Rodent Cage Space Requirements
Date Issued: June 23, 1999
Date Revised: December 16, 2011, 4-21-2010, 12-9-2008, 12-16-2003, 3-242003, 2-10-2002, 4-12-2000, 9-13-1999, 7-8, 1999
Comparative Medicine Resources
Purpose:
 define rodent housing population density standards for specific caging
 assign responsibility for management of rodent breeding cages
 assign responsibility for management of aggressive group-housed rodents
Introduction:
Overcrowding of cages may cause rodents to become stressed. Moreover, pups can die from
the inability to suckle the lactating mother. Overcrowding can also predispose to respiratory
disease when levels of ammonia from the urine are elevated.
Mice and rats have relatively short gestation times and large litters in comparison to other
laboratory species. Therefore, the individual responsible for managing the breeding colony
must be vigilant to wean juvenile mice from their mothers into a separate cage and to identify
pregnant mothers in harem breeding.
I. NON-BREEDING RODENTS
Housing density: maximum rodents allowed per cage style
SPECIES

STANDARD
MOUSE
Animal Care
Systems
CAGE TYPE
Animal Care
Systems
Small
Medium
Cage Floor
(in2)
67
75
83
MOUSE
4
4
5
RAT
Diabetic
rats and mice
3 mice
3 mice
4 mice
STANDARD
RAT
Animal Care
Systems
Large
153
180
12
 3 rats < 300 g
 2 rats 300-400g
 1 rat > 500g
 3 rats < 300 g
 2 rats 300-400g
 1 rat > 500g
 3 rats < 300 g
 2 rat > 300 g
 3 rats < 300 g
 2 rat > 300 g
HAMSTER
3
GUINEA
PIG
2
1
II. BREEDING RODENTS
A. Housing density: The maximum number of rodents allowed per cage style is in the
table below. The Attending Veterinarian may limit the number of litters on the basis of
specific animal health and welfare concerns related to high-density housing of litters.
SPECIES
STANDARD
MOUSE

Animal Care
Systems
Small
MOUSE
 1 female
 1 male
 1 litter
 1 female
 1 male
 1 litter
CAGE TYPE
Animal Care
Systems
Medium
Animal Care
Systems
STANDAR
D
RAT
Large
 2 adults
 2 litters1
 3 females
 1 male
 3 litters
(see below)
 2 females
 2 litters
1 female
1 male
1 litter
 1 female
 1 male
 1 litter
RAT
 1 female
 1 male
 1 litter
xa
HAMSTER
GUINEA PIG
B. Breeding location.
1. Mice breeding is only permitted in the mouse barrier rooms of the Central, Transgenic
and Cancer Center RAFs. Time-pregnant animals from commercial vendors are
permitted in the conventional rooms of the Central RAF.
2. Breeding of rats is permitted in barrier or conventional rat rooms.
C. Breeding schemes
1. For rats and mice, two different breeding schemes are acceptable
a. Monogamous pairs in all caging.
One male and one female are housed together; only one male per cage is permitted.
They are not separated when the female becomes pregnant. This model takes
advantage of postpartum estrus and allows the female to become pregnant and
nurse at the same time. Litters are born approximately 21 days apart. In rare
instances transgenic lines require up to 28 days prior to weaning. Should this be the
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case with specific mouse lines, the male must be removed prior to delivery. No postpartum estrus is allowed in this circumstance.
b. Harem mating
(1) One male and 1-3 females are housed together; only one male per cage is
permitted.
(2) For housing density, see chart above
(3) To avoid overcrowding of multiple litters in a cage, remove the apparent
pregnant female from the harem cage to her own cage before the litter is born.
Alternatively check females every morning for the presence of a vaginal plug.
This mucoid plug is only present for 12-24 hours post-coitus; the likelihood of
pregnancy is very high. After the pups are weaned, the female may be returned
to the harem cage.
This is the responsibility of the investigative staff, unless prior arrangements have
already been established.
D. Post-partum
1. She delivers her pups and nurses them for 21 days. Occasionally some transgenic
mouse lines have pups born with delayed neonatal development of up to 28 days.
2. Whenever possible caretakers will document the date of birth of litters on the cage card
during daily animal care rounds.
E. Weaning pups
1. Age of pups at weaning: 21 days
2. Segregate pups by male and female into separate cages. Brothers can and will mate
with sisters as young as 6-7 weeks of age. Sex animals by evaluation of the
anogenital distance rather than evaluating the sexual anatomy. When in doubt
consult with your caretaker or contact the CMR Training Coordinator for brief training.
3. Plan for sufficient shelf space on your rack. Order autoclaved cages in advance and
arrange with the veterinary technician should your rodent room be near or at full
capacity.
4. Observe whether the weaned pups are able to reach both the food and water bottle.
Request or place longer sipper tubes if the tubes are too short.
5. Always document addition of cages to the census sheet (CMR or research staff)
F. DEFAULT WEANING BY CMR:
Automatic weaning: a service fee will be charged for non-response to excessive rodent
housing density (overcrowding).
1.
CMR staff evaluate for overcrowding during routine rounds or during cage changes.
All cages that are overcrowded according to the above defined standards are marked
with a LIME GREEN Overcrowding Card, dated and initialed. Animal Caretakers report
findings of overcrowding to the responsible veterinary technician.
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2.
The CMR veterinary technician notifies the Principal Investigator or key staff member
within 24 hours. The investigator then has three calendar days from email notification
to separate the animals into the appropriate housing density. If this is not done after
three days, including holidays and weekends, a RAF staff member will complete the
task and identify the new cage based on the cagecard number of the source cage.
The CMR service charge is $20.00 per cage2.
3. Time allowed for correction of overcrowding is 3 calendar days i.e. this includes
weekends and holidays. Therefore it is imperative to establish proper rodent housing
before it becomes an overcrowding issue. If the correction is not done in the
appropriate amount of time, the RAF staff will do so and charge the lab a fee. If there is
insufficient rack space to wean the rodents, contact the Assistant Director of Operations
& Finance at least three business days in advance for consideration of waiver of the
$20 per cage separation fee. Please plan rodent breeding and colony management
responsibly. CMR accepts requests for colony breeding management by our veterinary
technical staff fee for-service.
G. DEFAULT SEPARATION BY CMR
1. Highly distended apparent late-term pregnant mothers not yet separated ($)
2. Injured rodents (NO $)
Rodents that are fighting or have fight wounds are separated immediately and are
labeled with orange “Under Observation” cards in order to monitor wounds and/or further
fighting. Caretakers must report injured rodents to the veterinary technician on the same
day so that the research staff will be notified within 24 hours of the observation.
H. COMPLETION OF DEFAULT WEANING OR SEPARATION
The card remains on the cage with the “resolved” section initialed and dated. This is a
record to alert the PI which cage had been corrected for overcrowding. The PI may then
remove the card.
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