When I was a kid, our house had rats. An exterminator came, caught
one, looked at it, and said that it was a particular type of rat that
always came in packs of sevens. There would be seven, he said; if
there were eight, it meant there were at least 14.

OK, I'm pretty sure I'm not imagining this, but I was young so I'm not sure. So:

Is there really a type of rat that comes in packs of seven? The answer
to this question must be definitive: either a pointer to some site
that discusses this type of rat, or a pointer to some site that says
something like "A common rumor about rat X is that it comes in packs
of seven; this is not true". Also please include your search strategy.

Thank you!

Perhaps this was a roof rat; they have about seven in a litter. They
are more common in Texas and the South than the Norway rat. Elsewhere
they tend to be outcompeted by the Norway rat.

From 

http://www.nsrl.ttu.edu/tmot1/rattratt.htm

"Distribution in Texas. Common over most of Texas, especially in towns.

Habits. Roof rats are largely commensals and live in close association
with man. They seldom become established as feral animals as do the
Norway rats; however, in Lavaca County they have been found throughout
the county, in the towns, and on the farms. They inhabited grocery and
drug stores, warehouses, feed stores, and poultry houses and were very
common in cotton gins and associated grain warehouses. On the farms
they lived in barns and corncribs. They may live near the ground, but
usually they frequent the attics, rafters, and crossbeams of the
buildings. They make typical runways along pipes, beams or wires, up
and down the studding, or along the horizontal ceiling joists, often
leaving a dark-colored layer of grease and dirt to mark their
travelways. Like the Norway rat, the roof rat is largely nocturnal and
only where populations are relatively high does one see them
frequently in the daytime. There is some indication that the larger
and more aggressive Norway rat is supplanting the roof rat in many
parts of the United States. In the southern United States, however,
the roof rat is by far the more common of the two."

"Roof rats breed throughout the year, with two peaks of production ?
in February and March and again in May and June. The period of least
activity is in July and August. The gestation period is approximately
21 days, and the number of young per litter averages almost seven."
-----
Re Norway rats:

http://www.ratbehavior.org/Stats.htm

Norway rats are said to have a litter size of "6-14 offspring." This
site doesn't indicate what the average litter size is.

-----
The following site has some information on the black rat (or roof
rat). It gives a variable litter size. It also has photos.

http://www.the-piedpiper.co.uk/th1b.htm

"This little fellow probably originated in south-east Asia from where
it has spread to large areas of the world. It arrived in Europe in the
early Middle Ages and became very widespread until it was displaced by
the brown rat. It can be identified by it's tail, which is always
longer than the combined length of the head and body. It is also
slimmer and more agile than the Norwegian or Brown rat. Its size
varies according to it's environment and food supply, and comparison
of the two pictures on this page show that colour is not a guide to
species. The Black rat is also known as the Ships rat and or Roof
rat."

"Black rats were responsible for spreading the Bubonic Plague  during
the Middle Ages as they carried the flea which transmitted this
disease. In most parts of Europe black rats have been completely
eliminated, but are still common in port areas where they find their
way ashore. They are associated with man to a greater extent than the
brown rat, but have a greater need for warmth and hence do not tend to
occur in the open in central and northern Europe, but are more common
in tropical and subtropical areas. "

"The litter size can be somewhat larger at between 5 to 10 per litter."

Search strategy: seven rats
Search strategy: rat litter (not a good criterion, gives too many "rat
litter" (ala "kitty litter" references))
Search strategy: rat litter size
Search strategy: black rat

FYI:  Average Litter size for a R. norvegicus is 10 pups. I have seen
litters as large as 18 in the lab.

Exterminators are notorious jokesters...  I have an old buddy that is
one, and he is always yanking the chain of one of his customers with
crazy stories, etc.  I don't know how old you are, or more
importantly, how old the exterminator is/was, but there was a very
popular movie in the mid-60's called "Robin and the Seven Hoods", and
guess who played the "Hoods"...  It was the members of the original
"Rat Pack", with Sinatra as one of the crime bosses.  Maybe his story
was just an attempt to be funny or timely.

