Neutering, from the Latin neuter ('of neither sex'),[1] is the removal of a non-human animal's reproductive organ, either all of it or a considerably large part. The male-specific term is castration, while spaying is usually reserved for female animals. Colloquially, both terms are often referred to as fixing.[2] In male horses, castrating is referred to as gelding. An animal that has not been neutered is sometimes referred to as entire or intact.

Neutering is the most common method for animal sterilization. Humane societies, animal shelters, and rescue groups urge pet owners to have their pets neutered to prevent the births of unwanted litters, which contribute to the overpopulation of unwanted animals in the rescue system. Many countries require that all adopted cats and dogs be sterilized before going to their new homes.

Methods of sterilization

Females (spaying)

See also: Pediatric spaying

Feline uterus
Closed spaying incision on a female dog, taken 24 hours after surgery

Spaying is the surgical removal of the ovaries and uterus in female animals. It is commonly performed as a method of birth control and behavior modification.[3]

In non-human animals, the technical term is an ovo-hysterectomy or ovariohysterectomy; while in humans, this is called a hystero-oophorectomy. One form of spaying is to remove only the ovaries (oophorectomy or ovariectomy), which is mainly done in cats and young dogs. Another, less commonly performed method is an "ovary-sparing spay"[4] in which the uterus is removed but one (or both) ovaries are left. A complete ovariohysterectomy may involve removal of the ovaries, uterus, oviducts, and uterine horns.

Surgical incision site of a female cat

The surgery can be performed using a traditional open approach or by laparoscopic "keyhole" surgery. Open surgery is more widely available, as laparoscopic surgical equipment costs are expensive. Traditional open surgery is usually performed through a ventral midline incision below the umbilicus. The incision size varies depending upon the surgeon and the size of the animal. The uterine horns are identified and the ovaries are found by following the horns to their ends.

There is a ligament that attaches the ovaries to the body wall, which may need to be broken down so the ovaries can be identified. The ovarian arteries are then ligated with resorbable suture material and then the arteries transected. The uterine body (which is very short in litter-bearing species) and related arteries are also tied off just in front of the cervix (leaving the cervix as a natural barrier). The entire uterus and ovaries are then removed. The abdomen is checked for bleeding and then closed with a three-layer closure. The linea alba and then the subcutaneous layer are closed with resorbable suture material. The skin is then stapled, sutured, or glued closed. For suturing the feline linea alba, the most appropriate suture bite and stitch interval size was suggested to be 5 mm.[5]

Laparoscopic surgery is performed using a camera and instruments placed through small incisions (ports) in the body wall. The patient is under anaesthesia and lying on the back. The incisions are between 5 and 10 millimetres (0.20 and 0.39 in) and the number varies according to the equipment and technique used. The surgeon watches on a screen during the operation. The first port is made just behind the umbilicus and the camera is inserted. The abdomen is inflated with carbon dioxide gas to create a space in which to operate. A second port is introduced a few centimeters in front of the navel and a long grasping instrument called a Babcock forceps is inserted. The surgeon finds the ovary with the instrument and uses it to suspend the ovary from a needle placed through the abdominal wall. This lifts the ovary and uterus safely away from other organs. The surgeon then removes the grasping instrument and replaces it with an instrument that cauterizes and cuts tissue. This instrument uses electricity to heat the blood vessels to seal them and to cut them. No sutures are placed inside. The ovary is separated from the uterus and round ligament. The cautery instrument is removed and replaced by the grasping instrument, which is used to pull the ovary out through the small abdominal incision (port). This is repeated on the other side and the small holes are closed with a few sutures. Another method uses ligatures and even the uterus is removed. In female dogs only removing the ovaries and not the uterus is not state of the art because this way the risk of pyometra persists.

The benefits of laparoscopic surgery are less pain, faster recovery, and smaller wounds to heal. A study has shown that patients are 70% more active in the first three days post-surgery compared to open surgery. The reason open surgery is more painful is that larger incisions are required, and the ovary needs to be pulled out of the body, which stretches and tears tissue in the abdomen (it is not uncommon for patients to react under anaesthesia by breathing faster at this point).

Spaying in female dogs removes the production of progesterone, which is a natural calming hormone and a serotonin uplifter. Spaying may therefore escalate any observable aggressive behaviour, either to humans or other dogs.[6][7][8][9]

The risk of infections, bleeding, ruptures, inflammation and reactions to the drugs given to the animal as part of the procedure are all possibilities that should be considered.

