Levensverwachting van de fret door Bob Church

[Karen]

Met toestemming van Bob Church een artikel over levensverwachting van fretten en (mogelijke) veranderingen.


By Bob Church

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I’ve been asked by several people to comment on ferret lifespan. This is the fulfillment of several promises.
This note is divided into three sections: the background to determining lifespan estimations, the lifespan curve for pet ferrets, and a discussion of the results. If you don’t care to understand lifespan estimations, simply skip down to the areas that interest you.
SECTION 1: BACKGROUND
The accurate estimation of ferret lifespan is difficult, prone to various interpretations, and with numerous problems that need to be understood and controlled in order to reduce error. Many of the problems can be traced to erroneous information from owners, but some is due to a basic lack of accessible data. All-in-all, a number of reported lifespans of ferrets have some degree of inaccuracy.
Problems: Memory
A person’s memory is a fickle thing. It is not uncommon for false memories to be incorporated into a person’s memory, making the event seem true to them. Even people with excellent memories might have a difficult time remembering when a particular ferret was brought into the home, much less born. Many breeders will supply some type of record that documents pedigree as well as date of birth, but the vast majority of ferrets lack such paperwork and few owners think of recording basic information.
Basically, memory is unreliable when it comes to aging; at best it has an error rate of plus/minus one year. That is a range of three years — one third or more of a typical ferret’s lifespan.
Problems: Perception of Reality vs Factual Reality
People believe what they perceive, that is to say, whatever a person experiences and perceives becomes their personal reality. It is safe to say the perception of reality is relative to the person. As much as people wish, perception is not factual reality, but rather it is a personal interpretation of reality. Factual reality, in contrast, is more absolute, truthful, and is not subject to personal perceptions.
It is not uncommon for ferret owners erroneously report age or cause of death based on their perception of what happened, rather than what actually happened.
Problems: Cognitive Bias
Cognitive bias is a tremendous problem. There are a number of types of cognitive bias, including the curse of knowledge, belief bias, false consensus effect, leveling and sharpening effect, false memory, and many more. Cognitive biases help create faulty thought that results in irrational thoughts or beliefs.
In short, many people will say what they believe, regardless if the statement is true or not. If they believe their ferret died at 5 years of age of insulinoma, even if it died at 3 or 4 or 6 or 7 of cardiomyopathy, they will report what they believe.
Problems: Breeder Prejudice
Perhaps the greatest obstacle to the study of ferret lifespans is simple “breeder prejudice.” There is a general acceptance of the idea that hobby and small scale breeders produce ferrets with healthier genetics and that live longer than those bred by commercial farms. Perhaps this is true for number of reasons, but there is absolutely no scientific evidence that any one breeder produces ferrets that have better genetics and that live longer than any other breeder. Granted, you might be able to show some groups of ferrets live longer than others, but the question is, “Why?” In most cases, environmental factors, such as husbandry, overcrowding, diet, long-term stress, dental disease, early neutering, lack of adequate veterinary care — and more — muddies the observational waters and skews any conclusions.
For example, insulinoma and adrenal disease are perhaps two of the most common diseases in ferrets. Both are predominately diseases caused by environmental factors, such as diet, husbandry, light exposure, and early neutering. For the sake of argument, assume there is evidence suggesting large commercially farmed ferrets have shorter lifespans than those produced by hobby and small commercial breeders. Are the shorter lifespans due to breeding or to environmental influences? Is there demonstrable evidence to prove either side of the question? There is, but it currently supports the environmental side.
At this point in time, it is extremely difficult to impossible to determine if any breeder produces ferrets with better genetics and that live longer than any other breeder — including those from large ferret farms.
Problems: Owners