I have studied a lot of biology, as a private endeavor, and have never
run accross a mammal that can predictably always have the same number
of multiple young, and the few rare examples are not in the rodent
families.  Further complicating the possibility of this being true is
the simple fact that outside the human species, the mortality rates of
infants is very high.  In a litter of rats, even in the best
conditions, it likely that only 50%-60% would survive to maturity, due
to competition amoung siblings, disease, predation, etc...  So, it is
A)unbelievable that a rat would predictably have exactly any number of
young, and B) highly unlikely to impossible to believe that all would
survive.

Chances are more likely that the exterminator was either scaring your
family into buying more services from him, or being funny and making a
off-the-cuff reference to the aforementioned movie..

Hope this helps!!

-Rob
bradfro@yahoo.com
http://www.robbradford.com

Brix: great research! Though I believe the seven would have included
the two adults - he wasn't talking about just the litter.

Pforcelli: thank you for the added info!

Tigger: you very well may be right about the exterminator pulling our
legs. But something I'd left out before: at the end, we'd caught
exactly seven rats.

I forgot to add that I first searched the urban legends site:

www.snopes.com

Actually, I don't know why I didn't think of this before, but this can
very easily and definitively be proven to be a hoax mathematically. 
The answer was right there in front of our eyes all along.  When you
mentioned in one of your responses that you were assuming that two of
the rats were sexually mature adults, it struck me that this is
actually very similar to classic poulation growth models in
mathematics.  So, here is definitive proof that this must be false....

Oh, before I get to the math, one even more obvious point.  If one of
the seven were to die for some reason, does this mean that another
would spontaneously appear or that the sexually mature adults would
have a litter of 1 or 8 to compensate and keep the ratio?  This seems
way far fetched...

OK, so the mathematical proof, and you can do this ina simple
spreadsheet.  Lets assume that we start off with a perfect seven
group, which consists of the 2 parents and 5 children.  The average
rat reaches sexual maturity in about 3 months and they tend to have
litters, quite conveniently, about 3 months apart, during sexual
maturity.  So, we can simply do a spreadsheet with four columns,
"Sexually Mature Females" (SMF), "Sexually Mature Males" (SMM), "Young
Females" (YF), and "Young Males" (YM).  So to start in month zero, we
have SMF=1, SMM=1, YF=3, YM=2.  I arbitrarilly split the 5 young 3/2,
but you can do it any way you like and the proof remains consistent. 
Now at Month = 3, all the young are sexually mature, as are the
parents still, so now we have 4 SMF's that will presumably have 5 pups
each, as this is what was necessary to make seven in the first
generation.  Again, I will arbitrarilly split the 50% YM and YF
populations, but the result looks like this.  SMF=4  SMM=3, YF=10,
YM=10.  Total Population = 27.  27 is not evenly divisible by 7, so we
have proof in the second generation that the theory is false.  I
carried this out in my spreadsheet to 20 generations, and did
different arbitrary splits of the yong populations, and as chance
would have it, only 2 of the generations in 20 were ever divisible by
7, which we would expect thru random chance anyway.  I even monkeyed
with mortality rates after one year, and still couldn't make it fit
the seven theory.  At this point, I'd be willing to wager a million
bucks that the statement was either false, a joke, or misleading.  My
exterminator always tells me that if he finds a single termite, there
are a thousand more we don't see, which I'm sure is true, but by 1000,
he means there are many many others and not that there are exactly
1000.  I've heard this comment about lots of pests.  For every
cockroach you see there are a hundred you don't...  etc.  It is
interesting that you caught exactly 7, but I am absolutely 100%
certain that it was random chance.

