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Aggressive Behavior

Aggressive behavior is shown here by means of facial expressions. The three illustrations in the upper row show both aggressive and dominant behavior. The wolf in the middle is neutral, i.e., neither showing particularly aggressive, fearful, dominant nor submissive behavior. The three illustrations at the bottom show various combinations of aggressive and submissive behavior. Notice the position of eyes, ears, forehead, and lips—they tell us of the wolf's dominant and submissive status in that particular situation. The same applies to domestic dogs. (Illustration by Alice Rasmussen. ©)

Aggressive behavior is behavior directed toward the elimination of competition from an opponent. It ranges from threats to attacks. A threat is a reliable warning of an impending attack—such as growling, snarling, roaring, and stamping—if the threatened individual takes no evasive action. An attack is an injurious behavior, such as biting, staging, punching, and kicking. An individual can counter a threat by escaping or displaying submissive behavior, whereas escape is the only option to neutralize an attack. Aggressive behavior is distinguishable from dominant behavior in as much as the latter precludes and excludes injury though it may require some degree of forceful action. Predatory behavior is not aggressive behavior, though it is attacking.[1]


Contents

Definitions

Growling and snarling are also aggressive behaviors (picture by askmen).
Aggressive and dominant behavior (erected ears, short mouth, big eyes) (picture by dogsquad).

Aggressive behavior is difficult to define. “Lack of agreement regarding definitions of aggressive behavior has been a significant impediment to the progress of research in this area,” writes Nelson in 2005 in his big book "Biology of Aggression.[2] There is no one globally accepted definition of aggressive behavior.[3]

The problem redoubles because we use the terms aggressive and aggression in daily language to characterize highly different behaviors, e.g., an aggressive state (the action of a state in violating by force the rights of another state), an aggressive tactic (a sports team focusing on attacking rather than defending), an aggressive attitude (someone showing an unfriendly disposition).[4] Clearly, this is not what aggressive behavior means in an evolutionary-biologic context.

Aggressive behavior is different from what is commonly called assertiveness, although the terms are often used interchangeably among laypeople, e.g., an aggressive salesperson. Assertiveness corresponds to dominance in ethology.

Moreover, the lay-person (and specialized literature as well) fails to recognize that not all attacks have the function to eliminate competition. Hence the misunderstood term predatory aggression, which involves attacking but not aggressive behavior as its function is to acquire a resource (food), not to eliminate competition. Predator and prey do not compete as to resources.[1][5]

“The beginning of wisdom is the definition of terms,” said Socrates (470 – 399 B.C.)[note 1][6][7] Why is a sound definition necessary? Because only then do we know what we are discussing.[8]

The requirements for a good (sound) definition are: (1) it defines something concrete and observable; (2) it states a necessary condition to distinguish it from a related technical term; (3) it does not include other terms needing a definition; (4) it includes enough conditions to justify the use of the term, not too few to risk being synonymous with another term, and not too many to risk losing its explanatory value by being too encompassing; (5) it gives examples of what is and is not what we are defining; (6) it does not presuppose any special knowledge of the reader to understand it.[9]

Review of Common Definitions

Aggressive and submissive behavior (not fearful). This is the behavior a cornered dog shows when pacifying, submission and flight don’t work (picture by doggies).
Aggressive and submissive behavior (ears down, long mouth, smaller eyes) (picture by Cesar’sWay).

The original definition in "Dog Language,"[10] was: “Aggressiveness (or aggressive behavior) is behavior directed toward the elimination of competition. It can range from displays of intent, like growling, roaring and stamping to injuring behavior like biting, staging, kicking.”

It is not a bad definition, according to the requirements above, but we can improve it. Therefore, we checked several other definitions to analyze their strengths and shortcomings, in an attempt at getting the necessary information to come up with a sound, good definition.

A— "Aggressive behavior can cause physical or emotional harm to others. It may range from verbal abuse to physical abuse.”[11] That is not a definition, unless we help it a bit: "Aggressive behavior is all behavior that can cause [...]" However the explanation may be enough for the layperson to have an idea of what we are talking about, it isn't precise enough to use in the biological sciences, though it recognizes, that the behavior ranges over a broad scope.

