Lifespan and longevity in rats

Expected lifespan

Lifespan predictions and averages for domesticated rattus norvegicus seem to vary depending on source. Most sources seem to quote average to maximum lifespan whilst ignoring the fact that if average lifespan is (for example) 2 years then 50% of rats die before this age. Some of those that I came across whilst researching this article include:
Source 1: Supreme Petfoods (Reggie rat) 2 to 4 years.
Source 2: Pets at Home care leaflet 2 to 2.5 years.
Source 3: Wikipedia 2.5 to 3.5 years.
Source 4: The Welfare Of Laboratory Animals Volume 2. 2-4 years.
Source 5: The Laboratory Rat. Vol II 2 to 3.5 years.
Source 6: http://www.ratbehaviour.org 2 to 3.5 years.
Source 7: Lifespan measurements in the male rat. Jones, D. C. ; Kimeldorf, D. J. Sprague-Dawley lab rats - average 707 day (2 years), range 6 months to 3 years (757 rats).
Source 8: Survey of pet rat life spans in the UK (figures taken from obituary notices in ProRatA), Clark, A. 2004 - average 21.6 months, and 95% percent had died by age 3 years.

Where lifespan data is based on evidence the average lifespan seems to be approximately 23 to 24 months. This means that 50% of rats die before this age and 50% die beyond this age. Angela Clark's survey included pet rats from all sources: pet shops, rescues and breeders. It is interesting that the lab research into the Sprague-Dawley strain produced similar results.

Lifespan comments and comparisons

There are two ways of considering how we can affect lifespan in pet rats. The first is to consider whether it is possible to extend maximum lifespan. In the lab survey (where results could not possibly have been skewed by human error or failure to keep proper records) all of the 757 rats had died before reaching a few weeks over 3 years of age. The age by which all of the rats had died would be considered maximum lifespan for that strain. Longevity studies then ask, "Is it possible (by the means being tested) to extend that maximum so that some rats from that strain live significantly longer lives?"

The second consideration is to question whether there are interventions that can extend the average lifespan. In this scenario, the maximum lifespan would remain at just over 3 but instead of 50% of rats having died by the age of 2, might it be possible to extend the average lifespan to nearer 28 or even 30 months?

It has been suggested that in comparative terms 1 rat year equates to approximately 30 human years, making a 2 year old rat approximately equal to a 60 year old human and a 3 year old rat approximately equal to a 90 year old human. This is a simple comparison based on comparative life expectancy and should not be taken too literally. Source: http://www.ratbehaviour.org

Factors that influence longevity

Many factors influence lifespan. These include:

• Genetic factors
• Environmental hazards
• Medical/surgical resources and expertise available
• Diet
• Stress factors
• Exercise
• Breeding practices
• Husbandry

Genetic factors

The process of aging varies considerably between individual members of any population. Probably the single most influential factor in causing this variation is the genetic make-up of the individuals concerned. These genetic influences seem to fall into two broad categories:

1. Genes that affect the rate of aging.
2. Genes that predispose an individual to a life shortening disease.

Genetic factors are unpredictable except where consistent inbreeding creates many individuals with similar characteristics. As well as the physical characteristics that can be seen, genes that influence aging and disease also become more predictable if comprehensive records are kept. Even when a genetic predisposition to a certain illness exists in an individual, positive steps can be taken to reduce the risk of developing the actual disease. Some of the factors that will minimise the risk of succumbing to illness are:

• Getting adequate exercise throughout life (in-cage exercise can be increased by providing wheels and spinners from infancy, removing ramps and easy access to higher levels, and providing many and varied climbing opportunities). Wheel running has been shown to increase longevity by just under 10% in caged rats, but does not extend maximum lifespan.1
• Living in a smoke (and other toxin) free environment.
• Getting adequate rest (having a natural rest pattern that is respectful of a rat's diurnal pattern, and allows for long, undisturbed daytime sleep periods).
• Eating a suitable diet with plenty of antioxidants. Antioxidants can significantly increase average lifespan, but have no effect on maximum lifespan.
• Reducing stress factors.

Environmental hazards

Accidental injury will account for a proportion of early deaths in any population. Within a carefully managed environment it is possible to reduce the influence of environmental hazards to almost zero. Freak occurrences, however, can occur even within the most well managed environment, for example, missed footing and an awkward fall within a well designed cage.

Medical/surgical resources and expertise

The availability of excellent health care for rats varies according to area within the UK. Interventions such as surgery, which might be seen as very low risk under one practitioner can become significantly more risky under another. The availability of treatment is also dependant on a vet's willingness to consider new and more complex treatments. Veterinarians who are very experienced can often do a great deal to treat illness in rats, and therefore extend lifespan.