Males (castration)

Main article: Castration § Animals

See also: Gelding

Closed castration incision on a male dog, taken 12 hours after surgery

In male animals, castration involves the removal of the testes (testicles), and is commonly practiced on both household pets (for birth control and behaviour modification) and on livestock (for birth control, as well as to improve commercial value). Often the term neuter[ing] is used to specifically mean castration, e.g. in phrases like "spay and neuter".

Surgical alternatives (vasectomy, tubal ligation, "gomerization")

Vasectomy: In a more delicate procedure than castration, the vasa deferentia – ducts that run from the testes to the penis – are cut then tied or sealed, to prevent sperm from entering into the urethra. Failure rates are insignificantly small. Breeders routinely have this procedure carried out on male ferrets and sheep to manipulate the estrus cycles of in-contact females. It is uncommon in other animal species. Because a vasectomy is usually a more expensive procedure, among pet-keepers it is more often performed on show animals, to cosmetically preserve their appearance (though depending upon the fancier organization, the procedure may invalidate the animal's candidacy for certain awards, or relegate it to a non-pedigree, generic "household pet" competition division, just as with full castration).

Tubal ligation: Snipping and tying of fallopian tubes as a sterilization measure can be performed on female cats, dogs, and other species; it is essentially the female equivalent of vasectomy, but a more invasive procedure. Risk of unwanted pregnancies is insignificantly small. Only a few veterinarians perform the procedure.

Like other forms of neutering, vasectomy and tubal ligation eliminate the ability to produce offspring. They differ from neutering in that they leave the animal's levels and patterns of sex hormone unchanged. Both sexes will retain their normal reproductive behavior, and other than birth control, none of the advantages and disadvantages listed above apply. This method is favored by some people who seek minimal infringement on the natural state of companion animals to achieve the desired reduction of unwanted births of cats and dogs.

"Gomerization" is breeders' informal term for surgical techniques by which male livestock, such as bulls, retain their full libido (and related effects like sex pheromones that would be lost through castration), but are rendered incapable of copulation. This is done to stimulate and identify estrous females without the risk of transmitting venereal diseases or causing a pregnancy by a male other than the one intended for selective breeding. Animals altered for this purpose are referred to as teasers (teaser bulls, etc.), or gomers. Several methods are used. Penile translocation surgically alters the penis to point far enough away from its normal direction that it cannot manage vaginal penetration. Penile fixation permanently attaches the penis to the abdomen so that it cannot be lowered for penetration. Penectomy is the partial or complete removal of the penis.[10]

Nonsurgical alternatives



Early-age neutering

Early-age neutering, also known as pediatric spaying or prepubertal gonadectomy, is the removal of the ovaries or testes before the onset of puberty. It is used mainly in animal sheltering and rescue where puppies and kittens can be neutered before being adopted out, eliminating non-compliance with sterilization agreement, which is typically above 40%.[21] The American Veterinary Medical Association, American Animal Hospital Association and the Canadian Veterinary Medical Association support the procedure for population control, provided that the veterinarian uses their best knowledge when making the decision about the age at neutering.[22][23][24] A task force recommends that cats are spayed–neutered prior to 5 months of age.[25][26]

While the age-unrelated risks and benefits cited above also apply to early-age neutering, various studies have indicated that the procedure is safe and not associated with increased mortality or serious health and behavioral problems when compared to conventional age neutering.[27][28][29][30][31] Anesthesia recovery in young animals is usually more rapid and there are fewer complications.[31][32] One study found that in female dogs there is an increasing risk of urinary incontinence the earlier the procedure is carried out; the study recommended that female dogs be spayed no earlier than 3 to 4 months of age.[28] A later study comparing female dogs spayed between 4 and 6 months and after 6 months showed no increased risk.[33]

One study showed the incidence of hip dysplasia increased to 6.7% for dogs neutered before 5.5 months compared to 4.7% for dogs neutered after 5.5 months, although the cases associated with early age neutering seems to be of a less severe form. There was no association between age of neutering and arthritis or long-bone fractures.[28] Another study showed no correlation between age of neutering and musculoskeletal problems.[30] A study of large breed dogs with cranial cruciate ligament rupture associated early-age neutering with the development of an excessive tibial plateau angle.[34]

