Hormones and Aggressive Behavior

Increased competitiveness and aggressive behavior are often associated with hormonal changes occurring around puberty, a biological change that may lower the threshold for several significant sex-related behavior patterns, including intermale and interfemale aggression. While lowering the threshold for general activity, urine marking, and aggressive behavior, the threshold for pain and fear may be elevated under the influence of these various hormones.

Stress Hormones and Aggression

The effects of endogenous hormones on aggressive behavior are evident in wild canids. A lower threshold for aggressive behavior is exhibited by both male and female wolves during the annual mating season, when both sexes show an increased tendency to engage in sex-related aggressive behavior (). This sharp increase in aggressive behavior is probably mediated by a number of interacting hormonal systems. The alpha female can be particularly intolerant and hostile toward her female subordinates. McLeod et al. (1995) have shown that the upsurge of aggressive activity among captive wolves is especially stressful (by cortisol measures) on the lowest-ranking female and the second-ranking male.

Increased corticosteroid levels may play an indirect role in regulating sexual and competitive activity among wolves. Only the dominant female comes into full estrus and whelps young. Estrus in subordinate females is suppressed by some external cause, possibly as the result of the dominant female’s continuous harassment and badgering before, during, and after the mating season. Estrus may be blocked by stress-mediated mechanisms involving corticosteroid secretions or related physiological mechanisms, such as stress-mediated suppression of luteinizing hormone (). In addition to impeding reproductive activity in subordinate females, stress produced by aggressive interaction between wolves appears to reduce the sex drive of subordinate males, as well. Consequently, the increase of aggressive interaction during the mating season may serve to “disable” rivals sexually and competitively, while helping to achieve an optimal physiological state for reproduction in the dominant or alpha pair.

It is interesting to note in this regard that Sapolsky (1990) found that dominant males among free-ranging olive baboons showed distinct differences in cortisol concentrations, depending on the presence or absence of five personality traits. Dominant baboons were most likely to have optimally low cortisol concentrations, if (1) they were able to differentiate between threats and neutral interaction, (2) they initiated the fight with the threatening rival, (3) they won the fight they initiated, (4) they exhibited differentiated behavior after winning or losing a fight, and (5) they redirected aggression toward another baboon when they lost a fight. Dominant males not exhibiting these traits tended to have cortisol levels similar to those of subordinate males.

Sex Hormones: Estrogen, Testosterone, and Progesterone

Estrogen (estradiol) levels are highest during proestrus, with progesterone levels increasing as the female enters estrus. Progesterone appears to exercise a modulatory effect over estrogen, and only after estrogen levels begin to fall will the female become receptive toward the male. Also, as estrus is approached, circulating testosterone in female dogs reaches plasma levels that are comparable to those in male dogs (). The various sex hormones are closely related steroidal compounds, with testosterone being easily biosynthesized from progesterone and estradiol synthesized from testosterone ().

Estrogen affects dog behavior in many ways: it increases general activity levels, promotes increased urine output and marking, increases vocalization, and stimulates nervous arousal in female dogs (). All of these changes are the result of estrogens threshold-lowering effects on the female brain, especially involving target areas mediating the expression of proestrus sexual behavior needed to attract a mate. Progesterone, on the other hand, appears to exercise an opposite effect to that of estrogen by generally elevating behavioral thresholds and asserting a calming effect on dogs and enhancing their receptivity to intimate contact. In high doses, progesterone may even induce general anesthesia. Not all practitioners agree on the anti-aggression effects of progesterone. For example, Overall (1997) directly implicates progesterone as an aggression-facilitating hormone, noting that “high levels of aggression in hamsters are associated with the presence of progesterone” (). Although progesterone may facilitate certain forms of aggression under the influence of certain hormonal environments in certain species (), the general contention that progesterone promotes aggressive behavior does not appear to be supported by the weight of experimental evidence () and the clinical impressions of many practitioners who use progesterone to control aggressive behavior in dogs. Several laboratory and clinical reports have noted the threshold-elevating effects of progesterone on aggression in both male (testosterone environment) and female (estrogen environment) animals, including intact dogs () and other domestic species exhibiting undesirable aggressive behavior (). As of 1991, Houpt described the progestins as the “most effective pharmacological treatment of aggression now available” (). Progestins in the form of megestrol acetate (Ovaban) or long-lasting injections of medroxyprogesterone (Depo-Provera) were frequently administered to control aggression in dogs. Unfortunately, progestins produce a number of potential side effects, including diabetes mellitus, mammary tumors, sterility in intact males, and excessive weight gain. Coupled with the growing popularity of psychotropics, the use of progestins has become much less common. In combination with appropriate behavior modification, however, progestin therapy remains a viable short-term adjunctive treatment for the control of some forms of intractable intermale aggression and other sexually dimorphic behavior problems ().

