What does neutering do?

Note: I am a biologist, not a veterinarian. This article is designed to give the pet rat owner some information about the effects of neutering on male rat behavior and fertility from the scientific literature. This article is not designed to solve unusual behavioral problems, nor is it designed to replace the advice of a veterinarian. Please consult your veterinarian regarding the care of your pet.

What is neutering?

Neutering, or castration, is the surgical removal of a male animal's testicles (testes). The testes produce sperm and a hormone called testosterone. Removing the testes therefore means the male can no longer produce sperm. Neutering greatly reduces the levels of testosterone, which changes behavior -- especially sexual and aggressive behavior.

What does neutering do?

Reduces aggression

Neutering tends to reduce male rat aggression (DeBold and Miczek 1981). Castrated rats tend not to initiate conflict (Barfield et al. 1972).

Neutered dominant rats tend to lose status. Castration results in a loss of position by the dominant rat and a disruption of the social hierarchy (Stewart and Palfai 1967). After the castration of the dominant rat, an intact subordinate male may become dominant and may display increased levels of aggression (Albert et al. 1986).

Neutering reduces aggression toward intruders. Under normal circumstances, the socially dominant male in a colony attacks intruders most (Blanchard et al. 1988). If the dominant male is castrated, he displays much less aggression toward an intruder. A former subordinate may subsequently become more aggressive toward intruders (Albert et al. 1986).

However, castration does not eliminate aggression. Aggression depends on many factors besides testosterone. For example, castrated males may still show low levels of aggression toward an intruder in their home cage. Such aggression is low, about 15% of pre-castration levels. When faced with an intact intruder, resident castrated rats are more aggressive than the intruder 50% of the time (Albert et al. 1986). Christie and Barfield (1979) found that resident castrates rarely displayed aggression unless an intruder initiated an encounter. Therefore, castration may drastically reduce aggression and may lead to loss of dominant status, but castrated resident rats may still display some aggression, particularly toward intruders in their home cage.

Reduces attacks on neutered rat

Neutered rats are attacked less by other rats.

Specifically, castrated male intruders are attacked by resident males about half as much as intact male intruders (Debold and Miczek 1984, Christie and Barfield 1979, see also Flannely and Thor 1976, Barfield et al. 1972).

Males castrated at different ages elicit different amounts of aggression from resident males. Flannelly and Thor (1978) presented aggresive resident males with adult male intruders that had been castrated at different ages (ages 1, 10, 30 or 60 days of age). Intruders that had been castrated before puberty (ages 1, 20 or 30 days) were immune from attack. Males that were castrated after puberty, at 60 days, were subject to attack by the resident males, but this attack was not sustained and they received few bites. In contrast, intact intruders were subjected to immediate, vigorous, prolonged attack and received many bites. Twenty-one percent of intact intruders were killed.

Therefore, castration of an intruder reduces the attacks he receives from a resident male rat. Castration of the intruder is especially effective if it occurs before puberty.

Reduces urine marking

Neutering tends to reduce urine-marking.

Price (1975) measured urine marking by male rats before and after castration. He found that urine marking declined after castration to 17% of precastration levels.

Taylor et al. (1987) also measured pre- and post-castration urine marking. The authors found in the month after the surgery (7 to 27 days post castration) castrated rats urine marked at 19.5% (+- 3.96%) of precastration levels. During the second month (27 - 47 days post castration), castrated rats urine marked at only 3.2% (+- 0.46%) of precastration levels.

Increases obesity

Intact rats eat less food and have a lower percentage of body fat than castrated rats. Specifically, intact rats eat 17% less food and they have 5% less body fat than castrated males (Drori and Folman 1976).

Slightly increases lifespan

Castration moderately prolongs life in rats.

Drori and Folman (1976) castrated males between ages 38 and 44 days. They found that castrated males lived, on average, 2 years, 2.9 months +/- 32 days while intact males lived 2 years +/- 26 days.

To examine longevity more closely, the authors divided the deaths into 'early deaths' (the first 25 rats to die in each group) and 'late deaths' (the last 24 to die in each group). The castrated rats' 'early deaths' were about 2 months later, and their 'late deaths' were about 4 months later, than the control group:


Average age at early death

Average age at late death

Average age (all deaths)


1 year, 9 months +/-30 days

2 years, 9 months +/- 22 days

2 years, 2.9 months +/- 32 days


1 year, 6.9 months +/- 22 days

2 years, 5.3 months +/1 16 days

2 years +/- 26 days

Note, however, that regular exercise, mating, and restricted food prolonged lifespan more than castration:

Rats in the exercise group, who experienced brief but strenuous activity every day, lived an average of 2 years and 4.3 months. Rats who mated regularly lived 2 years and 3.5 months, while rats who were on a restricted diet lived an average of 2 years and 5.2 months (Drori and Folman 1976).

