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Inbreeding and Genetic Weakening
© paul goodman, 2002 - 2012.

Because their life expectancy is short and breeding is prolific, mice make excellent subjects for observing Darwin's theories on evolution. While captive deer mice can live up to 5 years, in the wild only about 5% survive the first year and the oldest live about a year and a half. Generally only the healthiest, smartest, fastest, strongest and luckiest survive long enough to reproduce and pass on their genes. Even casual observers can quickly see the effects of raising these mice in a domestic environment.

Genetic weakening occurs with mice raised in a safe (captive) environment. When mice are protected from the natural culling of the wild, genetically "weak" mice survive long enough to reproduce and pass their genes on. Even in one generation you can start to pick out the mice that wouldn't survive in the wild. This isn't necessarily bad for pet mice that will never have to contend with the rigors of wild survival. Some of these domesticated mice will develop traits that make them better pets, like being less excitable and easier to handle. Breeders often select stock for these traits that make good pets.

If living in captivity can produce traits that make better pets, what's the problem with genetic weakening and what can be done about it? Well along with more domestic traits come things like reduced intelligence, lower resistance to disease and all the undesirable traits that are no longer culled out. Selective breeding can help reduce genetic weakening. When deciding on which mice to breed, the selection process should consider a diversity of desirable traits, not just one trait like color. Mice that show undesirable traits should be excluded from a breeding program.

Many breeders use inbreeding to create strains of mice with particular characteristics. A responsible breeder using this technique will understand genetics; know their goals and the proper steps to accomplish them. They will also be aware of the negative traits they may be promoting and take actions to minimize them. If you plan to breed mice, here are some things you should consider.

It's clear that in the long run inbreeding is bad, but how bad is it really. My experience with inbreeding is limited, but striking. I acquired Squeek with the intention of breeding her, she was caught wild and arrived pregnant. She had 3 healthy babies. Gordo (male), Mr. Earl (male) and C.C. (female). I accidentally allowed Mr. Earl to breed with his sister C.C. resulting in a double litter of 2 babies each. These are the results:

First Litter:

Second Litter:

These are the only deer mice that I've seen develop any health problems. I have one other inbred mouse, Pumpkin, that was the product of a mother-son breeding. Pumpkin is doing well and shows no signs of any health problems. I have 13 other deer mice from unrelated parents. They are all over a year old and healthy.

Update:

Pumpkin died at about 1 1/2 years old (18 months). Other than a short life span she showed no other health problems.

Ghost died at about 3 1/2 years old. She was by far my longest surviving inbred mouse.

Two brothers that were born to a wild caught mom and were known collectively as The Babies (because they were particularly small) lived an especially long time. They both lived to be 6 1/2 years old. I have no way of knowing if they were inbred, but I assume they were not.

I have never seen it myself, but one person wrote to me and told me their deer mice lived to be 8 years old.

I recommend that you avoid inbreeding without a solid understanding of genetics and a good reason to do so. If you choose to inbreed, only use high quality healthy mice. Out-breeding will generally provide healthier, smarter babies and is a much better choice for the casual breeder.

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Bucky Goldstein's G.W. Deer Mouse Ranch © paul goodman, 2000 - 2013.