Does castration really alter male dog behavior?

From ANIMAL PEOPLE,  July-August 2013:

PORTLAND,  Oregon––Does castration really make male dogs less dangerous? The return of an injectible zinc gluconate chemosterilant to the U.S. market––Zeuterin,  formerly called Neutersol––has rekindled a debate that most of the humane community,  most veterinarians,  and probably most people involved with dogs in any way thought was long since settled.

Writers about animal husbandry have recognized since Biblical times that castrating bulls to produce oxen and castrating stallions to produce geldings yields more tractable animals,  better suited to most work,  much less likely to injure the humans handling them.  Castrating human males to make eunuchs has similar results,  though eunuch warriors have often won distinction,  including the 15th century Chinese Muslim admiral Zheng He.

That castration should make male dogs safer has been presumed since 1923,  when the American Veterinary Medical Association first approved the safety of the surgical procedure for dogs.  But the presumption has been surprisingly little studied,  and what data there is turns out to be at best ambiguous.

“Much of our experience with the effects of castration on behavior has been with livestock––cattle and horses.  We do see substantial impacts in behavior.  We also see this with cats,”  summarized American SPCA science advisor Steve Zawistowski.  

But among dogs,  Zawistowski added,  “There have been studies by James Serpell and Anthony L. Podberscek,  and by Illana R. Reisner, among others,  showing that castration has a limited and variable impact on aggression.”

Castration & attacks

K.A. Gershman,  Jeffrey J. Sacks,  and J.C. Wright reported in the journal Pediatrics in 1994,  based on 1991 Denver dog bite data,  that non-castrated male dogs are about 2.6 times more likely to bite people than the average dog.  

This finding has been duplicated,  in gist,  by many subsequent reports.  But if non-castrated male dogs were 2.6 times more likely to bite at all times,  in all places,  dog bites in the U.S. should have steeply dropped since 1960,  when the first surveys of sterilization frequency found that barely 1% of the U.S. dog population had been sterilized.

In 1960 there were about 611,000 dog bites resulting in medical treatment,  according to the first Centers for Disease Control estimate of the incidence of severe dog bites.  There had been an average of under one dog attack fatality per year in the U.S. during the preceding three decades.   The U.S. dog population was about half of the present number.  Under 1% were pit bulls,  but pit bulls had killed nine people in 30 years.  Dobermans,  then the most feared breed,  had killed two people.

The hypothesis that castration makes dogs safer seemed to hold up for about 25 years.  By 1985 about half of all dogs in the U.S. were sterilized,  according to several different surveys,  while the CDC found that dog bites requiring medical treatment had dropped to 586,000. But pit bulls were up to 2% of the U.S. dog population,  and fatal dog attacks had increased to an average of 10 per year,  including five fatalities per year inflicted by pit bulls.

More than 70% of the dogs in the U.S. were sterilized by 1991,  and more than 80% today,  exclusive of pit bulls,  among whom the sterilization rate is circa 25% or lower.  Pit bulls are now 6% of the U.S. dog population.  Dog bites requiring medical treatment have increased to 4.8 million per year.  Fatalities in the present decade are averaging more than 30 per year,  with an average of 23 inflicted by pit bulls. About two-thirds of the fatal and disfiguring dog attacks occurring in the U.S. during the past 30 years have been by pit bulls.  The low rate of sterilization among pit bulls contributes mightily to the repeated finding that the majority of fatal and disfiguring dog attacks are by non-castrated male dogs.  

But about two-thirds of the fatal and disfiguring attacks by castrated male dogs are also by pit bulls,   including at least nine fatalities since 2010 inflicted by pit bulls who were adopted from animal shelters after both castration and passing standard pre-adoption behavioral screening.

Excluding pit bulls from the data removes any distortion due to pit bull proliferation.  But even with pit bulls excluded,  today’s mostly sterilized dogs appear to be about a third more likely to inflict a bite requiring medical treatment than the unsterilized dogs of 1960.

