The Effects of Spaying and Neutering on Canine Behavior

Spaying and neutering is an often suggested remedy for various behavior problems. This article will be a review of the effects of spaying and neutering
on behavior.

Neutering Male Dogs

Neutering (Gonadectomy) the male dog removes the source of circulating testosterone by removing the testicles, which is presumed to be the major influence
on observed changes in behavior following gonadectomy.

Testosterone affects the male dogs brain intensively at two points in their development.The first is probably prenatally (Hart and Eckstein, 1997) and
the second is during sexual maturity. In the uterus, testosterone can diffuse through the amniotic membrane and through uterine blood flow. Once it
reaches the male dogs brain, it brings about genderspecific structural changes that then relate to later development of sex-typical behaviors. Remember
that these structures start out female and develop into male structures. These changes masculinize the male brain structure. Prior to sexual maturity,
male dogs display male-typical behaviors. During the second significant surge of testosterone in the sexual adolescence period of development, increasing
testosterone levels continue to act on these already established anatomical changes to again increase the frequency and magnitude of male sexually
dimorphic behaviors. Testosterone titers start to rise by the time the male pup reaches 4 to 5 months, where after testosterone levels reach a maximum
at 10 months of age and then fall to adult male levels by 18 months of age.(Dunbar, 1999, p. 68) The fact that the dog experiences two significant
surges of testosterone, one of which has significant effects on the anatomy of the brain prior to gonadectomy, it is not surprising that gonadectomy
does not have total control over sexually dimorphic behaviors.

Ben and Hart at the University of California carried out one of the most extensive surveys on the effects of gonadectomy on dogs, finding that at least
in intermale aggression, aggression was reduced by neutering in 60% of cases with rapid reduction in 25%, and gradual reduction in 35% (Fogle, 1990,
p. 53). Neilson, Eckstein, and Hart, (1997) found that approximately 25% of adult dogs that were aggressive toward humans or other dogs in the household
can be expected to have a 50 to 90% level of improvement after gonadectomy. A 50 – 90% level of improvement can likewise be observed in 10 to 15% of
dogs that are aggressive toward unfamiliar people or human territorial intruders after gonadectomy. Neilson et al. also found that neither the age
at which the gonadectomy was performed or the duration the problem behavior existed for affected the behavior after gonadectomy. Hart and Eckstein
(1997) performed a review of the research and literature on the effects of gonadal hormones on objectionable behavior. They relay that gonadectomy
affects sexually dimorphic behaviors and that aggression toward other dogs and dominance over owner are particularly sexually dimorphic. They point
out that much previous research has largely been based on guardian survey studies and has not experimentally controlled for the placebo effect in that
guardians may either instate some form of changes that constitute behavior modification or they may imagine changes that in fact did not take place
or did not take place to the extent they believed they did. Although this is a serious lacking in the validity of the studies these same affects can
be expected in real life gonadectomies performed as part of behavior management plan. To the extent that the guardian makes changes in their behavior
or the environment that result in decreases in aggressive behaviors simply as a result of having the gonadectomy performed these same results can be
expected in behavioral cases. A large scale German study by Heidenberger and Unshelm (1990) found very similar results to those described above.

Testosterone tends to promote greater reactivity in dogs. They trigger a little quicker to aversive stimuli and respond a bit more intensely and for slightly
longer duration. Affecting the magnitude of aggressive behavior could be particularly helpful in many cases.

Some general statements and recommendations can be formed from these studies and notions:

  • Neutering cannot be expected to reduce aggressive behavior in all dogs
  • Neutering will not always completely eliminate aggressive behaviors
  • When the neutering is carried out cannot be expected to influence the rate or magnitude of changes in behaviors
  • How long the problem behavior has existed does not tend to affect the level or trend of change in behavior after neutering
  • If an intact dog demonstrates aggressive behavior neutering should be considered as an adjunct to other behavior modification practices
  • Early neutering is probably not helpful at preventing aggression and may pose certain medical risks.

Spaying Female Dogs

Spaying (ovariohysterectomy) is the surgical removal of the ovaries and uterus, the source of estrogen and progesterone in female dogs. Estrogen and progesterone
levels ebb and wane in cycles. The most significant influence cycling fluctuations in estrogen and progesterones have on female dog behavior is pregnancy
related behaviors (female sexually dimorphic behaviors). While estrogen increases in the dogs body for a short length of time, progesterone remains
in circulation, influencing the brain for two months after each estrous and can have a dramatic effect on canine behavior. The most common behaviors
are those associated with pregnancy, nest building, guarding possessions and milk production. (Fogle, 1990, p. 54)

The most notable problem arises when the dog guards items maternally. Other problems can involve irritability, conflict with other dogs and energy reduction.
Possessive guarding in intact females that occurs in cycles is usually a hormonal guarding.

With a lack of testosterone to masculinize the brain in utero, female dogs maintain their female brain structure. Both males and females start with the
basic brain organization for male and female behaviors. The phenotype results from the probability or frequency with which the feminine or masculine
system is activated in very early development.