Hope this helps...  Rob

Tigger (Rob): LOVE the proof! Though wouldn't it throw things off if,
say, there was some N% chance that one of the litter was sterile? Or
perhaps that the group of seven consisted of N adults and M children
(one of the others being a sister or a brother, let's say)?

Nonetheless, I'd bet with you that what the exterminator said was
either a hoax or a joke.

Thank you again,

Porkribs

As I think about it, at the time I sort of thought that the seven rats
were some random grouping. Like not necessarily family, just a group
of seven that always travel together. Whatever that means in the rat
world.

Porkribs

OK, We can finally put these rats to bed.  I have three final thoughts
that ought to "exterminate" this theory, once and for all.  =)

One, is an observation of rat behavior.  The second, is an improved
spreadsheet with your added hypothetical sterility and original mix
rates.  And the third is a quote from my old exterminator buddy.

OK, so first?.  I have to admit that I laughed out loud at the thought
of the rats traveling in packs of seven.  Not because of the thought
of seven rats per se, but because I started playing out the
conversations in my head.  I imagined a family of rats in the attic of
my house, and one day they do a quick count and realize that they are
no longer a multiple of seven.  So they sit down with all the family
and they have a tough conversation.  ?Your father and I just realized
that we now number 18, and as you all know, we can only have multiples
of seven at any one location, so I?m going to have to ask 4 of you to
leave immediately, so that we can number 14 here at this address.  So,
Minney, Mickey, Mindy, and Mikey, you will stay here with us.  Fred,
Barney, Linda, and George, I?m afraid we?ll have to ask you to leave. 
Oh wait a minute, if only four of you leave, you won?t be a group of
seven.  Darn?  Honey, what should we do??  LOL.  OK, I?m seriously
flicted for even having a thought like this, but it does prove my
first point, that if a pack ever reached a non-7-multiple, the pack
that would break away would have to be a non-7 multiple, or it would
leave the rest with a non-7.  Basically, if 7 ever left, they would
leave behind a non-viable combination.  Also, obviously, rats aren?t
smart enough to figure this out anyway.

Next, I adjusted my spreadsheet to represent your sterility theory and
your original-7 distributions.  My spreadsheet now allows for you to
make any assumption you like about the mix, the mortality rates, and
the sterility rates and model, and the likelihood of having
male/female offspring.  After trying about 20 combinations of
differing assumptions, it always ended the same way, about 1 in seven
generations would have an even multiple of seven in population (i.e.
random chance).  If you send me an e-mail address, I?ll send you the
XL sheet and you can play with the assumptions your self.  =)

Finally, I called my old buddy to ask him, and I think I got your most
definitive answer.  He told me that he uses this very story all the
time with a variety of pests.  His comment was that he needs to sell
the whole rodent protection plan and not just get paid to lay a trap. 
So, he tells people there are more than the one they saw.  Now, here?s
the clever part, that I hadn?t thought about.  He said that with large
pests, like rodents and snakes, he actually quotes them an exact
number ratio for two reasons.  One, it makes him look like he is
really knowledgeable, and therefore people will always call him back. 
And two, he can?t ever be wrong or run out of work.  If he says there
are seven and they eventually trap seven, then he looks like a genius,
but, here?s the real genius....  If they catch less than seven, they
will fixate on bringing him back and paying more to flush ?the rest?
of them out, and if they trap more than seven, same thing?  They will
fixate until they get the rest of them.  No matter what happens, the
customer will always need him and want him.  According to him, there
are no mice or rats that group in specific numbers, but he uses a
variation of that line all the time for a variety of different kinds
of pests to up-sell work.

Well, there it is?   Humans 7, Rats 0?   =)

-Rob

Seven having a  magical air to it, puts the exterminator in a position
to be a magician, and someone you need, to magically fix your problem.
Traps work well and cost a buck

Love your writeup, tigger! Thank you.

I'm fairly certain that some crows group in sevens. I'm sure there is
leeway involved too, esp. for a starting colony, or what have you.