“There is little research into the effect of neutering in pet dogs who do not have a pre-existing behavioural problem,”  summarized N. C. Guy and six corresponding authors in the September 2001 edition of Applied Animal Behaviour Science.  

Among the studies that had been done,  J.C. Neilson,  R.A. Eckstein,  and Benjamin L. Hart of the Veterinary Medical Teaching Hospital at the University of California at Davis reported in 1997 that about a third of 57 mature male dogs were less aggressive toward family members after castration.  The other two-thirds were not.  “Castration was most effective in altering objectionable urine making,  mounting,  and roaming. Castration may be effective in decreasing aggression in some dogs,”  they reported,  “but fewer than a third can be expected to have marked improvement.”

Hart and S.G. Hopkins had reported 21 years earlier that castrating dogs did not appear to reduce either territorial or fear-related aggression.

Serpell and Podberscek in 1996 reported a significant positive association between neutering and aggression among English cocker spaniels, though the effect was strongest in females.  “Many of the breeds showing up as most aggressive in these studies were small breeds,”  observed Zawistowski.  “If castration reduces testosterone levels,  this might result in reduced confidence and perhaps a reason for fear biting.  I think that attacks by larger dogs,  whether they are pit bulls,   Rottweilers,  or whatever,   appear more like predatory attacks than dog-versus-dog social aggression,”  Zawistowski added,  “based on reports that I have read for court cases, and videos of such attacks.”


As making male dogs safer appeared to be a persuasive argument for castration,  contributing to population control,  the presumed behavioral benefits of castration were little questioned by the humane community,  but may now undercut enthusiasm for Zeuterin,  and indeed for any non-surgical sterilization method which does not reduce testosterone production.

“While surgical neutering virtually eliminates testosterone,  Zeuterin lowers testosterone by about half,”  says the web site of Ark Sciences,  the Zeuterin distributor.  “Based on feedback from animal caretakers and dog owners,  zinc neutering has suppressed mating behaviors and calmed the dogs down.  

The U.S. Food & Drug Administration allows the following statement about zinc gluconate:  Like surgical castration,  it may or may not suppress mating behaviors.”

The Zeuterin formulation was originally marketed by Addison Labs in 2003 as Neutersol.  According to Ark Sciences,  Addison Labs “overestimated the growth in demand and created too much inventory.  

The excess inventory expired in two years and the manufacturer went unpaid,  shutting down production.”

Ark Sciences acquired the Neutersol distribution rights in 2011,  reintroduced the product as Esterilsol for international markets,   and is bringing it back to the U.S. in 2013 as Zeuterin.  

Behavioral genetics

“ACC&D stands for the Alliance for Contraception in Cats & Dogs,  not the Alliance for Behavioral Modification in Cats & Dogs,”  Zawistowski reminded the ACC&D conference on June 22,  2013.  “If we allow ourselves to become  sidetracked by concerns about behavioral effects,  as legitimate as these concerns are,  we may tend to lose our focus on the primary issue that ACC&D exists to address,  which is to reduce the numbers of the animals whose behavior concerns us.  If nothing else,  reducing the numbers of these animals on the streets and coming into our shelters will reduce the numbers of animals whose behavior raises public concern and may require our efforts to modify.”

Continued Zawistowski,  who holds a Ph.D. in behavioral genetics and is on the ACC&D board of directors,  “Inasmuch as we know that dogs are morphologically and physiologically the most varied of any mammal species,  especially those dogs whose physiognomy has been the most altered by humans through selective breeding for specific traits,  it would not be surprising if they showed variable responses to different drugs,”  including zinc gluconate,  calcium chloride,  and other chemosterilants for male dogs. Specifically,  Zawistowki told ANIMAL PEOPLE,  dogs bred to display predatory behavior may have a different response to either castration or chemosterilants than dogs whose “aggressive” behavior has origins in social conduct. “In any species in which this has been examined, including dogs,  fish,  and poultry,  enhanced aggression is among the easiest traits to breed for.  This is due to the fact that high levels of aggression are usually not adaptive,  and there is latent genetic variation available.  The alternative––the ability to select for tameness––is also true,”  Zawistowski said.  

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