B— “Aggression is a forceful behavior, action, or attitude that is expressed physically, verbally, or symbolically. It may arise from innate drives or occur as a defense mechanism, often resulting from a threatened ego. It is manifested by either constructive or destructive acts directed toward oneself or against others."[12] The passive voice weakens the definition. Who, what, when, why are not clear. It recurs to terms needing strong definitions as well, i.e. [Drive|drive], defense mechanism. Finally, it is too psychological for the evolutionary biologist—what is a threatened ego?

C— “Aggression is behavior that is angry and destructive and intended to be injurious, physically or emotionally, and aimed at domination of one animal by another. It may be manifested by overt attacking and destructive behavior or by covert attitudes of hostility and obstructionism. The most common behavioral problem seen in dogs.”[13] This definition is not good. It is more a list of synonyms (angry, destructive, hostility, obstructionism) than a definition. It mixes up concepts (aggression and domination?). Finally, the most common problem in dogs, for example in the files of Ethology Institute Cambridge (over 10,000 of them), is home alone problems,[14] not aggressive behavior. That statement requires a reliable reference in itself.

D— "Aggressive people often uses anger, aggressive body language [...]“[15] This one, we won’t waste any more time with. A definition that uses the definiendum is not a definition.[8]

E— “Aggression is a response to something/someone the animal perceives as a threat. Aggression is used to protect the animal through the use of aggressive displays (growling, barking, tooth displays, etc.) or protect the animal through aggressive acts (biting). Aggressive behavior is most frequently caused by fear.” (somewhere on the Internet). This definition is not good either. Again, it uses the definiendum in the definition. It is a circular definition.[16] We miss the definition of threat to be able to analyze the sentence conclusively. More seriously, it states that aggression is caused by fear, which from an evolutionary point of view doesn’t make sense. (See below.)

F— “Aggression is defined as behavior which produced or was intended to produce physical injury or pain in another person.”[17] This is a much better definition, but it could be more explanatory. If we substitute person with individual, we can apply it to animal behavior as well.

G — "Aggressive behaviour, [is] animal behaviour that involves actual or potential harm to another animal."[18] This is also a short and good definition. Even though definitions should be as short as possible, they must not sacrifice explanatory value for the sake of length. This definition needs to be expanded so we fully understand what we are talking about.


The Making of a Definition

This dog shows self-confident (dominant) aggressive behavior. This is instrumental aggressive behavior (photo from dog-adoption-and-training-guide).

Let us consider the definition at the beginning of this article.

Aggressive behavior is behavior directed toward the elimination of competition from an opponent. It ranges from threats to attacks. A threat is a reliable warning of an impending attack—such as growling, snarling, roaring, and stamping—if the threatened individual takes no evasive action. An attack is an injurious behavior, such as biting, staging, punching, and kicking. An individual can counter a threat by escaping or displaying submissive behavior, whereas escape is the only option to neutralize an attack. Aggressive behavior is distinguishable from dominant behavior in as much as the latter precludes and excludes injury though it may require some degree of forceful action. Predatory behavior is not aggressive behavior, though it is attacking.”[1]

That is a much better definition than the above reviewed. The short version, "Aggressive behavior is behavior directed toward the elimination of competition from an opponent" complies with all the requirements for a good definition. The expanded version defines something concrete and observable. It states a necessary condition to distinguish it from a related technical term, dominant behavior, even explaining a characteristic of the latter. It defines other terms needing a definition, i.e. threat and attack. It includes enough conditions to justify the use of the term, not too few to risk being synonymous with another term, and not too many to risk losing its explanatory value by being too encompassing. It gives examples of what is, and is not, aggressive behavior. It does not presuppose any special knowledge of the reader to understand it.

It is a good and sound definition because it defines the term, including and excluding the necessary conditions. Whether it will be the last word on the matter is another story. There is always room for improvement; and a good definition must also be able to accept reviews imposed by newer discoveries.


Types of Aggressive Behavior

This dog shows insecurity and aggressive behavior. This may be reactive-impulsive aggressive behavior, but may also be learned behavior (photo by petexpertise.com).
This dog (to the right) shows learned aggressive behavior. It may be impulsive-reactive, but it does not need to be (photo by onegreenplanet.org).

In the following, for the sake of brevity, we will use aggression and aggressive behavior interchangeably. When studying human aggression, it is common to subdivide it into two types: (1) instrumental aggression, which is purposeful or goal-oriented; and (2) reactive-impulsive aggression, which is elicited by a loss of emotional control and often manifests itself as uncontrollable or in inadequate actions.