Adult sensitivity to androgens and estrogens may be influenced by perinatal exposure to these sexual hormones. Simon and Whalen (1987) found that female mice treated with testosterone or estrogen on the day of birth exhibited an enhanced responsiveness to the hormone upon reaching adulthood. Testosterone-treated mice showed increased aggressiveness in response to testosterone but not to estrogen, whereas estrogen-treated mice selectively responded to estrogen but not to testosterone.

Male and female sexual hormones play an important regulatory role in the expression of sex-related intraspecific aggression. Whether such hormones play a significant role in the expression of interspecific aggression (e.g., toward people) remains an open question. Although testosterone has been often implicated as a facilitating hormone, its role in the expression of aggressive behavior is anything but clear and straightforward. Both androgens and estrogens appear to facilitate aggression, especially during the mating season. Perhaps the facilitative effects of sex hormones on aggression are mediated indirectly by the activation of sex-related emotions and drives, making aggression most likely to occur in the presence of species-typical triggers shown by conspecifics operating under the influence of similar hormonal changes.

Effects of Castration on Aggressive Behavior

The importance of sexual hormones for the modulation of aggressive behavior has long been recognized. However, the effect of hormones on dog behavior is ambiguous and highly variable. Endogenous sexual hormones appear to play a role in the development of some behavior problems (). The relationship between androgens and unwanted behavior is especially evident in the case of aggression, where male dogs present much more often than females — as much as 90% more often by some estimates (). In general, males also present more frequently than females with other common behavior problems (), including playfulness, destructiveness, snapping at children, territorial defense, and general activity excesses. According to the Harts’ study, females are more trainable, easier to house train, and more affectionate. Areas where no significant differences between the sexes were found include watchdog barking, nuisance barking, and general excitability. Voith and Borchelt (1996) reported similar findings indicating that male dogs present more frequently with behavior problems than females ().

From such data, one might suppose that a more or less direct causal connection exists between the presence of male hormones and increased tendency to behave aggressively. However, such a robust causal relationship between hormones and aggression does not appear to exist. This lack of a definitive cause-effect relationship is evident in the highly variable effect of castration on behavior. Contrary to common belief, castration often fails to affect offensive and defensive aggression significantly; neither does it typically have a significant effect on a dog’s general activity level. In general, castration appears to exert its strongest influence over sexually dimorphic behavior patterns, such as intermale aggression, urine marking, mounting, and roaming. Neilson and coworkers (1997) found that such behavior was reduced between 50% and 90% following castration.

Many studies have been performed to evaluate the effects of castration on male behavior. For instance, Beach (1970) carried out a series of experiments investigating the effects of castration on the sexual behavior of dogs.

The dogs included in his study were experienced copulators. If testosterone predominantly controls or mediates the expression of copulatory behavior, then one would expect to observe a sharp decline in sexual activity in castrated dogs. However, Beach found that castration had limited effects, with no apparent effect on sexual response latency or mounting frequency in the dogs he observed over the study period, though he did find a reduced frequency of intromission and more brief durations of coital lock.

These findings are consistent with the effects observed after castration on other sexually dimorphic male behavior patterns like aggression, roaming, urinary scent marking, mounting, and intermale fighting. Although such behavior patterns are not always entirely eliminated by castration, their frequency and magnitude may be reduced — occasionally very significantly so. In the case of agonistic displays, one should expect a slight general modulation in the direction of reduction, especially in terms of the intensity/duration of episodes and the tendency for aggression to escalate. Also, the denouement phase following an episode may be much more steep following castration than before. The effect of castration is one of degree and subtlety — an effect that is often unobserved and unappreciated by the owner.

Some hormonal factor probably exists in the etiology of dominance aggression, since males exhibit the behavior problem more often than females, but the cause and source of this effect may be largely independent of a dog’s adult sexual status. The most likely mechanism for the effect of hormones on aggressive behavior is androgen-mediated perinatal differentiation of neural tissue. Early ontogenetic exposure to sex hormones may facilitate the elaboration of sexually dimorphic circuits modulating respective threshold differences between males and females for the display of aggressive behavior as adults.