Of particular note are two recent studies from Lynette Hart's lab at UC Davis. The first study from 2013, published in a well-known interdisciplinary peer-reviewed journal demonstrated "no cases of CCL (cruciate ligament tear) diagnosed in intact males or females, but in early-neutered males and females the occurrences were 5 percent and 8 percent, respectively. Almost 10 percent of early-neutered males were diagnosed with LSA (lymphosarcoma), 3 times more than intact males. The percentage of HSA (hemangiosarcoma) cases in late-neutered females (about 8 percent) was 4 times more than intact and early-neutered females. There were no cases of MCT (mast cell tumor) in intact females, but the occurrence was nearly 6 percent in late-neutered females".[35]

The second study from 2014[36] highlighted significant difference in closely related breeds (retrievers), suggesting that inter-breed variability is quite high and that sweeping legal measures and surgical mandates are not the best solutions to canine welfare and health. Specifically the study states: "In Labrador Retrievers, where about 5 percent of gonadally intact males and females had one or more joint disorders, neutering at 6 months doubled the incidence of one or more joint disorders in both sexes. In male and female Golden Retrievers, with the same 5 percent rate of joint disorders in intact dogs, neutering at 6 months increased the incidence of a joint disorder to 4–5 times that of intact dogs. The incidence of one or more cancers in female Labrador Retrievers increased slightly above the 3 percent level of intact females with neutering. In contrast, in female Golden Retrievers, with the same 3 percent rate of one or more cancers in intact females, neutering at all periods through 8 years of age increased the rate of at least one of the cancers by 3–4 times. In male Golden and Labrador Retrievers neutering had relatively minor effects in increasing the occurrence of cancers."

In terms of behavior in dogs, separation anxiety, aggression, escape behavior and inappropriate elimination are reduced while noise phobia and sexual behavior was increased. In males with aggression issues, earlier neutering may increase barking.[28] In cats, asthma, gingivitis, and hyperactivity were decreased, while shyness was increased. In male cats, occurrence of abscesses, aggression toward veterinarians, sexual behaviors, and urine spraying was decreased, while hiding was increased.[27]

Health and behavioral effects

This article contains a pro and con list, which is sometimes inappropriate. Please help improve it by integrating both sides into a more neutral presentation if this helps improve article flow. (April 2017)

See also: Castration § Medical consequences


Besides being a birth control method, and being convenient to many owners, castrating/spaying has the following health benefits:



Current research

Various studies of the effects neutering has overall on male and female dog aggression have been unable to arrive at a consensus. A possible reason for this according to two studies is changes to other factors have more of an effect than neutering.[64][65] One study reported results of aggression towards familiar and strange people and other dogs reduced between 10 and 60 percent of cases,[66] while other studies reported increases in possessive aggression[67] and aggression towards familiar and strange people,[68] and more studies reported there was no significant difference in aggression risk between neutered and non-neutered males.[65][69] For females with existing aggression, many studies reported increases in aggressive behavior[6][7][8][9] and some found increased separation anxiety behavior.[68][70] A report from the American Kennel Club Canine Health Foundation reported significantly more behavioral problems in castrated dogs. The most commonly observed behavioral problem in spayed females was fearful behavior and the most common problem in males was aggression.[71] Early age gonadectomy is associated with an increased incidence of noise phobias and undesirable sexual behaviors[clarify].[72]

Terminology for neutered animals

A specialized vocabulary is used in animal husbandry and animal fancy for neutered (castrated) animals:

Pig castrated before maturity.[73]
Male castrated draft animal.[74]
Male castrated chicken.[73]
Male castrated horse,[75] or donkey.
Male castrated cat,[76] or ferret.[73]
Male castrated deer.[77]
Male castrated rabbit.[73]
Male castrated draft animal.[74]
Female neutered cat.[76]
Female spayed chicken.[73]
Female neutered ferret.[73]
Male cattle castrated before maturity.[73]
Male cattle or pig castrated after maturity.[73]
Male castrated goat or sheep.[73]

Religious views


There are differing views in Islam with regard to neutering animals, with some Islamic associations stating that when done to maintain the health and welfare of both the animals and the community, neutering is allowed on the basis of being in the interest of 'maslaha' (general good)[78] or "choosing the lesser of two evils".[79]


Orthodox Judaism forbids the castration of both humans and non-human animals by Jews,[80] except in lifesaving situations.[81] In 2007, the Sephardic Chief Rabbi of Israel Rabbi Shlomo Amar issued a ruling stating that it is permissible to have companion animals neutered on the basis of the Jewish mandate to prevent cruelty to animals.[82]

See also


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