The Effects of Neutering

Bruce Fogle, D.V.M., M.R.C.V.S.

Ovariohysterectomies (or spays) and oophorectomies (or castrations) are the most common operations that veterinarians perform.  Although owners like to feel they are having this surgery carried out to prevent unwanted pregnancies it is equally true that neutering operations are requested to alter the behavior of their cats. Owners simply don't want to put up with the fighting, roaming, spraying and unpleasant urine odor of intact toms, nor do they want to hear the plaintive cries or receive the pelvic presentations of females in heat. But how effective is neutering, and if it is to be carried out when is the best time for the surgery to be performed? And are there less dramatic alternatives?

Ben Hart at the University of California veterinary school at Davis was the first to investigate the value of neutering male cats. He interviewed cat owners twenty-three months after their cats had been castrated. The owners reported that in nine out of ten animals neutering reduced fighting, roaming and spraying. Sometimes the change took longer than in others, and by Hart's definition a 'rapid decline' meant within three weeks of neutering and a 'gradual decline' meant one that took up to four months. His findings were as follows:

Fighting        ---rapid decline            53%
                     ---gradual decline        35%
                     ---no change                12%
                             Effectiveness ---88%

Roaming       ---rapid decline          56%
                     ---gradual decline       35%
                     ---no change                 9%
                            Effectiveness ---91 %

Spraying       ---rapid decline          78%
                     ---gradual decline        9%
                     ---no change             13%
                           Effectiveness ---87%

Hart noted that it was not the most experienced fighters, roamers and sprayers who continued their activity but rather the most vigorous ones, and he attributed this variation to both inheritance and early learning. He also noted that a rapid decline in one behavior did not necessarily correspond to decline in the other two behaviors. Some cats would stop spraying and roaming but continued fighting. Others stopped fighting and spraying but continued roaming. Finally, and importantly, he observed that there was no relationship between the age at which a tomcat was neutered and the rate of decline in any of these behaviors. The cats in this study were all between one and seven years old and the effect of neutering was the same throughout the various ages.

Castration will almost immediately eliminate what is to the human nose the unpleasant odor of tomcat urine. And, contrary to what some owners believe, if it is carried out after five months of age it will not result in a smaller urethra (urine passage) or contribute to urethral blockages in cats. Castration does not affect either fear aggression or predatory aggression but significantly reduces male dominant aggression and territorial aggression. Most cats will stop breeding within two weeks of surgery but some experienced males will continue to mate with estrous females for as long as a year. Although domestic cats usually reach puberty before they are a year old, it often takes another year before they fully develop all their secondary male characteristics, such as bulky muscle mass and a thick neck. From what we know about the effect of castration, regardless of age, owners who want their tomcats to develop these secondary sex characteristics can wait a little longer before having them neutered, but they should remember that there is a high likelihood that their cats will start spraying, wandering and fighting.

Neutering females does not result in such dramatic changes as it does in males. It simply terminates estrous cycles. It does not affect either predatory or any other type of aggression. If a female house cat has not permitted another female on her territory before, she is unlikely to do so after surgery either.

In reviewing the literature on the behavioral consequences of neutering cats, I was struck by the lack of statistical information on the subject. Like other veterinarians I had my own feelings on the matter, but rather than rely solely on those I carried out a simple survey by sending a questionnaire to one hundred practicing small-animal veterinarians, asking them questions about ten different behaviors in male, female and neutered male and female cats. This was a 'forced evaluation' survey, an exasperating form to complete, but one that is scientifically valid if a large enough number respond. Well over two thirds did so almost immediately.

The survey compared the behavior of intact males and females and yielded the following results. Females are more playful, demand more attention, are more hygienic and are friendlier to other household cats than are males. They are also slightly more affectionate and excitable. Entire male cats are slightly more active and more destructive than entire females and both are equally vocal and tolerate handling.

The survey asked about behavior changes in both males and females as a consequence of neutering. Neutered males are much more hygienic, are much friendlier to other cats, tolerate handling better, give more affection, are more playful and demand more attention than entire males. Entire males are more active and more vocal. Neutering has no effect on excitability or destructiveness.

The same questions asked about the behavioral effects of neutering females produced results that were not as dramatic as those reported when males were neutered. Neutered females are slightly more playful, are friendlier to other household cats and tolerate handling better than entire females. Entire females are more vocal and more active, but neutering has essentially no effect on hygiene, demand for attention, giving of affection, excitability or destructiveness.

Finally, and perhaps most importantly, the survey compared the behavior of neutered male and neutered female cats. The results show that there are very few and only minimal differences in behavior. Neutered males are very slightly friendlier to other household cats, are slightly easier to handle and give a little more affection than females. There is no difference in their demand for attention, hygiene, level of activity, destructiveness, use of voice, excitability or playfulness.

These results conform well with the anecdotal comments of professional cat breeders. Whereas the difference in behavior between male and female dogs remains significant after neutering, this is not the case with cats. In fact there is some evidence to suggest that as far as friendliness to other cats, ease of handling and giving of affection goes, neutered males actually make easier pets than do neutered females. In any case our personal biases and preferences usually decide what sex we choose to keep as pets. The evidence shows that we should not use our knowledge of sex differences in dog behavior when making decisions about cats. The consequences of neutering are dramatically different.

Finally, are there any safe alternatives to neutering that avoid surgery but prevent unwanted litters and social behaviors? The answer is a very qualified 'yes'. Both males and female cats can have their 'tubes' tied, surgery that does not interfere with hormone production but that simply prevents eggs or sperm from getting where they are most useful. Neither operation can be seriously recommended as routine because they don't resolve what is for many owners the major inconveniences of their cat's sexual behavior, urine smell, spraying, fighting and calling. Because all of these behaviors are hormonally influenced, other hormones given either in tablet form or by injection have been used for almost twenty years to counter sexual behavior. Estrous can be suppressed in cats by the administration of progesterone or a related progestogen, the hormones of pregnancy. Progestogens are also used to treat spraying and aggression in tomcats or neutered males and females, although their use is based on clinical experience rather than on controlled trials. The actual mechanism of action of progestogens still isn't known, but they can have a potent effect on modifying various feline behaviors and may act centrally on the brain. These hormones also have an anti-inflammatory effect, causing adrenal gland suppression, and this too might result in behavioral changes. The down side is that, although estrous can be suppressed by these hormones, there are possible side effects including weight gain, lethargy, loss of hair, womb infections, adrenal gland suppression, diabetes and mammary gland development. Some cats develop a pot-bellied appearance. Others suffer a heavy moult after the hormone treatment has been stopped. My clinical advice is that hormones are excellent for use in short-term therapy but have too many potential side effects to warrant their use as a replacement for spaying or castrating.  Hormone treatment, and tubal ligation-vasectomy do not prevent mammary, ovarian, uterine, and testicle cancers.  These cancers can only be prevented by neutering and spaying.

Additional Reading: Spaying & Neutering

Main Subject Index