Let us note that, “Aggression differs from what is commonly called assertiveness, although the terms are often used interchangeably among lay-people (as in phrases such as ‘an aggressive salesperson'” in Quadri and Vidhate’s words.[19] They also state, “Predatory or defensive behavior between members of different species may not be considered aggression in the same sense.” We would go a step further and claim that these are not to be considered aggressive behavior in any sense.[5][18]

A distinction in types of aggressive behaviors is between (1) pro-active (also controlled and instrumental) and (2) reactive-impulsive. The former is not an end in itself, only the means to achieve a goal. There are no strong emotions involved. On the contrary, its effects depend on calculated and well-timed action. The latter has no goal in itself and is marked by intense emotions. In short, researchers of aggressive behavior in children have found it useful to distinguish between reactive (impulsive) from proactive (instrumental) aggression.[20]

Modern frustration-aggression theory claims that anger is a reaction to an aversive experience, including frustration. It emphasizes the importance of moral violation as justifying the expression of aggressive behavior.[21][22]

The question is whether we also find these types of aggressive behavior in other animals than Homo sapiens sapiens. Evolutionary biologists and Darwinists are always extremely suspicious of any statement claiming that any trait is only found in one single species. The odds of that happening are immensely poor, to the point of being able to be regarded as impossible.


Do Animals Other than Humans Have Morality?

Do animals other than humans have morality and will they fight for a cause? That is a tricky question because we cannot envisage any way of verifying it. In that sense, some would call it a meaningless question. Let us analyze the evidence we have. We have proof that some animals show empathy and altruism, widely recognized as conditions for morality. Shermer points out that humans and other social animals share the following characteristics: “[…] attachment and bonding, cooperation and mutual aid, sympathy and empathy, direct and indirect reciprocity, altruism and reciprocal altruism, conflict resolution and peacemaking, deception and deception detection, community concern and caring about what others think about you, and awareness of and response to the social rules of the group.”[23]

However, we can account for all these characteristics in terms of evolutionary costs and benefits, using models based on evolutionarily stable strategies.[24] We don’t need to invent a new term, morality, to explain that. Therefore: if humans show moral behavior, so do other species, albeit differently. What we might need to concede is that sometimes quantitative differences amount to a qualitative difference, and hence humans showing these traits in such a high degree justifies us in coining a new term, morality.

If that is the case (and we are only theorizing), it makes sense to label some human behavior as moral, and to disregard the possible morality in other animals (unless discoveries enlighten us differently).

If it does not make sense to analyze the behavior of other animals than humans, in terms of morality, then it follows that we can as well neglect reactive-impulsive aggression caused by violation of moral rules in those other animals.


Loss of Emotional Control

Despite our conclusions above, we cannot dismiss behavior caused by loss of emotional control because other animals than humans can also lose control over their emotions. The difficult part here is, as always, the term emotion, which is vague and, therefore, one that biologists prefer to avoid.

What is an emotion? According to Schacter, an emotion is a “[…] a positive or negative experience that is associated with a particular pattern of physiological activity”[23] caused by hormones, neurotransmitters, dopamine, noradrenaline, serotonin, and GABA. We find all these in some animals other than humans; therefore, if we can accept the above definition of emotion, we must concede that if we can show emotional behavior, so can they.


Reactive-Impulsive Aggressive Behavior

The only way in which it makes sense when dog people speak of dogs being reactive (meaning they growl or bite someone) is this: those dogs display reactive-impulsive aggressive behavior. It is still by all means aggressive behavior, just one type that may or may not exist in some considerable degree in other animals than humans, depending on whether we are right or wrong in our theorizing.

Recognizing and identifying reactive-impulsive aggression correctly may be advantageous because research shows that it may be easier deterred than instrumental aggressive behavior.[20] Reactive-impulsive aggression appears to result from a distorted perception of competition, not realizing that there are evading routes, and enhanced by the inability to control the associated emotions. There is also evidence that reactive-impulsive aggression (contrary to instrumental aggression) is related to low levels of serotonin in the brain.[25] On the other hand, classifying all canine aggressive behavior as reactive-impulsive, as it seems to be the practice these days, may prove to be a mistake with extremely severe consequences.

