American Fancy Rat & Mouse Association

This article is from the Spring 1998 AFRMA Rat & Mouse Tales news-magazine.


The question that has plagued the fancy for many years.

By Nichole Royer

* Culling is eliminating animals from a breeding program.

* Wild rats average 6–9 babies in favorable conditions; Domestic rats average 8–12 and often times can have 14–16 per litter. Domestic rats have been known to have as many as 22!

* Rats only have 12 nipples; Mice have 10.

* Baby mice are much more sensitive to being out-competed than are rat babies.

The dreaded “C” word. This topic has created more controversy, arguments, and knock-down drag-out fights than any other in the rat and mouse fancy. It has also spawned a huge amount of misunderstanding, some of which is based solely on heated feelings and incorrect information on both sides. Rather than hide from this unpleasant subject, or say, “It’s okay to do it, just don’t tell anyone,” I thought it was time to take a calm, mature look at this concept.

What Is Culling?

According to the American Heritage Dictionary, the definition of cull is “to pick out from the others; select.” The definition changes a bit though when used in connection with breeding animals. In that context it usually refers to eliminating animals from a breeding program, or reducing the size of a litter, often through euthanasia.

Culling Rats

Why on earth would someone choose to euthanize some of the babies from their litters? This is the question that has spawned so much controversy. I will try to explain.

Rats in the wild who live in a granary and have abundant food have an average of 6–9 babies; those living under less favorable conditions have fewer. If they have more, the babies are small and do not have as high a survival rate as those from small litters. Thus the rats from large litters are selected against, and more of the babies from small litters survive to reproduce.

In captivity, rats have traditionally been kept because they are prolific breeders. Scientists and reptile food producers systematically selected animals who have large litters. Without natural selection, these smaller babies grow up and themselves reproduce. Thus our domestic rats average 8–12 babies, and often more.

Female rats have a very strong maternal instinct. They will do their best to raise as many babies as they can, and will even raise those of other species. Rats have been known to attempt to raise as many as 25 babies, and though they usually remain on the small side throughout their lives they are reasonably healthy. Though raising a large litter puts a strain on the female, she recovers after the babies are weaned.

Female rats have 12 nipples. If they have more babies than that, they will divide them into separate piles. They will then spend their time running from pile to pile feeding the babies, snatching a drink and something to eat in between. In most cases they lose some weight, their fur gets thin, and when they see people, they beg to get out of their cages and away from their babies. When the babies are weaned, the female usually quickly regains her previous weight and condition.

In the wild it is likely (I have read that it happens in 90% of the cases) that a female with two piles of babies will have one of the piles eaten by a predator before they reach two weeks old. She herself would also be at an increased risk of being preyed upon due to the strain of raising a large number of babies. If she is eaten, all the babies die. This is simply natural selection at work.

The people I know of who cull do not do it because they are cruel and uncaring, nor do they do it because they see their animals as disposable.

In our homes natural selection does not work, so some breeders take it into their own hands. This is not because they don’t care about their animals, actually quite the opposite. They feel that they are responsible for their female being pregnant, and so it is their responsibility to see that she does not have so many babiesthat it causes undue stress on her. They also feel that it is up to them to make sure each baby has the opportunity to reach its utmost potential in size and health. This is backed up by the fact that scientists have shown that rats and mice with a quicker rate of growth (weight being kept in proportion to size, not overweight) do better on intelligence and sociability tests than their slower growing comrades.

Breeders who feel this way take it into their own hands to decrease the size of a large litter to the point they feel comfortable with. They do this by fostering babies on another female if available, or by euthanizing any babies with problems or who are particularly small. This mirrors natural selection. If the litter is still too large, they will choose to keep the babies who are the most likely to find good homes.

A small proportion of breeders cull because they refuse to take excess babies to a pet store since they are likely to be abused and used as reptile food. These breeders are concerned that they will not be able to find good homes for all of them, and would rather take it upon themselves to see their babies humanely euthanized at 3 or 4 days old than take the chance that they will be mistreated.

Culling Mice

Breeders cull mice for much the same reasons, though mice do have their peculiarities.

Mice are not the wonderful mothers that most rats are. In the wild, if a mouse baby is small, it often does not get enough food, gets weak, and is nudged to the edge of the nest. When this baby gets cold, the mother will kill and eat it. Often a mother mouse will kill and consume her entire litter if it is disturbed, or if it is large enough to put a strain on her. This works very well in the wild, and is a behavior that we occasionally see in domestic mice.

Baby mice are much more sensitive to being out-competed than are rat babies. Runts nearly never catch up in size or health with their larger littermates, and often end up simply starving to death. Some breeders let nature take its course, while others step in and put down the smallest.

One of the reasons English mice are so large is that some English fanciers culled down to 2 or 3 babies. This allowed those mice to reach their fullest potential, and they then selected the largest to use for breeding. Culling played a large part in creating English mice, and they really do not do well with large litters.

Male Mice

Male mice present their own dilemma. There are often more males in a litter than females. They are larger, and frequently the females have a difficult time competing. This leads to females who are small, and in a large litter some may die.

Unfortunately, it’s the female mice that are in demand for pets. Though some of us have a distinct preference for male mice as pets, most people like the girls. They have good reason. Male mice have a very distinctive odor which most people find very offensive. Male mice will also usually fight with other males, so have to be kept singly.

If a breeder has just one line of mice, and breeds the minimum number of litters to keep it going, that’s 4 litters a year. If the average litter is 8 babies, 4 male 4 female, that’s 16 males a year. With an average life span of 1½ years, that’s 24 males at a time. Twenty-four males, each in his own separate cage which has to be cleaned, handled, fed, and watered. For most breeders this would be beyond their means to maintain responsibly. Almost all of the people who have tried to keep every male they don’t place have ended up with so many that their resources have been stretched beyond the point of humane care.

Some breeders take their excess males to the pet store to be sold for reptile food. Others cull them at 2 or 3 days. On occasion we hear of someone who turns them loose in a field, but since these animals often starve to death, we strongly discourage doing so.

No matter what, if you breed mice, you will have to deal with the dilemma of what to do with the males.

To Cull Or Not To Cull

That is the question that has plagued the fancy for many years. Some say it’s murder to do it, others that it’s cruel not to. It’s right! It’s wrong! Neither! Both!

AFRMA’s policy as a club is very simple. AFRMA does not support or condemn the practice of culling. Instead AFRMA encourages each individual to learn about the topic (thus AFRMA provides forums like this) and then to make a decision based upon what is best for themselves and their animals.

AFRMA has Board members who cull, and Board members who don’t. If you ask a Board member their opinion on culling, that is what you get. Just their opinion, not the club’s recommendation.

The people I know of who cull do not do it because they are cruel and uncaring, nor do they do it because they see their animals as disposable. Instead they do it because they feel it is the responsible decision for them. In most cases it is a heart wrenching, emotionally upsetting decision which they make because they feel it is the right thing for their animals. The same can be said for the decisions made by those who don’t cull. *

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Updated March 20, 2014