Sexual dimorphism is a matter of degree rather than an all or nothing phenomenon. Most male dogs behave typically male and most female dogs behave typically
female in terms of sexually dimorphic behaviors. Some male dogs display fewer or lower magnitude male sexually dimorphic behaviors and in some cases
display some female sexually dimorphic behaviors. Likewise, some female dogs display fewer or lower magnitude female sexually dimorphic behaviors and
in some cases display some male sexually dimorphic behaviors. Think of it as a continuum. One explanation for this is the basic similarity and integration
of behavioral systems of males and females of the species. Another explanation is that the masculinizing effects of testosterone are a matter of degree
rather than a zero-sum trigger and that some individuals in the uterus are exposed to greater or lesser amounts of testosterone.

It has been found (OFarrell and Peachey, 1990; and also discussed in Hart and Eckstein,1997) that the only two behaviors affected by spaying of female
dogs was aggressive dominance toward guardians and indiscriminate eating. In this case, the aggressive behavior increased after spaying. This effect
was only noted in female dogs ovariohysterectomized before 12 months of age, and that already demonstrated aggression. Female dogs ovariohysteretomized
after 12 months of age presented no risk of increased aggression. Hart and Eckstein (1997) point out that female dogs are in a progestational state
for two months following an estrus period and spaying them during that time creates a sudden removal of the source of progestins (which tend to have
a calming influence on animals). It is proposed that this removal of progesterone may promote irritability or aggression in some individuals.

Another piece of this puzzle may involve androgenization of fetal female dogs. It is suggested that either or both of two mechanisms may lead to a slight
masculinization of female brains in untero. If a female is positioned between two males in utero, their brain may be masculinized by diffusion of testosterone
through the amniotic membrane. The other mechanism involves caudally (closest to the tail of the animal) positioned males androgenizing rostrally (closest
to the head of the animal) positioned females in utero through the fetal blood supply. For a good discussion of this, see Hart and Eckstein(1997).

In a study by Kim, Yeon, Houpt, Lee, Chang and Lee (2005, in press) it was found that female German Shepherd Dogs spayed between five and 10 months were
significantlymore reactive than an intact group. The intact group was not exposed to a sham operation so even though the measures were taken well after
the surgery it is possible that the results are due to the surgical procedure rather than the absence of ovaries and uterus. It is also possible that
the results are only reasonably generalizable to German Shepherd Dogs. This study does suggest though that spaying may cause more reactivity in dogs.
Replication will be important.

Some statements and recommendations can be derived from the above:

  • Females should not be spayed within two months after their estrus period to avoid sudden contrast between the presence and absence of progesterone
  • If a female demonstrates this consistent dominance related aggression pattern prior to spaying and prior to 12 months of age the risk of increased
    aggressive behavior must be balances against the inconvenience of an intact female dog and potential health detriments of remaining intact. Spaying
    can simply be delayed until behavior modification has achieved progress
  • Early spaying is likely not helpful at preventing aggression and may pose certain medical risks.

References cited

  • Dunbar, I. (1999). An Owner’s Huide to a Healthy Happy Pet Dog Behavior. New York:Howell Book House.
  • Fogle, B. (1990). The Dog’s Mind. New York: Howell Book House.
  • Hart, B. L., & Eckstein, R. A. (1997). The role of gonadal hormones in the occurrence of objectionable behaviours in dogs and cats. Applied Animal
    Behaviour Science,52, 331-344.
  • Heidenberger, E., & Unshelm, J. (1990). Changes of behaviour in dogs after castration.Tierarztliche Praxis, 13(1): 69-75.
  • Kim H. H., Yeon S. C., Houpt K. A., Lee H. C., Chang H. H., and Lee H. J. (2005).Acoustic feature of barks of ovariohysterectomized and intact German
    Shepherd bitches. J Vet Med Sci. 67(3):281-5.
  • Neilson, J. C., Eckstein, R. A., & Hart, B. L. (1997). Effects of castration on problem behaviors in male dogs with reference to age and duration
    of behavior. Journal of the American Veterinary Medical Association, 211(2), 180-182.
  • O’Farrell, V., & Peachey, E. (1990). Behavioural effects of ovariohysterectomy on bitches. Journal of Small Animal Practice, 31, 595-598.

James O’Heare

  • James OHeare, CABC., is the President of the Cynology College(www.Cynology,
  • Owner of the Applied Companion Animal Behavior Network (,
  • Director of the International Institute for Applied Companion Animal Behavior ( and
  • owner of DogPsych Publishing (
  • James personal web site is at
  • James holds a Doctoral level Post-Graduate Certificate in Comparative Psychology.
  • James is the Author of Aggressive Behavior in Dogs, The Canine Aggression Workbook, The Canine Separation Anxiety Workbook, Dominance Theory and Dogs,
    Raw Meat For Cats and Dogs?, and Canine Neuropsychology. The Canine Aggression Workbook will be discontinued after this edition and replaced by
    James new book Aggressive Behavior in Dogs.
  • James has been helping companion animal guardians solve behavior problems for more than 15 years.

Leave a Reply

Please Login to comment
1 Comment threads
0 Thread replies
Most reacted comment
Hottest comment thread
1 Comment authors
Sarah Smith Recent comment authors
newest oldest most voted
Notify of
Sarah Smith
Sarah Smith

Thanks for your information on spaying. I would like to spay my new dog because we already have a boy dog at home. We’ll start looking for a vet to help us.

Pin It on Pinterest

Scroll to Top