A dog displaying aggressive behavior can show it self-confidently (what ethologists call dominant behavior) or insecurely (showing submissive behavior—not fearful). The former is excluded from being reactive-impulsive—it is instrumental and goal-oriented. The latter may be if the dog does not realize that a clearer display of submissive behavior or flight would solve the problem. This kind of aggressive behavior may be: (1) the consequence of poor imprinting and socialization (the dog simply didn’t learn how to solve social conflicts), (2) the result of inadvertently reinforced behavior. Dog owners reinforce their dog’s reactive-impulsive aggressive behavior attempting to do what they call calming down the dog. The dog growls, they say, “quiet ” (or similar), the dog looks at them, and they reinforce that with a treat and a “good job.” It doesn’t take many repetitions for the dog to learn that displaying aggressive behavior provides it with attention and food, which are desired resources.

The term reactive does not belong to ethology, which classifies behavior by function. In ethology, reactive-aggressive behavior is aggressive-submissive behavior. We don’t know how it came into dog training, but we suspect a psychologist introduced it and people liked it because, apparently, it sounded better to say, “My dog is reactive” than, “My dog shows aggressive behavior.” Ironically, the term places the full responsibility for the unwanted behavior on the owners—reactive-impulsive aggression is either the result of poor imprinting/socialization or inadequate training. The same applies to encounters between two dogs. A dog displays aggressive-submissive behavior because it can't see any escape route.[5]


Aggressive and Dominant or Submissive Behaviors

These two dogs are both equally self-confident (showing equal dominant behavior). Any aggressive behavior deriving from this situation will not be reactive-impulsive, but instrumental (photo from dog time.com).

A dog can show both aggressive and dominant behavior simulataneously—as well as aggressive and submissive behavior—but it cannot show aggressive and fearful behavior at the same time. The two are mutually exclusive. They serve two distinct functions.

The function of aggressive behavior is to gain access to a resource by means of reliable and impressing warnings and threats, injuring an opponent if necessary be. The function of fearful behavior is to escape alive and well from a conflict situation, accepting the loss of any involved and possible resources.

An aggressive animal takes the initiative and shows its potential. A fearful animal keeps a low-profile, and tries to be imperceptible.

Konrad Lorenz explains that aggressive behavior—the so-called evil—is in social animals a catalyst to individual recognition and to develop bonds.[note 2][26] However, he fails to distinguish properly between aggressive and dominant, as well as between fearful and submissive, proposing a model where simultaneously aggressive and fearful behaviors are compatible. The first to recognize that aggressive and fearful behavior might not be compatible is Erik Zimen in 1981.[27]

The Fear-Biter and Other Misinterpretations

Wolf showing aggressive and submissive behavior. When an opponent does not accept the submissive display, and flight is impossible, wolf and dog will fight back (illustration by Alice Rasmussen ©).

The Fear-Biter misinterpretation is common. It states that aggressive behavior is caused by fear, which from an evolutionary point of view doesn’t make sense. Fear does not elicit aggressive behavior. It would have been a lethal strategy that natural selection would have eradicated swiftly and once and for all. A cornered animal does not show aggressive behavior because it is fearful. It does so because its natural responses to a fear-eliciting stimulus (pacifying, submission, flight) don’t work. (See above.)

Contrary to the popular belief, aggressive behavior is not caused by fear. Fear leads to pacifying or submissive behavior (active or passive) or to flight. If the dog perceives that its opponent does not accept its signals of submission and flight is impossible, it may recur to aggressive behavior as the last instance. If its opponent accepts its submissive and pacifying postures, and walks away, it will not bite. This is true for human-dog as well as dog-dog encounters.[5]

Predatoty behavior is not aggressive behavior. The Encyclopædia Britannica recognizes that: "This article follows the common practice of biologists by considering only intraspecific attacks under the title aggressive behaviour. The emphasis here is on biological context—that is, the roots of aggression in competition for food and mates; the influences of the nervous system, hormones, genetics, and environment; and scientific models for analyzing the likely outcome of aggressive interactions."[18] The function of predatory behavior (to acquire food, self-preservation) is quite distinct from the one of aggressive behavior (to eliminate competition).