Diet

Dietary considerations related to breeding practices will be reviewed in a later section. Dietary factors have been shown to strongly influence lifespan in rats, and there are two main principles of rat nutrition that will serve to extend the duration of life in an individual. These are:

1. Slow growth through puberty and into maturity.
2. Reduced volume of feed overall to less than 80% of what would be taken if fed ad lib.

In general, an early adult death age is associated with a high food intake prior to adulthood particularly when coupled with a high efficiency of food utilization during the post-puberty period, a rapid growth rate and early attainment of mature weight. Deviations from this pattern serve to increase the duration of life of the individual.

The trend within the fancy is probably to over feed our rats. This is particularly true for three reasons:

1. To produce lovely big kittens, that go on to mature early into lovely big adults. These rats can be bred from quite early and will succeed from a young age when shown (if good examples).
2. The tendency to feed the same diet throughout life without considering the changing needs of the individual rats.
3. The trend to overfeed in terms of quantity whatever diet is chosen.

All of these trends lead to overweight middle-aged rats who have a shortened life expectancy. Radical calorie restrictions (up to 50% less than ad lib intake) from post-weaning age have been shown to increase both maximum and average lifespan in mice and rats.2 However, they are rightly rejected by pet owners as they effectively chronically under nourish the rats, sometimes to the extent of causing lethargy, poor condition and stunted growth. However, some studied have looked as moderate restrictions of around 80% of the volume of food taken if fed ad lib, and these restrictions also extend lifespan, just by a lesser degree. It seems that the tendency of a rat fed ad lib is to overeat, and this can reduce longevity and increase disease occurrence. Hence, modest restrictions can have a positive effect.

It should be noted that commencing restrictions in middle age have been shown to actually reduce lifespan. Good feeding practices should ideally begin before birth and where this is not possible should start as soon as possible after weaning age.

When many breeders began to raise whole litters there was a need to learn to feed them well in order to meet their nutritional needs. The pendulum perhaps swung too far the other way, with over feeding becoming more of a problem than under-nutrition. Neither is desirable. Moderation, to promote slow and steady growth, with reduction in food intake as soon as the rapid growth phase begins to slow down, is ideal.

Reducing intake in a pet colony can't be 'scientific', but it is certainly possible for our rats to have a lean, active period over the evening (without available food) by feeding late in the evening before we go to bed. This coupled with moderate amounts of food that they are expected to finish before their lean hours will effectively reduce overall food/calorie intake. When feeding fresh food always reduce the quantity of dry food available to compensate.

The problem with suggesting a move towards less food is that there will always be people who take advice to the extreme, and some rats may become undernourished. Please be careful about any restriction on diet. Observing a rat's tail for signs of thinness/square edges (as well as overall fitness and condition) can help to determine the nutritional status of an individual rat.

Antioxidants in the diet (e.g. from fresh fruit and vegetables) have been shown to significantly extend average lifespan, but not maximum lifespan. Blueberries for example have been linked to a reduction in brain aging in rats.

Low protein diets (around 9%) given to pregnant and lactating does cause a reduction in average lifespan of around 11% in the offspring. Protein levels to maintain reproduction should be nearer 20-25%. Protein for maintenance (crude not pure) should ideally be between 12 and 15%. Rats are not able to easily excrete the breakdown products of high protein diets and kidney damage occurs frequently when high protein maintenance diets are given. This will shorten average lifespan. Egg protein is considered the best protein for rats.

Stress factors

Stress is a complex area of study, with daily stress, psychological stress (such as loss/change of cage mates, change of home, being bullied) and heightened anxiety reducing lifespan and increasing disease processes. At the same time, some research shows that some exposure to physical stresses such as cold and noise can actually increase both average and maximum lifespan.

Interestingly, males housed in larger groups have shorter average lifespan than those housed with females or individually. This is thought to be due to stress related factors. Clearly, for the majority of males the benefits of same gender group living outweigh any loss of lifespan. This is a point that can be translated to other areas of intervention (such as the extreme calorie restriction diets). The life of a pet rat must to a large degree be measured in quality rather than quantity, and only measures that maintain both quality and quantity of life should be considered.

