Survival of the fittest underpins natural selection. In the wild, animals are subject to a variety of pressures, including predation, disease, and competition for food or mates. The individual who is able to survive the longest and reproduce the most is the individual who is the fittest. Pets, however, are not subject to the same pressures as their wild counterparts. Our husbandry can prevent health issues and we can take individuals to the vets, they don't have competition for resources, nor do they have the risk of predation. But they do have their own pressures, such as sociability, adaptability and even desireability in terms of appearance.

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Nevertheless, the concept of survival of the fittest still applies in some way when breeding companion animals. If we breed an animal, we should breed them with improvement in mind. This includes the improvement of health, type (or conformation) and temperament. They should also be sociable with people and other rats, be good keepers (ie. not require copious amounts of food to maintain good body condition), and should have good fertility levels. There are times when you may make minor compromises for the sake of saving a particular line, but there should be rapid improvement thereafter. Starting with rats from good stock is the best way to begin breeding as you will have a known entity and can work from there. Starting from imports or other unknown entities is much harder work and even experienced fanciers can have a hard time producing quality rats from these sources. It took 7 years for the chinchilla variety to be suitable for exhibition and homing out after being imported, for example!

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A breeder should be able to identify the strengths and weaknesses of their breeding animals. For example, if the rat has a well-shaped head but a chopped rump, or if they are sociable with other rats but could be more forthcoming with people. The breeder should then make appropriate decisions in order to improve the line overall. This may involve removing the rat from the breeding programme, or, if the fault is manageable, mating them to a rat who makes up for their weaknesses. Inbreeding, linebreeding (a form of inbreeding) and outcrossing are all approaches to matings that have their own benefits and drawbacks depending on the circumstances, and a breeder should have a good working understanding of these.

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You do not need to understand inheritance patterns or genetics to make breeding decisions. You do, however, need the ability to make practical choices. No matter the approach you take, use the best of each generation and be quite strict with your selection. What is considered the 'best' may depend on the breeder's goals, but a general rule is to not use any weedy offspring that, under different circumstances, likely would have perished in the nest or shortly thereafter. You want rats who have been good competitors at the milk bar, so to speak, and for them to be robust.

 

Be aware of the weaknesses of your line, but remember that you are spinning multiple plates. These plates can be temperament, stamina, eye size, variety, or head length, among many others. Do not focus on spinning one or a few plates so extensively that you neglect the other plates and they fall. This is how you end up with rats who are unbalanced; they may have fantastic ears, for example, but they may have very short heads, or they may have great type, but they are unsociable. Ideally, as each generation passes, you want the bar to gradually rise in terms of the quality of rats that you are producing. You will experience setbacks that may knock this bar back down a touch, but evaluate and adapt as these setbacks come.

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When making pairings, aim to maximise on strengths and minimise and improve upon weaknesses. For example, if you have a rat with small eyes, avoid mating them to a rat who also has small eyes. Also consider the traits of the parents of each rat as this will give you an idea of what traits may be underlying in the rat you are currently breeding from. For example, a rat with good muscle tone may have a parent who lacked it, and they may be more likely to have offspring who lack it also. Try to pair this rat to an individual who also has good muscle tone to fix this trait into the line.​

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Your breeding rats should also be in very good condition even before the matings are completed so that they can produce the best possible offspring. If they are underweight, for example, the buck will not have the nutrients available to produce high quality sperm. Stress can also have a negative impact on the quality of offspring, even before they are conceived. This is through epigenetic changes, which are alterations in gene expression that can be inherited. Stress during pregnancy can lead to a higher risk of reabsorption and complications as well.

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​Occasionally, you will find rats who produce offspring who are better than themselves. They may not be anything special, but the offspring they produce are. These are the rats you want to breed from the most, and a buck who is like this can be called a 'founding sire'. Pairing them to poorer quality rats will produce improved offspring, however do not expect these kittens to be good breeding stock as they still have the influence of the poor quality individual in their genetics. You may choose to inbreed to the founding sire after mating to a poorer quality individual to fix improved traits.​

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Do not be too concerned if your rats do not show excellence in all areas in the span of a few generations. Continue making the best breeding decisions possible and keep an eye out for individuals who fit your aims. Consider the famous racehorse, Northern Dancer. His more distant predecessors weren't particularly successful but they continued to be bred to the best and most complementary horses. As such, the right traits were all in there and finally aligned in Northern Dancer. He was then a hugely popular and successful sire, producing excellent offspring. Just make sure that the right 'ingredients' are in your rats, make the best breeding decisions available, and keep the best of each generation.

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Avoid breeding from old rats. You may choose to breed from an old rat if they are exceptional, your only option, or in order to save a line, but this should be the exception. Bucks produce sperm into old age, but this sperm is more likely to mutate and the quality of it will have decreased. Does should be fit enough to have a litter, but the quality of their eggs will also deteriorate with age. Older does are also more likely to have birthing complications. If you are breeding away from a health issue that only manifests at an older age, select using the bucks and mate them to young does. That way you're not doubling up on the same potential issue with regards to reduced constitution.​​​

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There are a few different approaches that can be taken when making pairings in terms of the relatedness of the rats. Each approach has its own benefits and drawbacks, so should be used based on the needs of the line.​​​​​​​​

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Inbreeding

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Inbreeding is the breeding of two closely related individuals, who may be siblings or parent and offspring. It does not add genetic variation, nor does it create good or bad traits. It simply provides the opportunity to weed out negative traits and fix positive traits. However, it also provides the opportunity to weed out positive traits and fix negative traits. This is why strict selection is required when inbreeding. Over time, inbreeding can produce consistency in quality of health, type and temperament too. If you do not strongly select for the best in the litters, then you risk producing rats of low quality with weak constitution and fixing negative traits. Your goal is to bring out all of those positive traits and have them align in your breeding rats.