Aggressiveness—Inheritance and Environment

Heritability studies attempt to determine whether a trait passes from parent to offspring. Some genetic lines in many species of birds, dogs, fish, and mice seem to be more prone to aggression than others. Through selective breeding, we can create animals with a tendency to show more aggressive behavior.[28][29]

Some aggressive behavior is evolutionarily advantageous, and some is not and may be an impediment to social cohesion. Maynard Smith says that it is not surprising for aggressiveness to have a strong genetic correlation given the high likelihood of both potentially positive and negative selective discrimination throughout evolution.[24]

Research has uncovered many factors that contribute to aggressive behavior. Disruption of the serotonin system is a highly significant feature in predisposing aggression.[25] There is a correlation between testosterone levels and aggression.[30] On the other hand, there is doubt whether low levels of blood sugar (hypoglycemia) may elicit physiological changes and aggressive behavior.[31][32]

Most researchers agree that we must consider the influence of genes, not in isolation, but as functioning in the whole genotype, as well as the effect of the environment.[33] Future research in the genetics of aggressive behavior may very well focus on epigenetic factors.[34]

Doubtless, most behavior traits, except simple reflexes, contain a genetic, plus an environmental component. No behavior will develop without the appropriate genetic blueprint and no behavior (again except for a few simple patterns) will show in the absence of the right environmental stimuli.

It is probable that each individual filters and selects stimuli from a wide range in its habitat according to its genetics, thereby creating its uniqueness of experiences. As Bock and colleagues say, we create our own environment.[35] We have no reason to suspect that the same does not happen with other animals.


Notes

  1. Even though this quote is consensually attributed to Socrates, we have no evidence he ever said that. We can't read it anywhere in Plato's dialogues. However, we know from Aristotle that Socrates was very keen on definitions, so he might have said something similar.
  2. The original title, Das sogenannte Böse, meaning 'the so-called evil' was translaated into English as 'On Aggression.'


