Breeding mice are more successful when bred in preferable age
ranges. This can vary between strains, but a broad
generalization can be made. Males can normally be mated from six
weeks of age and have a reproductive lifespan of 12 to 18
months. Females on the other hand can be mated a little younger
than males at only five weeks of age. Pregnancies may occur
earlier with it being rare so should not be relied upon in a
breeding program. Females may continuously mate up to 10 months
and sometimes longer with the size of litters produced
diminishing after 7 or 8 months of age. Virgin female mice that
are not mated before 3 months of age often have diminished
fertility and may breed poorly or not at all.
A female mouse will reach sexual maturity and begin cycles at
around 35 days of age with inbred strains of mice reaching
sexual maturity a little later than outbred strains. As a result
of this, many labs begin breeding females at 5-6 weeks of age.
Male mice mature slightly later than females and are normally
placed for breeding at 6-8 weeks of age. The age at which male
mice are weaned may affect their ability to successfully mate.
It’s recommended that inbred male mice intended for breeding are
weaned at 28 days to improve their reproductive performance.
- Gestation time: 19 - 21 days
- Age at weaning: 3 weeks
- Age at sexual maturity: 6 - 8 weeks
- Approximate weight at birth: 1g
- Approximate weight at weaning: 8 - 12g
- Approximate weight at adult: 25 - 35g (male > female)
- Lifespan: 1.5 - 2.5 years
- Average litter sized: 4 -12
- Total number of litters per breeding female: 4 - 8
- Useful breeding life of females: 6 - 8 months
- Useful breeding life of males: 18 - 24 months
Systems of Breeding
- Out breeding
- Line Breeding
- Cross Breeding
Monogamous breeding involves one male and one
female selected and paired together for the length of their
breeding life. This system streamlines record keeping and lends
itself well to sustaining inbred or outbred colonies.
Polygamous, also referred to as harem breeding is where one male
is kept with two or more females. This results in a large number
of young from the least amount of breeding animals. It is the
most economical and used method of laboratory animal production.
Inbreeding uses Brother/sister or parent/offspring mating for a
minimum of 20 generations. This type of breeding is used to
produce animals that are very genetically similar. The
reproductive performance and behaviours can vary depending on
Out breeding, also referred to as random breeding avoids the
mating of close relatives and produces the maximum amount of
genetic heterogeneity and large litters. Mice of the same stock
are mated, producing a more energetic mouse by maintaining
genetic diversity. Precise records are essential to prevent
breeding animals that are related to each other.
Line breeding is where mice are breed to produce specific
traits. This will normally be done to produce mutant or
transgenic lines or for a trait needed for research.
Cross breeding is where mice of different breeds or strains are
breed and are also called hybrid cross.
Backcrossing is the breeding of successive offspring to pure
otherwise known as wild mice giving you a mutation or phenotype
on a “pure” background. Pure background mice are only so, when
at least 20 generations has occurred.
Density of cages
Overcrowding of cages can result in estrous cycle suppression
and trampling of new born mice. Overcrowded cages will also
become soiled faster and may lead to harmful levels of ammonia
and other waste gas accumulation within the cage resulting in
poor mouse health.
To help reduce overcrowding of cages, litters should be
separated by sex at weaning. If they are not separated by sex at
weaning, you may end up with unwanted pregnancies, unnecessary
new born losses and overcrowding.
Male mice are normally housed alone as more than one male
together are likely to fight, or with one or more females for
breeding. Male mice from the same litter may be housed together
however fighting will often occur as they reach sexual maturity.
As soon as a male mouse has been used for breeding, he should
not be returned to another group as fighting will often ensue
and can also result in the loss of another valuable breeding
mouse. Before breeding, a male mouse should be placed in his own
cage for a few days before females are introduced to allow the
male to mark the cage and establish his territory.
Keeping your mice healthy and in a healthy environment is very
important as there are many infectious agents (bacterial and
viral) that can cause decreased reproductive performance,
neonatal mortality or result in unhealthy pups.
The best way to determine the sex of mice is by comparing the
distance between the anal opening and genital papilla. The
distance is shorter in females and longer in males. An advantage
of mice over 12-14 days old there is an absence of fur in a band
across the anogenital (relating to the region of the anus and
the genitalia) space. Sexing of very young mice can be difficult
for beginners and it is often helpful to compare a number of
littermates to one another. In older mice, the presence of
nipples indicates a female as male mice do not have nipples.
Possible weaning variances
Pups are weaned at around three weeks of age. At about three
weeks, the mice should be fairly active with their eyes open and
eating food, however they may still be suckling. An easy test to
see whether or not they are ready to wean is their reaction when
the lid is removed. If they stay perfectly still then they are
still too young. If they poke around during this test like
normal then they are old enough to be removed. If pups are
weaned too early they will not survive. It’s important to make
sure they are removed at the correct time and not too early. If
you discover a runt or otherwise underdeveloped mouse, there is
a benefit in removing other mice that are ready so that the
remaining mouse has time to get better. It’s important to make
sure pups are weaned by the time of the next litter. This will
prevent “double littering”, and the risk of older pups trampling
the younger litter, competition for milk and undesirable ammonia
A mouse can be annoying where mice can be a bigger problem!
Getting rid of mice can be done in many ways such as the use of
traps that kill or catch and poisons. Controlling mice in and
around your home must be taken seriously as they can introduce
illnesses to your family as well as transfer fleas to pets.