Sadly, owners are generally prejudiced, embarrassed, filled with guilt, confused, uninformed, or simply uncooperative when it comes to questions about the death of their ferrets. Some may say a ferret lived shorter than actual because of an underlying prejudice against commercial ferret farms. Others may say a ferret lived longer or shorter than actual because of poor age estimations when adopting. Some have even exaggerated lifespans from their own litters or from other hobby breeders. There are perhaps hundreds of other reasons.
This is generally not done maliciously, but it nonetheless occurs.
Problems: Records
There is an overall lack of record keeping by ferret owners. Reporting births and deaths are a legal requirement for humans, but not for animals. A breeder could have a very high mortality rate with newborns, yet never report it — perhaps might even hide it from outsiders. Ferret owners might lament the loss of some ferrets, but not report the loss of others in their care. Embarrassment, feelings of privacy, guilt, or fear of public condemnation might make many not want to report the loss of a ferret at all. All these factors — and more — could significantly impact the accurate reporting of ferret lifespans.
When humans are born and die, records are created that can be mined for data that are extremely helpful in longevity studies. There are no such centralized records when it comes to ferrets. Nobody really knows how many ferrets are in the USA or other countries, nobody know how many are birthed, nobody knows how many die, and nobody knows the ages when they die. At this point in time, such statistics are simply conjecture.
Problems: Overlapping Risk Factors
Confusion between environmental factors, genetics and epigenetics is common. Suppose a ferret died at 7 years of age from advanced adrenal disease. So what caused the adrenal disease? Was it genetics? The environment? Epigenetics (non-DNA factors that change gene expression)? Or is it some combination of the three? What about diet? See the problem?
There are many factors that influence lifespan; a discussion of all is beyond the scope of this note. Since individuals are not isolated in protective bubbles, they are subjected to all those factors simultaneously, which combine to limit or extend lifespans. These factors include (but are not limited to) diet, genetics, environment, evolution (actually part of genetics, but separated here for the sake of discussion), epigenetics (those environmental factors that directly influence genetic expression), infectious disease, poisoning, accident, predation, exposure to tobacco smoke and parasite infestation. There are certainly more.
Misunderstanding cause of death creates a circle of confusion where establishing one causation is hindered because there is a great deal of overlap with other possible causations. This introduces error into the results, making it difficult to map risk factors or to determine cause of death.
Problems: Cause of Death
The vast majority of ferrets die at home, many without veterinary diagnosis and treatment. Few undergo scientific necropsies, so the cause of death is open to misinterpretation. Many times the cause of death is a result of multiple problems, such as concurrent heart and liver disease. Which killed the ferret, liver cancer or cardiomyopathy? Statical investigations into the causes of death in ferrets must be based on bonafide veterinary or other scientific diagnosis, not on conjecture or supposition.
For our purposes, the cause of death really has little impact in creating a mortality table for ferrets. The goal here is to determine death rates over time. Regardless, problems with determining cause of death make it hard to determine disease rates in the ferret community.
Problem: Age at Death
How to categorize age a problem with age-at-death studies. Some of the ferrets might be only a few months old, while others might be close to 1 year old. A good way to get around this problem is to segregate individuals into age groups or classes. An age class is usually defined as yearly classes, such as 0-1, 1-2, 2-3, 4-5, 5-6, etc. Or, you can group a species by groups, such as newborns, infants, juveniles, adults, and seniors. The more precise the cohort class or group, the more accurate the data.
Reports of ferret age-at-death are prone to error due to two common problems: poorly kept records and error-prone age estimates. If the records are good enough, it is possible to study lifespans for each cohort year to determine if ferret lifespans are lengthening or shortening.
Problem: Disease and Environmental Impact
Aging an unknown ferret depends on a few assumptions that are prone to error. Common assumptions include the supposition that ferrets from every age group or cohort are similar in their age determination traits, such as muscle tone or tooth condition. Two main problems with this approach are the impacts of disease and various environmental factors, such as diet or fabric chewing. An even greater problem is the ability of people to perform accurate and precise age determinations repeatably.
A sick adult ferret might be grouped in the senior class, or an old infant might be grouped as a juvenile depending on impact of disease or various environmental factors. For example, a common method to age ferrets is to look at teeth and judge wear, color and transparency. Each of these parameters can be influenced by diet and behavior, introducing minimally up to a one year rate of error. This means using this method to come up with an age of 3 really means the ferret is 2 to 4 years old. That is a third of a long lifespan and almost half of a typical one. While that doesn’t have much meaning in a shelter, it is a tremendous amount of error when scientifically determining ferret lifespans.
Similar problems can be seen in ferrets suffering from long term inflammation (who look older), ferrets that have vigorous enrichment and physical activity (who look younger) and those that have the unfortunate trait of early aging (called senescence).
Problem: Quantitative vs Qualitative Age Determination
The majority of age-at-death reports are qualitative, that is, they are estimations made by someone who might (or might not) be experienced in age estimation. The problem is, regardless of the reputation of the person making the age determination, there is no way to check if the results are correct. If a shelter operator estimates 100 ferrets, what is the rate of error? It is impossible to say because there are no controls and the rate of error is unknown.
For example, if a person estimated the age of 100 known-age ferrets, then compared those estimations to the known ages, they would know how many times they got it right, how many times they got it wrong, and by how much. This would yield a rate of error so others would know the trustworthiness of the estimated ages. This type of quality control is never done; age estimations are usually accepted without question.
Quantitative age determinations are based on known birth dates or cohort years. A newly purchased Marshall Farms ferret is almost certainly a member of the 0-1 cohort group (that is, 1-year-old or less); these ages can be added to precise birthdates from breeders to form a single cohort group.
Quantitatively determined ages have lower error rates giving more precise lifespan estimations that qualitative ones.
Problem: Small Errors in Short-Lived Animals are Large
Ferrets are small animals that live short lives. What would be a small error in a long-lived species is magnified in short-lived ones. For instance, in a species that lives 9 years, an error of +/- 1 year constitutes 1/3rd of the species lifespan (2 - 3 - 4, or 3 years). In a long-lived species that lives 90 years, that same rate of error is 1/30th of the lifespan.
Ferrets are short-lived, so even small error rates can be quite significant in determining lifespans.
Problem: Differences Between a Population and an Individual

Invariably, when the discussion of ferret lifespans arises, someone will interject their ferret died earlier or lived longer. The general suggestion is that if one ferret is different, then the results must be flawed. This is because of a basic misunderstanding of the difference between the individual and a population of individuals.
Imagine a jar filled with different colored balls. Each ball represents an individual, but all the balls in the jar are a population. If the balls are counted by color and it is found that red is the most common, a green ball from outside the jar will not change the results within the jar. Neither will pulling out a green ball from inside the jar; it might not be red, but it is still a part of the population and is part of the results.

Lifespan studies are created from studies of populations and the results — if done correctly — represent the studied population, regardless of any differences for any one individual to how different they are.
Problems: Domestic vs Wild
Life mortality tables for domestic and wild populations might or might not correlate well and could potentially negatively influence observations. For example, the majority of a population of feral ferrets might have died in 5 years, but most domestic ferrets live longer. Does that mean one group of ferrets have an inherent ability to live longer? Or is it that life is harder for wild populations and they just live shorter lifespans? Could it be a bit of both?

Certainly the life mortality table for a wild population will have great importance when compared to that of a domestic group, especially in the determination of the beginning of old age (senescence), rate of aging (rate of senescence), longevity, and mortality baselines. Those comparisons can give great insight for understanding ferret lifespans by providing baselines for study.