Cold weather in Miami brought a warning of iguanas falling from trees, the National Weather Service tweeted last week. Low winter temperatures can put certain lizards into a coma-like state. Reptiles have adapted to land in wondrous ways.
As the National Weather Service’s Miami account mentioned, the local meteorologist doesn’t often call for cloudy with a chance of iguanas. “This isn’t something we usually forecast, but don’t be surprised if you see iguanas falling from the trees tonight as lows drop into the 30s and 40s,” the tweet said. An attached picture explained that due to iguanas being cold-blooded, they “slow down or become immobile” when temperatures drop below 50 degrees. They may remain still and presumed dead, but they are very much alive. This bizarre account explains just one of the many ways reptiles have adapted to modern times.
Baby, It’s Cold Outside
Aside from going into coma-like states, the cold weather affects reptiles in another life-altering manner in a phenomenon called temperature-dependent sex determination (TSD).
“In short, the sex of many reptiles is determined by the ambient temperature in the nest,” said Dr. Donald E. Moore III, Director of the Oregon Zoo and Senior Science Adviser at the Smithsonian’s National Zoo and Conservation Biology Institute. “In many turtle species, eggs from warmer nests result in all-female hatchlings, while eggs from cooler nests result in all-male hatchlings. Some crocodilian species show TSD that is just the opposite: low nest temperatures less than 30 degrees Celsius produce only females while high nest temperatures above 34 degrees Celsius produce only males.”
According to Dr. Moore, other species experience genotypic sex determination (GSD). While TSD occurs during the egg’s incubation, GSD happens at fertilization, like mammals and birds.
“What’s really interesting is TSD and GSD are not mutually exclusive,” he said. “Zoologists have shown temperature reversal of genetically-determined sex. These studies suggest that some reptiles may show transitional evolutionary states between GSD and complete TSD.”
Dressing for the Weather
Dr. Moore said that reptiles have adapted to life on land by “enclosing the watery environment that gives life with a water-retaining egg and a watertight skin.” Contrasting reptilian skin with that of sea-dwellers is a helpful way to understand how reptiles do so well living in our environment.
“Reptiles have dry skin that feels like your dry sneaker, unlike the moist skin of amphibians that makes them vulnerable to dehydration on dry land,” he said. “Unlike the bony, dermally-derived scales we find in fishes, reptile skins are made of keratin from the epidermis. The epidermal hard form of keratin in reptile skin that results in their characteristic scales not only makes the skin watertight, [but it also] provides protection against wear and tear in the terrestrial environment.”
Among the 10,000 species of reptiles, many different types of scales abound. Dr. Moore said that turtles have plate-like scales called scutes, which “develop new layers of keratin as they wear down.” Crocodiles and alligators feature scales that grow gradually over their lives, repairing damage and so on. Lizards and snakes, of course, shed. “New keratinized epidermis grows beneath the old outer scale layer, then the old is shed,” he said.
“The skin and eyes of [reptiles] have chromatophores, color-bearing cells that give them their amazing colors. These skins are prized by humans for alligator and snakeskin leathers, which are then made into handbags and shoes, sometimes causing conservation threats for the desired species.”
Inside and out, reptiles are walking reminders of evolution at work. Hopefully, Miami residents will forgive them for the occasional scare.
Dr. Donald E. Moore III contributed to this article. Dr. Moore is Director of the Oregon Zoo and Senior Science Adviser at the Smithsonian’s National Zoo and Conservation Biology Institute. He earned a bachelors degree in Wildlife Management and Zoology and a doctoral degree in Conservation Biology from the State University of New York College of Environmental Science and Forestry.