See Also

References

  1. 1.0 1.1 1.2 Abrantes, R. (2014). "Aggressive Behavior—the Making of a Definition". Ethology Journal Online. Ethology Institute Cambridge. 2014:06-12. 
  2. Nelson, R. J. (2006). Biology of Aggression. Oxford: Oxford University Press. 
  3. Zirpoli, T. J. (2014-05-01). "Aggressive Behavior". Education.com. Pearson Allyn Bacon Prentice Hall. Retrieved 2017-10-10. 
  4. "Aggression". TheFreeDictionary. Huntingdon Valley: Farlex, Inc. 2017. Retrieved 2017-10-10. 
  5. 5.0 5.1 5.2 5.3 Abrantes, R. (2004). The Evolution of Canine Social Behavior. Naperville: Wakan Tanka Publishers. ISBN 0966048415. 
  6. Beebe, J. R. (2003). "Socrates' Conception of Definition". Dept. of Philosophy, University at Buffalo. Retrieved 2017-10-10. 
  7. Zalta, E. (2015-04-20). "Definitions". Stanford Encyclopedia of Philosophy. Retrieved 2017-10-10. 
  8. 8.0 8.1 Pepper, M.; Driscoll, D. L. (2015-07-30). "Writing Definitions". Purdue OWL. Retrieved 2017-10-10. 
  9. [1]Purdue OWL: Writing Definitions.
  10. Abrantes, R. (1997). Dog Language—An Encyclopedia of Canine Behavior. Naperville: Wakan Tanka Publishers. 
  11. Erickson Gabbey, A.; Jewell, T. (2016-03-07). Legg, T. J., ed. "What Is Aggressive Behavior?". Healthline. Retrieved 2017-10-10. 
  12. Mosby (2012). Mosby's Medical Dictionary. Amsterdam: Elsevier. (in https://medical-dictionary.thefreedictionary.com/Aggressive+behavior). ISBN 9780323112581. 
  13. Blood, D. C.; Studdert, V. P.; Gay, C. C. (2007). Abutarbush, S. M., ed. Saunders Comprehensive Veterinary Dictionary, 3rd edition. St. Louis, Missouri: Saunders Elsevier. (in http://medical-dictionary.thefreedictionary.com/Aggressive+Behaviour). ISBN 0-7020-2789-8. 
  14. Abrantes, Roger (1999). Dogs Home Alone. Wenatchee, WA: Dogwise Publishing. ISBN 0966048423. 
  15. [2]Changing minds: Aggressive behavior.
  16. [3]Wikipedia: Circular definition.
  17. Nelson, R. J. (2006). Biology of Aggression. Oxford: Oxford University Press. ISBN 0195168763. 
  18. 18.0 18.1 18.2 Huntingford, F. A. (2011-09-08). Aggressive Behaviour. Encyclopædia Britannica (Online Encyclopedia). Encyclopædia Britannica. Retrieved 2017-10-10. 
  19. Javeed, Q. S.; Vidhate, N. J. (2012-08-07). "A Study Of Aggression And Ego Strength Of Indoor Game Players And Outdoor Game Players" (PDF). Indian Streams Research Journal. 2, Issue 7. 
  20. 20.0 20.1 McCauley, C. (Spring 2000). "Some Things Psychologists Think They Know about Aggression and Violence". The HFG Review of Research. 4, No. 1. 
  21. Akert, R. M.; Aronson, E.; Wilson, T. D. (2010). Social Psychology (7th ed.). Upper Saddle River, NJ: Prentice Hall. ISBN 0134012399. 
  22. Blair, R. J. R. (2014). "The roles of orbital frontal cortex in the modulation of antisocial behavior". Brain Cogn. 55: 198–208. Retrieved 2017-10-10. 
  23. 23.0 23.1 Shermer, M. (2004). The Science of Good and Evil. New York, NY: Times Books. ISBN 0805077693. 
  24. 24.0 24.1 Smith, M.; Harper, D. G. (1988). "The evolution of aggression: can selection generate variability?". Philos Trans R Soc Lond B Biol Sci. Jul 6;319(1196): 557–70. 
  25. 25.0 25.1 Siever, L. J.2 (2008). "Neurobiology of aggression and violence.". Am J Psychiatry. 165: 429–442. Retrieved 2017-10-10. 
  26. Lorenz, K. (1963). Das sogenannte Böse: Zur Naturgeschichte der Aggression [Published in English as "On Aggression"] (1998 ed.). dtv Verlagsgesellschaft. ISBN 3423330171. 
  27. Zimen, E. (1981). The Wolf: His Place in the Natural World (1st edition ed.). Souvenir Press. ISBN 0285624113. 
  28. Tremblay, R. E.; Hartup, W. W.; Archer, J. (2005). Developmental Origins of Aggression. New York, NY: Guilford Press. ISBN 1593851103. 
  29. Miles, D. R.; Carey, G. (1997). "Genetic and environmental architecture of human aggression. 72:207–217.". J Pers Soc Psychol. 72: 207–217. Retrieved 2017-10-10. 
  30. Batrinos, M. L. (2012). "Testosterone and Aggressive Behavior in Man". Int J Endocrinol Metab. 2012 Summer; 10(3): 563–568. doi:10.5812/ijem.3661. 
  31. Andrade, M. L.; Benton, D.; et al. (1988). "A Reexamination of the Hypoglycemia Aggression Hypothesis in Laboratory Mice". ResearchGate. Retrieved 2017-10-09. 
  32. Benton, D. (1988). "Hypoglycemia and aggression: a review". Int J Neurosci. 1988 Aug; 41(3-4): 163–8. Retrieved 2017-10-09. 
  33. Craig, I. W.; Halton, K. E. (2009). "Genetics of human aggressive behaviour" (PDF). Hum Genet. Springer-Verlag. 126: 101–113. doi:10.1007/s00439-009-0695-9. Retrieved 2017-10-09. 
  34. Waltes, R.; Chiocchetti, A. G.; Freitag, C. M. (2016). "The neurobiological basis of human aggression: A review on genetic and epigenetic mechanisms". Am J Med Genet B Neuropsychiatr Genet. Jul;171(5): 650–75. doi:10.1002/ajmg.b.32388. Retrieved 2017-10-09. 
  35. Bock, G. R.; Goode, J. A. (1996). Genetics of Criminal and Antisocial Behavior. Chichester: John Wiley & Sons. Published Online: 28 SEP 2007. doi:10.1002/9780470514825. 


External Links

  • [4] Abrantes, R. (1997). Dog Language—An Encyclopedia of Canine Behavior. Naperville: Wakan Tanka Publishers. ISBN 0966048407.
  • [5] Abrantes, R. (2003). The Evolution of Canine Social Behavior (2nd Edition). Naperville: Wakan Tanka Publishers. ISBN 0966048415.
  • [6] Chance, P. (2013). Learning and Behavior (7th Edition). Wadsworth Publishing. ISBN 1111832773.
  • [7] Ethology Journal Online. Ethology Institute Cambridge.
  • [8] Ethology Institute Cambridge—Books.
  • [9] McFarland, D. (1998). Animal Behaviour: Psychobiology, Ethology and Evolution (3rd Edition). San Francisco: Benjamin Cummings. ISBN 0582327326.