Breeding

Contents

A: Breeding FAQ

Why won't my mice breed?
My mice didn’t breed. Can I get a credit?
How do I plan a colony? How many breeders do I need to set up?
What is the mating age? When should I set up my mice to breed?
What is the weaning age?
How long is gestation?
How many pups do mice have in a litter?
When should breeders be replaced?
How do I set up timed matings/timed pregnant females?
How do I foster mice?
How well does this strain breed at JAX? Where can I find information about how my specific strain breeds?
Can this transgenic breed as a homozygote?
How many pups per litter should I expect?
How do I know when to refresh my colony?
Can I breed my humanized mice?
What is Love Mash?
My mice are fighting – now what?

B. Tips for Poor Breeders

C. General Husbandry Tips

D. Mouse Room Conditions

E. Breeding-Related Articles and Resources

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A: Breeding FAQ

1. Why won’t my mice breed?

Please read our Tips for Poor Breeders (Section B) and General Husbandry Tips (Section C).

Also see Mouse Room Conditions (Section D), which includes suggestions to reduce stress.

2. My mice didn’t breed. Can I get a credit?

No, unfortunately non-productive breeders are not covered by our credit policy. Learn more about our credit policy.

3. How do I plan a colony? How many breeders do I need to set up?

A minimum of two to four breeding pairs are recommended for most strains; additional pairs are suggested for strains challenging to breed, or to expedite colony expansion. 

4. What is the mating age? When should I set up my mice to breed?

Most strains reach sexual maturity at ~6 - 8 weeks of age

5. What is the weaning age?

Most strains are weaned ~21 days; up to 28 days if preferred

6. How long is gestation?

Gestation is ~19-21 days

7. How many pups do mice have in a litter?

Litter size:  2-12 pups; highly strain-dependent

8. When should breeders be replaced?

Replace breeders:  ~7-8 months of age (several mutant strains have considerably shorter windows of optimal breeding performance)

9. How do I set up timed matings/timed pregnant females?

• House stud males (individually) for 2 weeks prior to mating
• Use 8-15 week old females
• Add 1-2 female(s) into each stud male’s cage in the late afternoon, prior to the dark cycle
• Check for vaginal plugs early the next morning (plug will dissolve over time)
• The date a plug is observed is gestational day 0 to 0.5

Note:  Not all females will plug and not all plugged females will become pregnant; success rate varies by strain, male’s experience, conditions, etc.

Reference: Nagy A, Gertsenstein M, Vintersten K and Behringer R.  2003. Manipulating the Mouse Embryo:  A Laboratory Manual, 3rd ed.  Cold Spring Harbor Laboratory Press.  ISBN 0-87969-574-9.

Also see the following article: 6 steps for setting up timed pregnant mice

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10. How do I foster mice?

• If possible, select a foster mother with a different coat color and a natural litter as close as possible to the same age as the foster pups.
• Remove as many pups from the natural litter as you have foster pups, to keep the final litter size manageable for the foster mother. If the litter to be fostered is large, consider dividing it between two foster mothers.
• Combine the remaining natural litter with the foster pups and gently intermingle them with soiled shavings (including feces) from the foster cage, then place them together with the foster mother.
• Replace the cage and do not disturb.
• Check the pups after a few hours (without disturbing the cage). If the foster mother has gathered the pups together, she will likely care for them.

11. How well does this strain breed at JAX? Where can I find information about how my specific strain breeds?

Some data on your strain may be in the Handbook on Genetically Standardized Mice; get a copy here. There may be additional information in the Technical Support section of the strain datasheets under Breeding Considerations.

Reproductive performance can vary significantly at different institutions due to environmental factors.

12. Can this transgenic breed as a homozygote?

If we distribute a given transgenic as hemi (and/or noncarrier), we may not have information as to whether the strain can be bred to homozygosity without adverse effects on fertility or viability. If known, this information is commented on in the strain datasheet in the Technical Support section under “Breeding Considerations.” 

13. How many pups per litter should I expect?

Litter sizes vary by strain. Some data on your strain may be in the Handbook on Genetically Standardized Mice; get a copy here.

 14. How do I know when to refresh my colony?

We recommend refreshing the breeders every 5 – 10 generations. For more information, please see the Genetic Drift section

15. Can I breed my humanized mice?

The fertility may be affected by the irradiation at 3 weeks. Please note the human immune system is not passed on to the next generation.

16. What is Love Mash?

Love Mash™ is a diet designed to support reproduction.

17. My mice are fighting – now what?

Consult with your facility veterinarian. Below are some general guidelines that can help to reduce the risk of aggression.

• Males may be combined at weaning age (3-4 weeks), but should not be combined at older ages.
• Males shipped in separate compartments of the same shipping box or in individual boxes should not be combined upon entry into your facility as they may fight.
• Separate group-housed males that are fighting—at minimum the dominant male (often the mouse without wounds).
• Males from some strains (e.g. SJL/J) are extremely aggressive and may need to be removed until pups are weaned.
• Housing density of mice can affect aggressive tendencies. Consult your Institutional Animal Care and Use Committee for housing density guidelines.
• Change gloves frequently and use disinfected forceps to reduce scent cues.
• Providing environmental enrichment such as nesting materials, Nestlets (Animal Specialties and Provisions, LLC), NestPaks (WF Fisher and Son), and Shepherd Shacks (Shepherd Specialty Papers), which can help alleviate stress and aggression.

