What Makes A Rain Garden?

Why Is It Raining Iguanas in Florida?

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If you're looking to have a myth debunked, you've come to the wrong place. "Frozen Iguanas Falling From Florida Trees" is neither the name of a schlocky B-rated horror film nor an urban legend. It's something that really happens, which, if you're a Floridian yourself, you might be somewhat familiar with. But the rest of us may just be getting used to the fact that it rains more than cats and dogs in The Sunshine State. In addition to hurricanes and alligators, there's another form of reptilian precipitation to watch out for.

But just why does this phenomenon happen? The short answer is that iguanas simply don't belong in Florida; they're not native to the state, and those living there aren't used to the extremes of Florida weather yet. But there's a longer answer, and it's a fascinating tale of invasive species, animal physiology and one of the strangest weather reports you'll ever see.

Iguanas Are Cold-Blooded, Which Induces Lethargy

When a creature is cold-blooded, its body temperature changes along with shifts in the ambient temperature that occur in the air around the animal. This lies in contrast to warm-blooded animals, which are able to maintain internal body temperatures higher than those of their surroundings due to their differing metabolic processes. Snakes, crocodiles, alligators, turtles and lizards, all of which are reptiles, are generally cold-blooded. When temperatures around them drop, so does their internal temperature. This process also happens to iguanas — even the iguanas that call Florida home.

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As the temperature in the air — and, thus, the iguanas' blood — drops, they become increasingly inactive. When external temps reach about 45 degrees Fahrenheit, iguanas exposed to these conditions enter a stunned or dormant state. They'll gradually become so sluggish and so immobilized that they may look dead — but aren't. These lethargic lizards are actually still breathing, and all their bodily functions are continuing. But those functions are taking place much more slowly because the iguanas' blood is moving around their bodies at a greatly reduced rate.

That said, if it stays in the 40s longer than eight hours, those persistent cold temperatures can become fatal to iguanas. But just how cold does it have to be to trigger lethargic responses? That depends. Ron Magill, Zoo Miami's communications director, told CNN, "The temperature threshold for when iguanas begin to go into a dormant state depends greatly on the size of the iguana… Generally speaking, the larger the iguana, the more cold it can tolerate for longer periods." That may have to do with the fact that the larger lizards have more blood in their bodies so they can retain warmth in their blood a bit longer than the smaller reptiles.

The Lizards Are Diurnal — and They Have Unconventional Sleeping Spots

There may not be many things that people and iguanas have in common, but the period of time when they're awake each day is one. Diurnal animals like iguanas are active during daylight hours and inactive at night when they sleep or rest. Because iguanas are already slow or sleeping at night when temperatures are most likely to reach their lowest points, that's when iguanas are most vulnerable to the lethargy-inducing effects of a cold snap. The nighttime temperatures and the cold ambient temperatures compound.

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There's one more thing about iguanas' diurnal nature to know about, though. It's where they tend to sleep that matters — and that leads to "iguana rain." Iguanas typically wander the ground or stay slightly secluded in brushy areas during the day. But they then sleep up in the relative safety of tree branches.

A typical slumbering iguana is perfectly capable of remaining safe and secure in a tree until morning. However, when iguanas are rendered lethargic or comatose by cold temperatures, their immobility causes them to lose their grip on the branches. Iguanas that succumb to the coldest overnight temperatures in Florida simply fall out of bed — and onto the ground to be found by startled Floridians when the sun rises.

They're Invasive and Aren't Suited for Florida's Climate

One might think that iguanas would've evolved to deal with Florida's temperatures without going through this issue — they're native to rainforests, after all. But even if that were ordinarily the case, there are a few factors working against iguanas in this regard.

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First, temperatures low enough to trigger this effect are pretty uncommon in Florida, so the lizards aren't exposed to these dips often enough to develop any kind of evolutionary response. Low lows happen occasionally — it's often January when they do occur — but Florida temperatures in the 40s are by far the exception rather than the rule.

While Florida does have a small number of native iguana species, the vast majority of these lizards in Florida — including the most common green iguana, a species that's helpfully named Iguana iguana — aren't native to Florida at all. They're actually invasive, so they haven't adapted to the state's (very) occasional chilly weather.

According to the Florida Fish and Wildlife Conservation Commission, there are over 40 non-native iguanas and relatives calling The Sunshine State home. These transplants were introduced to Florida as a result of the pet trade. In 1995 alone, over 800,000 green iguanas were imported into the United States from their native homelands — much warmer countries like Honduras, El Salvador, Panama and Colombia. Over time, so many iguanas escaped or were released by pet owners into the wild that they established a presence throughout the state.

No, That Iguana Is (Probably) Not Dead

In most cases, an iguana that you might find lying on the ground under a tree first thing in the morning isn't dead and won't die from the cold snap. Rather, it's simply immobilized or comatose due to the cold. As the temperatures increase around the iguana and it's exposed to sunshine, the iguana's blood temperature will increase, too.

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Gradually, the iguana will become more energetic and scamper away. As the Miami Zoo's communications director mentioned, though, very cold temperatures can kill small iguanas, but many simply shake off the cold (and any falls from trees) with the arrival of warmer temperatures and sunshine.

With this in mind, it probably won't be so startling next time you hear about weather forecasts — yes, the Miami National Weather Service has issued them before — for raining iguanas in Florida. In addition to having the benefit of this general introduction to the reptile-related implications of cold snaps, though, you can sometimes count on Florida weather forecasters to give you all the information you need even if some of it is definitely not information you want. (Check out this story about a Florida weather forecast that went way beyond the probability of precipitation, humidity and expected high and low temps.)

So, if you ever should hear the telltale slap of an iguana hitting the ground in the cool temperatures of a January Florida night, don't be alarmed. Iguana rain is normal. Weird, but normal.