Lifespan and longevity

There are two aspects to consider when talking about lifespan:

1. The maximum lifespan of a strain (the longest that any rat of that strain will live).
2. The average lifespan of a strain (the age by which 50% of the rats will have died and 50% will still be alive).

If you would like to read about this in more detail, some of the research that has looked at which factors affect the lifespan a rat is explored here

It is also necessary to consider the effects of inbreeding and outcrossing in this regard.

Only when a line is inbred to the point where the rats produced are consistent does lifespan become predicatable. Even in this situation lifespan is represented by a typical bell curve - put simply, there is always a spread of 'normal' length of life in any line. Sadly early deaths will occasionally occur even when rats are from a generally long lived line.

Loosely inbred lines are more difficult to predict, but trends can certainly be observed.

Outcrossed lines and outcross litters are much more unpredictable. While the outcross itself can bring new vigour into the line, lifespan will not necessarily be increased (though it can be, especially in the offspring of the outcrossed pair). Only once the rats have been inbred again will it be possible to see the extent of any problems that have been 'brought in'. And it will take several generations before lifespan trends can be realistically assessed again.

Lifespan is affected not only by the rate at which individuals age, but by the health problems that the rats are prone to. Poor immune system health is a major problem in rats, and can be the cause of many early deaths in rats that have not aged normally. Mammary tumours (if not removed) and pituitary tumours can also cause early death in otherwise fit and healthy rats. Some conditions are specifically age related and if they occur early with cause premature aging. These include hind leg degeneration, kidney disease, some neurological problems and heart disease.

It seems much easier to extend the average lifespan of a strain of rats than extend the maximum lifespan. Increasing average lifespan is therefore a good place to begin when breeding for longevity. In effect this means that if the maximum lifespan of a line is (for example) three years, and the average lifespan is 24 months, then begin by trying to extend the average to 28 months, then 30 months. Once this is achieved you could work at aiming for the average lifespan to be as close as possible to the maximum lifespan. It should be noted that lifespan is not solely down to breeding, but is greatly influenced by husbandry and health care as well.