When George Lucas was filming scenes of Luke Skywalker's home planet of Tatooine for the original Star Wars, his cast included an Asian elephant named Mardji. Although her species is adapted to hot, tropical environments in Laos, Thailand, and Sri Lanka, Mardji wasn't ready for the heat (and, more significantly, the dryness) of Death Valley. Dressed in heavy shag to play a furry beast of burden called a Bantha, she was clearly uncomfortable. So uncomfortable that she kept trying to remove her costume.





This anecdote about filming a sci fi movie in the pre-CGI era becomes a lot more important if you're trying to take Star Wars semi-literally, as an accounting of alien worlds and the animals and sentient beings that live there. From this perspective, there are at least 15 animal species native to desert-covered Tatooine plus another five whose origins are either otherworldy or unclear. (The two most-famous beasties — the Rancor and the Saarlac — aren't actually natives.) Most of these animals are megafauna, big enough that a human could ride them. And you can probably guess what I'm going to say: This is scientifically unrealistic. But not necessarily because of the heat. Get too hung up on whether big animals can survive under hot and dry conditions, and you'll miss the major reason scientists raise an eyebrow at Tatooine's fauna.

Tatooine

Evolution is a powerful thing. Even on Earth, there are plenty of animals that have adapted to survive and thrive in desert environments. Dale DeNardo at Arizona State University studies them, focusing particularly on the Gila monster, which is a great example of how biology can change to meet the needs of a very dry climate. Ability to withstand high temperatures is important, DeNardo says, but the ability to deal with a lack of water is actually more important. "In Arizona, for instance, illegal immigrants crossing the desert don't die from heat. They die from dehydration," he said.

Water is a necessary part of metabolic function. Cells contain water and need a certain quantity to function properly. Circulatory systems, too, rely on water. If the body loses too much, blood flow gets sluggish and veins constricted. Meanwhile, though, bodies in the heat are constantly losing moisture — because evaporating water away from the body, whether through sweat or some other mechanism, is a great way to cool down.

Humans are forced by biology to find water or carry it with us externally, but Gila monsters have an interesting alternative tactic. "It uses its urinary bladder as a canteen," DeNardo said. When liquid enters the human bladder, it can only go one direction — out. But Gila monsters are able to leave the waste that needs excreting in their bladders while re-using the water. Fully loaded, the monster's bladder canteen can make up as much as 25 percent of their body mass — providing lots of water storage.

Camels, the real-world pack animals of the sands, have a totally different way of dealing with this problem. Instead of being round, their red blood cells are more ovoid — an elongated shape that allows them to function with less water and to move more easily through constricted blood vessels.

In fact, camels, like most desert animals, have a whole host of strategies for dealing with their native climate. Besides the strangely-shaped cells, camels are also big, furry animals. That sounds like a drawback, but it's not. The larger something is, the more thermal inertia it has — the longer it takes to change its temperature. That's why it takes longer to heat a big, thick steak to a certain temperature, compared to heating a fillet of cod. As for the furry coat: While we usually associate fur with keeping things warm, what it really does is insulate. And insulation works to keep things cool, as well. The result is an animal that takes a long time to heat up in the sun — long enough that the camel can be out all day, with its body temperature slowly rising, and still be okay in the end. It doesn't hurt that camels are also capable of withstanding far higher increases in body temperature than humans can without getting sick — they can experience 7 degrees Celsius increase, compared to just 2 or 3 degrees for us. "On hot days, camels like to cuddle," DeNardo says.

On Earth, deserts are not places where lots of different species congregate. Nor are they really home to wild animals larger than, say, a small deer. (Camels, it should be noted, cheat by being domesticated. Humans supply them with food and water.) Even reptiles, which do better in hot climates because their metabolic processes don't produce a lot of excess internal heat, are limited by desert living. In desert regions of Arizona, the Western Diamondback rattlesnake only grows to be a little over 3 feet. In more comfortable parts of Texas, it can reach 7 feet.