Exercise

Physical activity is fundamental to fitness and long term health. Many caged animals are under exercised, partly due to poor cage design with the routine use of shelving with ramps. In cage exercise should be encouraged from infancy with varied climbing opportunities and cage set ups that do not allow easy access from one level to another. Obviously some adaptations will be needed in cases of disability and old age. Many rats will wheel run if given access to a wheel from infancy. This activity is to be encouraged as studies show voluntary wheel running alone can raise average life span by almost 10%. Out of cage activity is best designed to tie in with a rat's normal diurnal wakeful periods as far as is possible. Not only will the rats be more active and alert but their rest periods won't be artificially disrupted. It should be noted that calorie restriction to 70% of ad lib intake at the same time as voluntary wheel running can actually decrease average lifespan, though still extends overall life span.3

Breeding practices

Breeders can influence longevity in a number of ways:

• By keeping excellent records of longevity for all babies bred and continue to breed from lines that perform well for average and maximum lifespan.
• By using records to select away from the predisposition to life shortening illness and obesity.
• By selecting neophilic rather than neophobic rats for breeding. Neophilic rats (those that embrace new environments with exploratory behaviour) have been shown to live longer (by up to 6 months) and develop mammary tumours later than their neophobic siblings. This is thought to be hormonally influenced.4
• By feeding high quality, adequate nutrition in the early weeks of life, but not over feeding youngsters. Interestingly, significant dietary restriction during the rapid growth phase does not extend, but rather shortens lifespan. Nevertheless, fat babies become fat adults who tend to live shorter lives.
• By encouraging new owners to slow down nutritional input to mirror the slowing of growth.
• By raising babies in a suitable cage environment, with climbing opportunities and enrichment that encourages voluntary exercise (like wheels and spinners). If these are used in infancy they are more likely to be used throughout a rat's life.

Husbandry

Husbandry includes some of the aspects of care already discussed above such as providing adequate daily exercise (both in and out of the cage), suitable nutrition that changes throughout the rat's life and a minimally stressful environment particularly in terms of psychological stress. Maintaining a safe environment is also important in preventing accidental injury, and the best available health care should be sought when injury or illness does occur.

A number of aspects of day to day care have yet to be discussed, the most obvious of which is probably cage substrate. Much has been written about the effects of using a variety of cage litters and beddings but the research available of UK litters is relatively poor. Whatever litter is chosen ammonia will begin to build up within a day or two of cage cleaning, and high levels of ammonia are known to cause damage to respiratory and eye damage, so cages should be cleaned out regularly around every 7-10 days, depending on colony v cage size. Some litters such as aspen shavings have been shown to produce increased sneezing and lung damage over paper litter. 5

One laboratory supplier says the following about wood based litters (despite being manufacturers of softwood litter themselves): "Regardless of the amount of heat processing, some residual resins and aromatic hydrocarbons remain in all types of softwood bedding materials, including pine and cedar. These have been shown to cause elevated liver enzymes, and some respiratory irritations and ailments in several species of small mammals. For this reason, we do not recommend the use of pine shavings as a contact bedding material for rodents, rabbits, guinea pigs or other small mammals." 6

No comparative studies involving card based beddings seem to be available.

Environmental factors such as ongoing exposure to loud noise and cold have been shown not to affect longevity. A cool background temperature might however, help to reduce weight gain in predisposed rats.

There is some evidence to suggest that enrichment has a profound effect on lifespan. In laboratory rats even assumed enrichments for pet rats such as cage mates and daily human handling significantly added to lifespan. It was found that the brain of a stimulated, enriched rat aged at a slower rate than the brain of an under stimulated rat.

References

1 Dietary habits and the prediction of life span of rats: a prospective test MH Ross, ED Lustbader and G Bras. American Journal of Clinical Nutrition, Vol 41, 1332-1344
2 Longevity, body weight, and neoplasia in ad libitum-fed and diet-restricted C57BL6 mice fed NIH-31 open formula diet. Blackwell BN, Bucci TJ, Hart RW, Turturro A. Toxicologic Pathology. 1995 Sep-Oct;23(5):570-82
3 Exercise and Food Restriction in Rats. John O. Holloszy. Journal of Nutrition Vol. 122 No. 3_Suppl March 1992, pp. 774-777
4 Infant temperament predicts life span in female rats that develop spontaneous tumors. Sonia A. Cavigelli, Jason R. Yee and Martha K. McClintock Hormones and Behavior. Volume 50, Issue 3, September 2006, Pages 454-462
5 Long-term effects of cage-cleaning frequency and bedding type on laboratory rat health, welfare, and handleability: a cross-laboratory study. Charlotte C Burn, Alan Peters, Michael J Day and Georgia J Mason. Available online: http://seis.bristol.ac.uk/~frccb/papers/longterm_2006.pdf
6 Harlan Teklad product sheet for laboratory grade pine shavings.