If testosterone actually plays a significant role in the expression of aggression, one might reasonably expect to see increased signs of it between 6 to 8 months of age, when dogs undergo an endogenous surge of androgen activity (). Although many dogs do appear to go through an adolescent adjustment phase around this period, it is not a statistically significant time frame for the expression of dominance-related aggression, although dogs may become more competitive and difficult and become more aggressive toward other male dogs. Tinbergen (1969) describes some of these apparent and dramatic effects of the adolescent hormonal surge observed among free-ranging huskies in Greenland:

We followed the behavior of two young males carefully and found, to our surprise, that when they were about eight months old they suddenly began to join their pack in fights with their neighbors. In the very same week their trespassing upon other territories became a thing of the past. And it was probably no coincidence that in that same week both made their first attempts to mate with a female in their own pack. ()

Castration is often recommended as a means for controlling dominance-related and other forms of aggression (). The most commonly cited study concerning the therapeutic efficacy of castration on behavior was performed by Hopkins and colleagues (1976) (). Unfortunately, the study examined a very small sample of dogs (N = 42) and was poorly controlled. The authors noted striking improvement in dogs exhibiting various behavior problems, including roaming (16 dogs, 90% improved), mounting (15 dogs, 67% improved), inter-male fighting (8 dogs, 62% improved), and urine scent marking (10 dogs, 50% improved). Both territorial aggression (8 dogs) and fear-related aggression (4 dogs) showed no improvement following castration: “The subjective reports of the present study substantiate the contentions by others that only aggressive behavior toward other males is altered by castration” (). One potential source of error in the study was the fact that most of the dogs (37 of 42) involved were castrated to curb an unwanted behavior problem in the first place, perhaps biasing the owners’ observations to some extent in the direction of a placebo effect. The owners might have also picked up a few tips on how to control their dog’s unwanted behavior, thereby confounding the results. Additional support for the putative benefits of castration on dominance-related aggression have been reported by Neilson and colleagues (1997), who found that 25% of dogs exhibiting aggression toward family members improved between 50% and 90% after castration.

Finally, testosterone appears to be released following competitive victories, whereas a decrease of circulating testosterone follows defeats (). The differential increase or decrease of testosterone may affect the relative physical size (anabolic effect) of dominant and subordinate animals, lower aggression thresholds, and increase the magnitude of aggressive behavior. Increased testosterone levels may also provide a source of positive reinforcement for the successful combatant, perhaps promoting feelings of well-being and elation that occur as the result of the victory. Testosterone appears to facilitate aggressive arousal and preparatory reflexes conducive to agonistic success. For example, the direct stare and focused readiness commonly preceding dominance contests may be mediated by testosterone. Many studies involving a variety of species have shown that testosterone enhances selective attention in the direction of the target while simultaneously reducing distraction to extraneous stimuli (). Attention control is a significant factor in the modification of such behavior. Once attention is frozen on the target, it is very difficult to disrupt or divert it, making it of utmost importance to capture the dog’s attention during the earliest stages of aggressive arousal. In addition to possibly reducing the reward value of successful aggressive competition, castration may serve to reduce preparatory arousal and decrease the dog’s ability to focus its attention fully on the target of attack, thereby making it more easy to divert or disrupt the dog’s agonistic intentions and direct the dog into incompatible counterconditioning activities. Consequently, although castration alone may not eliminate aggression, it may make aggression problems more responsive to management and control efforts.

Effects of Prepubertal Castration on Behavior

Some veterinarians and humane groups have promoted early castration as a viable population and behavioral control measure, claiming that prepubertal castration produces superior effects over the current practice of performing castration and spay surgeries at 6 months of age. The evidence supporting this opinion is mixed and controversial. For example, on the pro side of the debate, Lieberman (1987) reports findings based on the results of a questionnaire generated by the Medford, Oregon, SPCA spay and neuter program. The study collected and compared information on about 400 dogs that had been castrated at different ages. The sample was divided into two groups: (1) 200 puppies castrated at 6 to 12 weeks of age, and (2) 200 puppies castrated after 6 months of age. According to Lieberman’s survey, the male dog’s unwanted sexual and aggressive behaviors were significantly reduced by prepubertal castration when compared to the group of dogs castrated after 6 months of age. If valid, these findings contradict the observation by Hopkins and colleagues (1976) that the “age of the dog does not seem to have a pronounced influence on the effectiveness of the operation” ().