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B: Tips for Poor Breeders

• Minimize noise. Keep in the quietest area possible, away from doors, sinks and heavy traffic.
• Minimize handling, especially with females close to delivering or with new litters. Stressed mothers are more likely to cannibalize or abandon their young.
• Keep males and females together whenever possible to maximize breeding and reduce stress. If a male needs to be removed, do not remove him when the female is about to give birth, and do not return to the cage until pups are weaned.
• Handle mice gently with forceps. Disinfect forceps between cages and change gloves frequently, to avoid spreading scents. 
• Maximize darkness. Move to the darkest location available, away from exit signs and other night light sources.  Ensure that the dark cycle is maintained once the lights are turned off.
• Provide nesting materials. Nestlets, Kimwipes or other soft, fibrous material provide security and enrichment that may enhance productivity.  See ‘Enrichment’ suggestions in the Mouse room conditions section.
• Consider changing diets.  Diets with a higher (or lower) fat or protein content, compared to your standard diet, may improve productivity of a challenging strain.

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C: General Husbandry Tips

• Consult your institution’s Animal Care and Use Committee (ACUC) for guidelines to determine the best husbandry practices at your institution.
• Obtain 2-4 breeding pairs to establish a new colony.  Additional breeders can significantly reduce the time needed to expand a colony.
• Combining males from different litters is only appropriate at weaning (3-4 weeks).  Males combined later are likely to be aggressive and to fight, causing wounds and/or death to their male cage mates. Males shipped in separate compartments likewise should not be combined.
• Mate mice early, at 6-12 weeks of age. Mice tend to gain weight and be less productive if mated later.
• Replace non-productive breeders. If matings have not produced a litter within ~60 days, replace them to maximize your colony production.
• Retire breeders at 7-8 months of age.  Rotating mice on a regular schedule will maximize colony production.
• Breeding characteristics are strain- and environment-dependent.  Establish normative breeding data for each strain at your facility to detect colony changes, identify deviations and maximize breeding efficiency.
• Genetic drift can alter strain phenotype over time. Cryopreserve your unique strains with The Jackson Laboratory and refresh inbred colonies regularly to protect against genetic drift, natural disasters, breeding errors or disease outbreaks. See our Disaster Planning for more information.
• Expect seasonal changes in breeding performance. Some strains produce more litters in the spring and summer than in winter and fall.
• Weather & air pressure changes can alter behavior and production. Reduced breeding performance and/or hyperactivity may coincide with changes in weather.
• Hybrid mice generally breed more efficiently than inbred mice. F1 and F2 hybrids and mice with mixed genetic backgrounds display hybrid vigor, producing more, larger, and healthier litters than strains on a pure inbred background.
• Anticipate changes in breeding performance when transferring a mutation (knock-out) or transgene to a new genetic background.  Breeding performance and phenotype of interest can change when a new background is introduced. 
• Avoid selection pressure.  Be careful not to choose breeders that might select against your phenotype of interest. For example, if older mice develop a phenotype that limits their life span or breeding productivity, select breeders for the next generation from early 2nd or 3rd litters.  Similarly, don’t select breeders based only on good breeding performance; you may unknowingly alter their phenotype.

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D: Mouse Room Conditions

• Light cycle:  A 14-hour light/10-hour dark cycle or 12 light/12 dark cycle is commonly used.  Ensure lights are not used and that researchers and technicians do not enter the mouse room during the dark cycle.
  
• Temperature and humidity:  Temperatures of 65-75°F (~18-23°C) with 40-60% humidity are recommended.
• Diet:  Fat content ranges from 4% to 11%; consult your facility manager and Institutional Animal Care and Use Committee to determine preferred diets at your institution.
• Water: Water should be accessible at all times; consult your facility manager and Institutional Animal Care and Use Committee to for treatment guidelines.
• Minimize handling: Handle mice gently and as little as possible, especially when females are pregnant, close to delivering, or have new litters.
• Minimize noises and vibrations: These can cause stress and decreased breeding performance.
• Minimize odors: Perfumes and other strong odors can reduce breeding performance and induce stress.
• Use gloves and forceps:  Can minimize scent cues. Refer to your Institutional Animal Care and Use Committee and/or facility guidelines for handling practices.
• Enrichment:  Nestlets (Animal Specialties and Provisions, LLC), NestPaks (WF Fisher and Son), and Shepherd Shacks (Shepherd Specialty Papers) can help alleviate stress and improve breeding for some strains.

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E: Breeding-Related Articles and Resources

1. 5 Reasons Why Your Mice Aren't Breeding
2. Most Efficient Breeding Scheme for Generating Cre/lox Tissue-Specific or Inducible Knockouts
3. Breeding Strategies for Tissue-Specific Knockout Mice
4. Six Steps for Setting up Timed Pregnant Mice
5. Get a copy of the Handbook on Genetically Standardized Mice here.
6. Mouse Genome Informatics email list – connect to other mouse users
7. CompMed^TM email list – connect to other comparative medicine professionals
8. Online book: Mouse Genetics by Lee Silver
9. Online book: Biology of the Laboratory Mouse, Earl Green, editor
10. Download breeding colony size planning worksheet

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