Aggressive behavior: evolutionary
considerations of biological and social role
Aggressive behavior plays a significant role in the fitness of
animals, and it is widespread in the animal kingdom. Animals
use aggression to defend themselves and their progeny from
attack by predators, to fight for females, to feed, and to
maintain the social hierarchy. The study of aggressiveness is
complicated, however, by the fact that aggressive behavior is
not a unitary trait. The first and most influential classification
related to preclinical aggression was proposed by Moyer.(1)
The classification was based on the eliciting stimuli and
included the following types of aggression: predatory (attacks
on prey), intermale, fear-induced, irritable, territorial, maternal
and instrumental aggression.
Although most of the research on the genetics of aggression
has been done with mice, much of the scientific and public
interest is sparked by concern about the role of heredity in
human aggression.(2) One of the main reasons is the apparent
increase in levels of aggression in human society. According to
a World Health Organization Report,(3) violence is a major
public health problem worldwide. It has to be noted that the
number of victims of interpersonal violence (almost one
person every minute) is almost twice as high as the number
of people killed in armed conflicts. Although it is widely
recognized that it is difficult to define the different types of
aggression in humans, it is evident that offense and defense,
infanticide and even, in some situations, predation do occur in
humans.(2,4) A genetic contribution has been found for nearly
all behavioral disorders that have been investigated in
humans, including panic disorder and antisocial personality
disorder.(5,6) Understanding the mechanisms responsible for a
predisposition to aggression and violence is therefore an
important goal in modern neurogenomics. At the same
time, a lot of data demonstrate the validity of animal
models for the study in behavioral genetics. The various
Genome Projects revealed very high homology of human,
mouse and rat genes as well as syntenic similarities. The
Rat Genome Sequencing Project Consortium showed that
rat, mouse and human genomes encode similar numbers of
genes and suggested that 86–94% rat genes have orthologous
genes in the mouse and 89–90% in the human.(7)
Although the mouse became the dominant model for
geneticists, the rat has been the favorite model for behavior
physiology.
Another interesting aspect of aggressive behavior is the
evolutionary role played by this kind of behavior in the
domestication of animals. Domestication is one of the greatest
achievements of man as well as the greatest biological
experiment. The first chapter of Charles Darwin The Origin
of the Species was devoted to the transformation of wild
animals into domesticated species, serving as an example of
artificial selection motivated by the practical needs of man.
Animals were first domesticated in ancient time, with most of
the species in the Stone Age. Although the history of
domestication of animals does not go back more than
15 thousand years, domestic animals differ from their wild
ancestors much more than do even some genera. At the
same time, different species of domestic animals exhibit a
homologous variability with respect to many phenotypic
features, and the main criterion and the common feature of
all domestic animals is their ability to have direct contact
with man and not to be afraid. This means that fearinduced
aggression has been reduced in domestic animals.(8)
Dmitry Belyaev(9) proposed that selection for behavior was
carried out in the earliest stages of animal domestication with
the tamer animals being retained for breeding and the more
difficult or even impossible to handle aggressive animals
being discarded.
To test this hypothesis, a unique experiment on the
domestication of silver foxes was started almost 50 years
ago.(9) The main aim of the experiment was, by means of
selection for tame behavior, to obtain animals similar in their
behavior to the domestic dog, so the main selection criterion
was the reaction of foxes to human contact. The selective
breeding for the lack of aggressive response to man was
quite effective.(10) In contrast to wild-type animals, the foxes of
the selected population are not afraid of people, and display, like
dogs, an active positive reaction to human contacts (Fig. 1).
Similar selective breeding for high and low aggressiveness
was also performed on another species, Norway rat.(11,12)
These experiments, firstly, developed the experimental
models to study the process of domestication, and, secondly,
demonstrated a significant role of the genotype in
aggressive behavior.
However important the elucidation of the contribution of
genetic factors to aggressive behavior might be, this approach
does not give an answer to the principal questions: which
candidate genes might be responsible for differences between
highly aggressive and nonaggressive animals, and in what
way can gene activity change behavior and make an animal or
a man more or less aggressive?