Thursday, October 19, 2006

fable of 420,000 cats

The myth: (whether the word "theoretical" is used or not)
Two unaltered cats and all their descendants can number 420,000 in just seven years.
Well, a few of us HAVE persistently tried to track down sources for, question, compile, and share the few facts and statistics available in the field of so-called animal protection, a dysfunctional system, or non-system.

Several years ago while researching the biological efficacy of spay/neuter, I compiled what information existed on the topic including feline reproduction. Since then I’ve seen endless quotes of 420,000 cats in publications and on websites of humane societies and animal groups, large and small, as well as in news articles.

These helpful references were what I found in 2002:

Karen Johnson in her 1995 National Pet Alliance report:
Realistically, over 12 years, one unspayed female, with all her unspayed female offspring, reasonably can be expected to be responsible for over 3200 kittens if there is no human intervention.

On the Cats and Wildlife webpages prepared by Ron Jurek of the California Department of Fish and Game:
Q. What's wrong with this statement about domestic cats?
"In seven years, one female cat and her offspring can produce 420,000 cats."
A. That result is not biologically possible. This is only a mathematical exercise. Cats certainly have a well-deserved reputation for multiplying rapidly when their health, safety, and forage conditions are good, but nowhere can cat populations grow to the magnitude and at the rate this statement portrays. This theoretical population growth rate would be possible only if there were no limits to population growth. This is termed the "biotic potential" of a population. What keeps this potential from being realized--in everything from bacteria to whales--are the many factors that cause mortality and restrict breeding, such as disease, starvation, predation, and behavioral and physical abnormalities. These create an "environmental resistance" to uncontrolled population growth.

Nathan Winograd (now of No-Kill Solutions) , pointing out the importance of using factual information in COMPASSION IS THE WAY: The Care & Feeding of Feral Cats, October 2002:
Millions of Cats?
The humane movement speaks for those who can’t. That is why it is important that we chose our words carefully. Presenting the public with incorrect or exaggerated information undermines our credibility and worse, allows for own words to be used against us.
According to a popular animal welfare brochure, two unaltered cats can produce 420,000 more in just seven years. But if this were true, our streets would literally be teeming with cats. In actuality, environmental carrying capacity, mortality and other factors limit this type of exponential birthrate.
However, this claim has been seized upon by those advocating against feral cat colonies and their caregivers. Instead of exaggerating to make a point, our efforts to make spay/ neuter an affordable and convenient alternative will be more beneficial to cats in the long run.

In 2002 I contacted Merritt Clifton of Animal People News and he assured me that the 420,000 cats calculation was unrealistic!

I saved this October 2002 letter from the Stovers to Animal People News regarding a similar oft-quoted non-fact:
15 dogs and 45 cats born for every person each year in the U.S

In spring 2006 I was happy to find "Dispel the Myths: 420,000 cats?" and "How Many Kittens in Seven Years?" in the February issue of Feral Cat Times, newsletter of the wonderful Feral Cat Spay/Neuter Project in Seattle. They enlisted mathematicians to help clear up the confusion.
The consensus of these experts based on these assumptions is this: one female cat and her offspring could produce between 100 and 400 cats by the end of seven years. This is astronomically lower than the old estimate of 420,000 and much more aligned with what we see in colonies that we’ve known.

Furthermore, take into consideration that this projection is based on an unrealistically high survival rate, 100% survival of all adult cats for all seven years. Thus, we’re still overestimating the potential off-spring of one unaltered female cat in seven years but closer to reality than 420,000. We can put that urban legend to rest!

Thank you, UW Math Department!


In recent months, a new statistic appeared in two news articles .....
If one female produces two litters per year, and if just two kittens per litter survive to reproduce, and none of these cats are ever spayed or neutered, the total population in five years, or 10 generations, multiplies to 59,049.
..... quoted by Alley Cat Allies and attributed to the University of Auburn / School of Veterinary Medicine in Alabama. Brenda Griffin, DVM and the model program Operation Cat Nap are there!
(See "Too many feral felines pester Putnam Lake", June 2006 and ("'Sea of cats' a problem in Bangor neighborhood", October 2006. Here it is in an article I posted today: "Rescuers establish their first permanent sanctuary for cats".)

Last week Carl Bialik (The Numbers Guy) of the Wall Street Journal wrote his column on "How to Herd a Cat Stat" and drew some interesting comments on the companion forum.

I’ve tracked cat / feral cat and animal issues for awhile now. The daily news and cat rescuers as well as humane society or animal control agency reports and statistics reveal cat problems in every area. There are "too many cats" starving, injured, abused or ‘humanely’ and inhumanely killed (even one is too many!). But compassionate, progressive individuals and groups everywhere have been taking non-lethal action. They are saving cat lives using Trap-Neuter-RETURN-Manage (TNRM) as well as affordable spay/neuter for 'owned' cats, adoption and fostering.

October 21 additions:
more on 420,000 cats fable.

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related post:
Too many cats, two few rescuers
24 Oct 2006