Many vertebrate brain structures involved in the control of aggression are richly supplied with receptors that bind with hormones produced in the endocrine system, in particular with steroid hormones produced by the gonads. In a wide range of vertebrate species, there is a clear relationship between a male’s aggressiveness and his circulating levels of androgens such as testosterone, a hormone produced in the testes. From fish to mammals, aggression levels rise and fall with natural fluctuations in testosterone levels. Castration has been found to reduce aggression dramatically, while experimental reinstatement of testosterone—for instance, through injection into the blood—restores aggression. Circulating testosterone can even influence the structures and signals used during fights. In stags the neck muscles needed for effective roaring enlarge under the influence of rising testosterone levels. In male mice the scent of another male’s urine, which contains the breakdown products of testosterone, elicits intense aggressive responses.

The close link between aggression and testosterone is not surprising, given that males of many species fight over access to fertile females, but the connection is complex. For instance, the more elaborate the social structure of a species, the less drastic are the effects of castration on aggression. In addition, testosterone of nongonadal origin (i.e., produced by the adrenal gland) may be important in aggression outside the breeding season, as in the case of birds such as the song sparrow that maintain nonbreeding territories in the winter. Furthermore, hormones other than testosterone and its derivatives also may be involved in the modulation of aggression. For example, in several species of mammals and birds, the distribution of the neuropeptide hormones arginine vasotocin (AVT) and arginine vasopressin (AVP) in the pre-optic and septal regions of the brain differs between the sexes. Aggression in males is facilitated by implants of AVT in the limbic system and inhibited by implants of AVP. Finally, while a causal link between circulating testosterone levels and aggression has been well established, it is also clear that the link can work in the opposite direction, with participation in a fight having rapid effects on hormone secretion. In particular, many vertebrates that win fights show increased testosterone levels, while losers exhibit not only reduced levels of testosterone but also elevated levels of the stress hormone cortisol. Changes in hormonal levels in turn modulate future aggressiveness. Such multiple and multidirectional links between brain biochemistry, circulating hormone levels, and aggression are a key part of the mechanisms whereby behaviour in conflict situations is adapted to both past experience and current circumstances.

Hormonal effects

The interaction between hormones and the expression of aggressive behaviour described in the previous section are reversible influences in adult animals—so-called activational effects. Hormones, however, can also influence aggression through long-term organizational effects that occur during development. Pre- and postnatally, at times specific to each species, the developing testis of young male mammals produces a brief surge of steroid hormones that is responsible for the development of male reproductive structures and mating behaviours. The hormones also have a lasting effect on the development of the brain structures that control aggression in adult animals, making the structures more sensitive to the aggression-facilitating effects of testosterone. The effects of early exposure to gonadal steroids have been described for a variety of vertebrate species. Early exposure to other, nongonadal hormones, such as AVP, has been shown to increase levels of aggression in adult males. Thus, the well-documented gender differences in aggressiveness seen in many species are the result of the lasting effects of exposure to hormones early in development.

Developmental effects can also generate the marked natural variation in aggression observed in many species among individuals of the same sex. To illustrate, young mice are exposed to different hormonal environments during development depending on their position within the uterus. Because connections exist between the placental circulation systems of neighbouring embryos, male embryos situated between two females experience relatively low androgen levels and remain relatively unaggressive when treated with testosterone as adults. Conversely, female embryos situated between two males experience relatively high androgen levels and become particularly aggressive to males when treated with testosterone as adults.

Environmental and genetic influences

The example of differential exposure to hormones in mouse embryos illustrates a point that is true for all behavioral traits—i.e., that aggression develops as a result of interaction between genes and the environment in which the genes are expressed. Genetic factors on the Y chromosome of mice determine whether the embryonic gonad secretes androgens and hence whether aggression-promoting brain regions are sensitized to testosterone. This process, however, is modulated by conditions experienced in the uterus. Individual genetic differences in aggressiveness have been identified in many species. In crickets, sticklebacks, and mice, selective breeding for high or low levels of aggression in males produces a marked and rapid response, indicating that at least some of the original variation in aggressiveness in the parental population is the result of genetic differences. In mice it has been shown that major differences in aggression are the result of variation in a specific region of the Y chromosome identified as the “pairing region.” Additional effects of the autosomal chromosomes (i.e., the nonsex chromosomes) have also been identified. The Y chromosome probably exerts its effect on aggression via an influence on early hormone secretion. Use of molecular genetic techniques has further demonstrated the importance of genetic differences in generating variation in aggressive behaviour and has shown how these effects may be mediated. In genetically engineered “knockout” mice, which lack both copies of the gene coding for a particular serotonin receptor, aggression is markedly higher than in nonaltered mice, confirming several other lines of evidence for an aggression-inhibiting effect of serotonin in vertebrates.

The well-known effects of genetics on aggression notwithstanding, the environment in which a young animal is raised also has profound effects on whether, and how, it fights as an adult. These environmental factors are not always directly related to social experience. For example, mice that are deprived of food during development become particularly aggressive as adults. On the other hand, environmental effects on the development of aggression may depend on social interactions, but in contexts other than fighting; for instance, mouse pups that have been roughly handled by their mothers are particularly aggressive as adults, as are individuals from a range of species that have been reared in social isolation. Finally, and perhaps not surprisingly, direct experience of victory or defeat during fights has a profound effect on subsequent aggressive behaviour in animals as different as crickets and chimpanzees; animals that lose regularly become increasingly less likely to initiate attacks. Such effects form the basis of dominance hierarchies, and they may be the result of short-term neuroendocrine changes, longer-term reward-based processes based on conditioning and learning, or both.

Whatever their nature, environmental effects may interact with the genetic make-up of the animals concerned. For example, gentle early handling by humans reduces aggression in mice that come from nonaggressive strains but not in mice from aggressive strains. More interesting perhaps is that female mice from aggressive strains tend to handle their pups roughly, so that the baby mice not only inherit genes that predispose them to be aggressive but also experience an aggression-promoting environment early in life. So for aggression, as for most other behaviours, how an animal behaves as an adult is not the expression of blind instinct in the adult individual, nor is it simply the result of experiences during development. Instead, it is the result of a continuous and complex interaction between inherited genetic material and the environment (pre- and postnatal) in which the genes are expressed.