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Today, the local newspaper breathlessly reported on an unusual development in Tehran:

TEHRAN has deployed snipers to fight an invasion of hyper-evolved "mutant rats", which officials say grew freakishly big after being exposed to radiation and chemicals. The cat-sized rats now outnumber humans in the Iranian capital.

City official and university professor Ismail Kahram said the physically changes in the rats would normally have taken "millions of years of evolution", according to io9.com.

A few elements of this story don't ring true:

  • that rats have grown to the size of cats (no actual measurements are given)
  • that the growth is due to "radiation and chemicals", which is right out of a comic book.
  • that "hyper-evolved" has any meaning.
  • that using sniper rifles to kill rats is cost-effective.

I'll settle for an answer to the first item. Is Tehran suffering a plague of R.O.U.S.?

  • 1
    Chasing through the sources: The news.com.au source cites io9.com who cite the International Business Times which cites QudsOnline. Unfortunately I don't read Arabic well enough to find the article on that site. – Ladadadada Mar 5 '13 at 10:41
  • NPR also has a story about how snipers are being used to control the rodent population. That story is cited by the Huffington Post which also reports that there are claims that "[the rats] seem to have had a genetic mutation, probably as a result of radiations and the chemical used on them." – rjzii Mar 5 '13 at 12:47
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    It could be that western media is faithfully reproducing incorrect information that originated in Iran. – Andrew Grimm Mar 5 '13 at 21:51
  • @Ladadadada It is not Arabic but Persian. Persians write and talk in Persian not Arabic. I think an enthusiast of Iran news in skeptical site at least should know more about the source of his news. :) – Persian Cat May 8 '13 at 1:22
  • I have never seen such cat-size rats! There are very big mice in Tehran but not cat-sized. &^) – Persian Cat May 8 '13 at 1:25
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Rats are probably a long term problem in Tehran, but reports of their size are probably exaggerated.

A news source closer to Iran is Dubai based The National which reports

Ismail Kahram, an environmental adviser to the city council ... said cats are now smaller than some of Tehran's rats, which can weigh up to 5kg.

In some versions of this story and in some unrelated reports, Ismail Kahram is described as a professor at Tehran Azad University.

Wikipedia isn't a great source but claims of Rattus Norvegicus (brown rat)

Adult body weight averages 550 g (19 oz) in males and about 350 g (12 oz) in females, but a very large individual can reach 900 g (32 oz). Rats weighing over 1 kg (2.2 lb) are exceptional, and stories of rats as big as cats are exaggerations

and of Rattus Rattus (black rat)

A typical adult black rat is 12.75–18.25 in (32.4–46.4 cm) long, including a 6.5–10 in (17–25 cm) tail, and weighs 4–12 oz (110–340 g)

my emphasis

University of Michigan Museum of Zoology says of Rattus Norvegicus (brown rats)

On average, these rats reach nearly 400 mm nose-to-tail, and weigh 140 to 500 g. Males are usually larger than females.

Brown rats (R. norvegicus) raised in captivity can reach 800g but it seems unlikely that large numbers of wild rats do so.


Update:

In Investigation of Contamination of Wild Rats (Rattus rattus) from Tehran City to Antibiotic Resistant Enterobacteriaceae in 2009 says

Background and purpose: Wild rats are considered as an important spreading factor of contamination in urban areas. During recent years, the wild rats were extensively distributed in Tehran

and

Materials and methods: Forty wild rats were trapped in the North, South, East, West and Central areas of Tehran and transferred to the laboratory alive.

So the species prevalent in Tehran is Rattus Rattus - the black rat, which is normally significantly smaller than the brown rat.

A Google translation of the Persian language report doesn't reveal any suggestion that the rats were over ten times bigger than "normal" specimens in 2009.


Update:

The origin of the story is unclear. It seems a bit circular.

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    Gambian pouched rat typically weighs between 1 and 1.4 kg which is about double your 800g figure. – Sardathrion - Reinstate Monica Mar 5 '13 at 12:47
  • @Sardathrion: which is likely why the link went to "brown rat". Two different species, different measurements. – 0xC0000022L Mar 5 '13 at 13:16
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    @Sardathrion: Apparently, the Gambian pouched rat isn't a member of the rat genus (Rattus). It is mainly found in Africa although some escaped pets are now breeding invasively in Florida. It doesn't appear to be present in the wild in Asia or Middle East to any noticeable extent and is an unlikely candidate for the animal in these stories. – RedGrittyBrick Mar 5 '13 at 15:11
  • @RedGrittyBrick - this is plausible, but isn't it possible that more than one different rat species is present in the locale? – user5341 Mar 5 '13 at 16:40
  • @DVK: I agree it is possible that Tehran may be home to several species of mammal that the public might imprecisely describe as rats (assuming that the Persian language has a noun that is used for the same range of animals that look vaguely rodentlike). Other than this single much-repeated story about 5 Kg rats, I have not yet found any other reports that support the claim. – RedGrittyBrick Mar 5 '13 at 19:53

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