In similar experiments, Drori and Folman (1986) found that castration between 38 and 44 days slightly increased lifespan, from 21.6 months +/- 28 days to 22.1 months +/- 32 days, while Asdell et al. (1967) found that intact, never-bred males lived an average of 20.3 months +/- 21 days while males castrated at age 45 days lived 21.5 months +/- 26 days.

One study found that age at castration was important: Talbert et al. (1965) found that castration at birth prolonged life from 15.1 months to 17.4 months, but that castration at weaning or at age 100 days had no effect on lifespan.

Conclusion: these studies found that castrated rats lived slightly longer than a similar group of intact rats. Castration is not, of course, a guarantee of longer life for an individual rat. Rather, considered as a group, castrated rats have a moderately longer average lifespan than a similar group of intact rats. However, other factors, such as exercise, mating, and restricted food can prolong life even more than castration when compared to similar groups of control rats. Longevity is subject to a wide variety of factors, and hormonal state is just one of the many variables that plays a role in determining how long an individual might live.

Why does castration prolong life?

Castration may prolong life by eliminating the life-reducing effects of testosterone. For example, testosterone may induce tumors, so castration tends to reduce the incidence of tumors. In Drori and Folman (1976), 10.4% of intact males developed tumors, compared with 8.7% of castrated males -- a small decrease. Castration may also prolong life by by lowering the metabolic rate, which may delay death in the event of a terminal illness.

Neutered rats are sterile

The testes produce sperm, so removing the testes eliminates the sperm producing organ and renders the male infertile. However, sterility after castration isn't immediate.

How long does it take after castration for a rat to become sterile?

There are very few scientific studies that examine fertility after castration. So far, I have been able to locate only one such study in the literature.

Pholpramool and Sornpaisarn (1980) castrated 21 male rats. The authors found that for two days after castration, fertility in these castrated rats was normal. After the second day fertility started to drop quickly, and by the eighth day after castration all the rats in this study were sterile.

Specifically, Pholpramool and Sornpaisarn (1980) castrated 21 male rats and divided them into groups of 2-5 rats. Males from each group were then individually housed overnight with 2 females in heat on the day of castration or 1, 2, 3, 4, 5 or 8 days afterwards. Mating was judged by the presence of sperm in the vagina on the following morning. Males which didn't mate were given another opportunity the following night and were placed in the subsequent group.

Females were examined 10 days later for pregnancy. Male fertility was measured for each male as a percentage: the number of eggs he fertilized / the number of eggs a female ovulated x 100. Specifically, %male fertility = #implantation sites / #corpus lutea x 100. Therefore, if a male's score is 100% then he fertilized all the female's eggs. If his score is 31.5% then he fertilized a little less than a third of her available eggs.

Day after castration

% male fertility

Fertility notes

Behavioral notes



Normal fertility

Normal sexual activity



Normal fertility

Normal sexual activity


89.2% (+- 2.4%)

Slight decrease in fertility



51.6% (+-22.7%)

Sharp dropoff in fertility



45.5% (+- 46.7%)

Fertility continues to decline



31.5% (+- 2-.3%)

Fertility continues to decline

Sexual activity declines.
Most males no longer
interested in copulation



Males completely infertile.

Only one male mated

Why does it take so long for castrated males to become sterile?

Under normal circumstances, sperm leave the testes and enter a long tube called the epididymis. They travel down this tube, mature, and gain their fertilizing ability. Sperm are stored at the end of the epididymis, where they remain fertile for 42 days in the rat, then die (White 1932).

After neutering, sperm move rapidly through the epididymis (Sujarit and Pholpramool 1985). The epididymis atrophies, becomes hostile to sperm, and slowly destroys them (Arya and Vanha-Perttula 1985). Within 3-4 days after castration the sperm in the epididymis lose their mobility, lose the ability to fertilize, and die (Dyson and Orgebin-Crist 1973).


A natural experiment

A rat rescue organization, Rattenvermittlung, which is part of the Swiss rat club named Club der Rattenfreunde Schweiz, neuters all healthy male rescue rats before finding homes for them. The neutered males are placed in cages with females 10 days after surgery. Over 100 males have been neutered and placed with females in this way. No pregnancies have occurred (C. Schenk, pers. comm.).


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