dog’s diet is an important aspect in the treatment of canine behavioral disorders, particularly aggression. These studies were based on the use of two types of diets: high protein (HP) and low protein (LP) in dog nutrition. The use of an LP diet in combination with the addition of L-tryptophan (Trp) may be effective in reducing dominant aggression in dogs. The effect obtained with a low-protein diet may be related to obtaining an appropriate ratio of Trp to other large neutral amino acids (LNAA) in the blood plasma. In addition, a low-protein diet rich in carbohydrates may adversely affect the ratio of Trp and LNAA, similar to an HP diet. The imbalance between the above-mentioned amino acids may result in their increased competition for blood-brain transfer. Disruption of Trp transfer from the blood-brain pathway may reduce the production of serotonin (5-HT), which in turn may lead to impulsive aggression. L-tryptophan is a biosynthetic precursor of the neurotransmitter serotonin, as well as melatonin and niacin (B₃). Therefore, it plays a key role in the diet of dogs and prevents the occurrence of, inter alia, impulsive aggression ^{[77][78][79][80][81][82]}[77,78,79,80,81,82]. In addition, long-term supplementation at a dose of 1 g/dog per day resulted in an increase in voluntary food consumption, which also indicates its role in influencing the hormones involved in food intake and satiety regulation, such as leptin or cholecystokinin [82]. Another important aspect of the diet is the level of lipids, in particular the number of fatty acids such as eicosapentaenoic acid (EPA), conjugated linoleic acid (CLA), or docosahexaenoic acid (DHA). As mentioned earlier, human studies have found a link between dietary DHA levels and aggressive behavior. Lipids perform many key functions, including they are components of cell membranes, precursors of chemical messengers, and are used as a source of energy or stored in adipose tissue. Additionally, they are highly concentrated in the central nervous system. Additionally, a component of the brain’s gray matter is derived from dietary CLA (18: 2n-6) and α-linolenic acid (18: 3n-3). As a result of an enzymatic reaction, linolenic acid can be converted in the body into arachidonic acid and then into docosapentaenoic acid (22: 5n-6). In contrast, α-linolenic acid gives EPA (20: 5n-3), which can then be metabolized to DHA (22: 6n-3) ^[83][84][85][83,84,85]. Brain membrane phospholipids contain high levels of polyunsaturated fatty acids (PUFA), especially arachidonic and docosahexanoic acids [85]. Dietary PUFA levels affect cognitive functions in mammals, including dogs ^[86][87][86,87]. Combinations of DHA and EPA in a study by Kidd et al. [86] showed that they have a significant impact on various psychological functions, such as protection against hyperactivity and aggression. In addition, many studies in mammals (mainly mice) suggest that an altered lipid profile may be responsible for the occurrence of aggression and impulsiveness in animals [87]. A study by Re et al. [84] on German Shepherds showed that low cholesterol, bilirubin, docosahexaenoic acid, and a higher omega-6/omega-3 ratio might be correlated with the occurrence of aggressiveness in the tested dogs. Confirmation of the obtained results by Re et al. [84] can also be found in Sentürk and Yalçin [85], where the lipid levels of aggressive dogs were found to be lower than those of non-aggressive dogs. Both studies suggest that this was associated with lowering cholesterol and triglycerides and HDL–C, which negatively affects serotonin reuptake but requires further investigation. Another important aspect of nutrition is the ability to influence the microbiome of the digestive system. Recent studies link the diversity of the composition of the gut microbiome with behavioral and psychological regulation in other mammals such as mice and humans, including aggressive behavior ^{[88][89][90][91]}[88,89,90,91]. A study by Kirchoff et al. [91] on dogs of the pit bull type showed a correlation between the composition of the digestive system microbiome and the incidence of aggression in dogs. The authors of this study suggest the possibility of using specific probiotics in the future to stabilize the microbiome of aggressive dogs because, according to preliminary analyzes, the microbiome of aggressive dogs are characterized by a higher level of Firmicutes, Fusobacteria, Bacteroidetes, Proteobacteria phyla, including Lactobacillus, and a lower level of Bacteroides, however, it requires for further research ^[92][93][92,93]. In addition to probiotics, it is possible to administer substances of plant origin that may positively affect the microbiome of the digestive system, such as proanthocyanins derived from grapes. In a study by Scarsella et al. [93] where the aforementioned substance was used, it was shown that it affects the microflora of the digestive system in dogs, as well as the neuroendocrine response—an increase in serotonin levels after 28 days of using the supplement. The use of polyphenols in dog nutrition as a way of manipulation of the microbiome is associated with the supposition that intestinal microorganisms may be involved in the absorption of the polyphenolic structure due to the transformation into low molecular weight compounds ^[94][95][94,95]. However, this topic requires further research in dogs. However, it should be considered that most of the presented studies were carried out on a small number of individuals, which requires further research.