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It is worth noting that rats have a higher tolerance to inbreeding than other species, and are not considered to be inbred until the 20th generation of sibling pairings. There are laboratory-based lines who have been inbred for over 100 generations! Selection for the fittest individuals is crucial. However, the general recommendation is to inbreed for up to 6 generations.​​​

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Inbreeding tends to be recommended after outcrossing. This allows for the introduction of new genetic material to address any issues within the line, then selection for positive traits and against negative traits.

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Linebreeding

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Linebreeding is a form of inbreeding where related individuals are bred together, but they are not closely related. This includes cousins, aunt-nephew/uncle-niece, and similar. Half-siblings can be considered as linebreeding, though my personal stance is that it depends on the relatedness of the other individuals involved. If they share a father and the mothers are siblings, then I would consider this to be inbreeding. If they share a father and the mothers are cousins, for comparison, then I would consider this to be linebreeding. Linebreeding fosters the maintenance of some genetic variation, especially in a larger breeding population, which allows for more varied selection opportunities. For example, if only inbreeding has been used and an issue crops up later on in an influential rat's life, there is little scope to then select away from them. By comparison, if you have linebred and kept a wider pool, you can better minimise or eliminate the influence of the affected individual. Linebreeding also helps to balance the benefits and drawbacks of inbreeding and outcrossing.

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Consider the strengths and weaknesses of each rat as well as the strengths and weaknesses of both sets of parents. This gives you an idea of what traits run in each 'strand', and is useful when deciding how complementary a mating would be. You want to capitalise on the strengths of each strand and try to bring them together while minimising any weaknesses.

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If you have inbred for a few generations, it can be helpful to linebreed for a generation or two afterwards to balance out any potential issues. Having said this, the suitability of the rats is the most important factor to consider. Do not mate a rat to a more distant relative purely due to relatedness if there is an all-round much better closely related option available.​​​

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Outcrossing

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Outcrossing is the breeding of two unrelated animals. Soft outcrossing (or semi-outcrossing), which is referred to here, can be defined as the breeding of two more distantly related animals. These types of pairings introduce additional genetic diversity, which can be beneficial but can also be a drawback. Strict selection for the best and fittest individuals is just as important here as when inbreeding or linebreeding, as indiscriminate outcrossing will never consistently produce good quality offspring and can introduce all sorts of issues into the rats.

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Any outcrosses made should be with a specific aim in mind (though outcrossing for temperament isn't as reliable). It should bring in new blood with strengths that your current breeding rats lack. This then allows for the introduced and better trait to be selected for within your line. Do not outcross to any rat for the sake of an outcross, especially if they have an unknown background as you will likely do more harm than good by introducing negative traits. Equally, do not only look at who is winning at shows (though this is a useful starting point), but also consider how complementary the rats are to yours. If they are winning but have the same fault as your own or have an underlying issue, why breed to them? Do not knowingly bring in detrimental traits just because the rats are winning based on their appearance. You may make short-term gains, but then you will have larger issues to contend with in the long-term.

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It can be difficult to find multiple quality outcrosses over time, so this is a practical restraint for a breeder who wishes to continually outcross. Including soft outcrosses and linebreeding may be more practical. Also remember that genes for different varieties are often associated with other effects such as reduced size or a tendency towards a cobby body type; it is rarely purely the colour or fur type of the rat that is affected. For example, there have been papers published suggesting that the hooded gene may reduce total weight and body length, and may subtly alter brain structure and glandular balance. If you layer more and more colour genes and produce "rainbow" litters you may be at risk of inadvertently producing kittens who are less robust.

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In the first generation, the outcome is very much so an exercise of chance. The offspring can vary in their physical characteristics, health and temperament, so selection with your aims in mind is important. Losing a recessive variety gene for a few generations is normal and an understanding of genetics and inbreeding to the parent line of the desired variety can increase the chance of reoccurrence.

 

You may also find that a new problem is introduced, even a problem that wasn't seen in either of the parent lines! Such is the joy of genetics. You can reduce the risks associated with outcrossing through soft outcrossing as there is already a degree of relatedness and compatability. If you find yourself in a tricky spot, perhaps due to an infection in your rattery or personal issue that delayed pairing, a soft outcross is a good option.​

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When you outcross, you should ideally continue breeding from them as a strand separate from your original line until you are happy with the outcome. This may take a couple of years while you evaluate them and following generations. If you merge them completely with your original line straight away and don't like the results, you land yourself in a tricky situation.​​

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It is often recommended to inbreed for a few generations after an outcross to fix the positive introduced traits and remove any negative traits. Of course, some traits may manifest over time and this should be taken into consideration when planning.

 

Remember that outcrossing tends to hide deleterious traits instead of removing them. Inbreeding is what allows you to remove the trait altogether and if inbreeding isn't used then the trait can just continue to persist. Below are some graphics to very crudely illustrate this.

 

Let's say that you have a line where there is a high incidence of heart issues, and it is making it difficult to select away from. If you were to represent this as a graph, it would look as below. The arrow marks where the, grossly simplified for illustrative purposes, 'heart issue trait' is. There is a high frequency of individuals exhibiting this trait, and not many who aren't. There is little wiggle room for inbreeding away from the trait.​​​​​​​​​