On the con side, Lieberman’s findings have been challenged by a controlled study carried out by Salmeri and coworkers (1991), who found that puppies castrated at both 7 weeks and 7 months exhibited little positive difference in significant behavioral parameters (for example, barking, playfulness, aggression toward other dogs, affection toward people, and outgoing nature) — all significant traits were unaffected by castration. The only behavioral traits influenced by castration were excitability and general activity, but both in a direction opposite to what one might expect; that is, dogs castrated early in life tended to become more excitable and active than intact controls. Even in cases involving intermale fighting, few dogs exhibited significant improvement after castration, although the tendency to fight appears to have been modulated to some extent. Perhaps castrates are less attractive as aggressive opponents for intact males. Finally, Jagoe and Serpell (1988) question the effectiveness of prepubertal castration in 6- to 12-week-old puppies, arguing that the surgery may be detrimental to a dog’s health, but they present no significant evidence to support their concern and warning. Although the scientific evidence is mixed, the selective use of early castration might be seriously considered in puppies exhibiting signs of excessive aggression at an early age.

Effects of Spaying on Female Aggressive Behavior

Voith and Borchelt (1982) reported observing a tendency of some female dogs to exhibit an increase in dominance-related aggressive behavior after spaying. The authors speculate that female dogs displaying such tendencies may have been exposed to fetal androgenization, resulting in their malelike behavioral characteristics (see Perinatal Androgenization). Spaying may predispose such dogs to express these undesirable androgynous traits. O’Farrell and Peachey (1990) observed a similar effect in a subgroup of spayed female dogs. They compared the behavior of 150 spayed females with a matched (breed and age) control group of 150 nonspayed females. Spayed females showed a significant increase in dominance-related aggression following surgery, especially if they were under 1 year of age and had exhibited aggressive behavior prior to spaying. In addition, Podberscek and Serpell (1996) have reported that females spayed before exhibiting aggression were more likely to exhibit aggression toward children.

Progestin as a Testosterone Antagonist

Joby and colleagues (1984) performed a series of studies to investigate the effects of oral prog-estins on the behavior of intact male dogs. The sample included 163 dogs with a variety of behavior problems, ranging from dominance aggression to destructiveness. The dogs were administered a daily dose of megestrol acetate (1 mg/kg or 4 mg/kg — 30 dogs required the higher dose) over the course of 2 or 3 weeks of treatment, depending on the dog’s response to treatment. Most of the dogs exhibited more than one unwanted behavior problem. Of the 163 dogs presented for treatment, 123 (75%) showed improvement at the conclusion of a brief exposure to megestrol acetate. An interesting aspect of the study is the broad effect that progestin treatment had on remote behaviors not directly related to a sexual motivation, such as dominance-related aggression (79% improved), fear-related aggression (71%), destructiveness (79%), and excitability (73%). The primary side effects reported (in 36 dogs) by the authors was an increase in appetite and lethargy. After 3 months off medication, many of the dogs continued to exhibit lasting improvement, although some had relapsed somewhat. Recidivism was especially evident in the case of dominance aggression and household urine marking. The broad benefits of progestin are probably due to its general tran-quilizing and anesthetic effects ().

Note: Although the authors reported minimal side effects, the use of progestin therapy is now widely criticized because of the availability of alternative medications and the potential for serious side effects resulting from long-term use of such hormones.


Pseudopregnancy ox pseudocyesis is a hormone-mediated condition that may reduce functional thresholds for aggressive behavior. Pseudopregnancy occurs in female dogs, usually 6 to 8 weeks after estrus, but in some cases not presenting for 4 or 5 months after estrus. In addition to mammary enlargement or lactation, various behavioral signs may present with the condition, including toy adopting, nesting behavior, hyperactivity, destructiveness, and aggression. Aggression thresholds may be generally lowered during the period of pseudopregnancy, with aggressive behavior being particularly likely to occur when the dog’s nesting area or toy surrogates are approached. Destructiveness involving digging into sofas and carpeting may also occur during pseudopregnancy. Voith (1980) has speculated that pseudopregnancy may have served an adaptive function for the dog’s ancestors. She argues that the physical and behavioral changes associated with pseudopregnancy may be the result of evolutionary pressures favoring the communal care of young by closely related females belonging to the mother’s group. Spaying is commonly recommended in the literature to control the problem (); however, spaying a dog while she is still exhibiting signs of pseudopregnancy may significantly protract the condition (), perhaps causing it to persist for years in some cases (). Consequently, spaying should be undertaken only after signs of pseudopregnancy have disappeared (approximately 4 to 6 weeks). Further, some females may develop signs of pseudopregnancy only after spaying, perhaps helping to explain some of the increased incidence of aggression in females after the surgery. In this regard, Borchelt (1983) found that spayed females were significantly more likely to engage in possessive aggression than intact counterparts. Further, in the only cases involving dominance aggression in intact females, it was later discovered that the females were under the influence of pseudopregnancy. Symptoms of pseudopregnancy are often controlled with sex hormones [e.g., progestins ()].

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