But, if we look at Earth, we also see that life finds a way. It's reasonable to suspect that, on a desert planet unlike anything we've ever seen in the real world, there might also be some evolutionary adaptations that we've never seen. For instance, some of Tatooine's water comes from mists that appear at night, the provenance of which is not yet understood by the fictional scientific community. Maybe some of the planet's animals have developed a way to collect and use this moisture. On Earth, we see Namibian desert beetles that harvest water from the fog that rolls in off the Atlantic Ocean, allowing droplets of water to condense on their forewings and flow into their mouths. Why couldn't the massive, rideable Tatooinian lizards known as "dewbacks" do something similar?

It's not the heat or the dryness that really makes scientists like DeNardo lose their grasp on that all-too-necessary sci-fi suspension of disbelief. Instead, it's the food supply.

"The problem with movies is there's always this giant, T-Rex type thing. But there's no food there," DeNardo said.

Sure, the T-Rex-type-thing might eat smaller creatures. But, the way food webs work, you eventually get down to the nitty-gritty of needing plants for the herbivores to eat — and Tatooine doesn't seem to have any. In fact, according to Wikipedia (which I'm going to trust on all matters Star Wars universe), what sparse vegetation the planet has is invasive, planted there by sentient beings exiled from other, more-habitable, places.

From the banthas, to the dewbacks, to the reptilian camel stand-ins called eopies, to the not-particularly-inventive "jackrabs" (they're bunnies) and "profoggs" (prairie dogs) — it's not at all clear how Tatooine's herbivore population eats. Omnivorous womp rats — famous for being shot for fun by a teenaged Luke Skywalker — have adapted to scavenge the garbage of Tatooine's sentient population. But they probably didn't evolve with that source of food available. The planet's native sentient populations — Jawas and the Sand People — both seem to make careful re-use of waste.

But if the herbivores and the omnivores don't eat, then how do animals like the krayt dragons and the carnivorous flightless birds called "woodoo" survive? That's the conundrum — the wobbly leg that kind of throws Tatooine's whole ecosystem into question. I can buy that native animals evolved ways around the heat and the lack of moisture. I can't buy that herbivores evolved on a planet with no herb.

Which brings us to the sketto, the animal that, to my mind, is Tatooine's most realistic native species. Unlike most of the others, it's pretty obvious that the sketto has adapted to its environment and those adaptations are right in line with the kind of things we see in Earth-bound desert animals.

The sketto is a reptile. That makes sense. In Earth deserts, reptiles are the most diverse species, thanks in part to low metabolic rates that reduce their risk of overheating and allow them to get by without frequent meals. The sketto can fly, something that DeNardo pointed out is a key advantage for Earth birds, which are able to quickly traverse the terrain to find water sources. The sketto comes out at night, a key behavioral adaptation for many of Earth's desert animals, which spend the hottest part of the day in relatively cool burrows and caves. Finally, the sketto eats blood. It's not a perfect solution to the lack-of-vegetation problem, but it does allow the sketto to make use of sentient beings, both native and transplant, which, using their sentience, would be able to get food from off-planet, if necessary.

The takeaway: Ecology and evolution matter. Even when you're talking about science fiction. They can break a story — pulling readers out of the world you're trying to create and making it seem fake, rather than fictional. Or, they can make a story work a little better, by giving you the tools to invent animals and places that seem more real. On Tatooine, there's a little of both.

Read the Rest of the Tatooine Science Carnival



This post is part of a series, spread over several blogs, all about the animals, climate, and oceans of Tatooine. Be sure to check out the rest of the stories!

• Why a Bunch of Science Writers Are Writing About a Fictional Planet — Matt Shipman at Communication Breakdown.



• The Tatooine Intergovernmental Panel on Climate Change — David Ng at Science Creative Quarterly.



• Functional Anatomy of Tatooine Megafauna (Hyperspace Transmission Received) — John Hutchinson at What's In John's Freezer?



• Science of Tatooine: Water — Adrienne Roehrich at GeekGirlCon



• Cascading planetary-wide ecosystem effects of the extirpation of apex predatory Krayt dragons on Tatooine — David Shiffman at Southern Fried Science