Luke Skywalker’s Green Milk: “It’s Not a Breast. It’s an Udder.” And Other Science Facts You Didn’t Ask For

This past weekend, nerds worldwide set down their game controllers and curled up with their plush Chewbaccas to watch the DVD/digital release of the latest installment of everyone’s favorite space opera, Star Wars: The Last Jedi. Since the film was released in theaters, it has generated much discussion, from the multiple U-turns director Rian Johnson pulled with the storylines that the previous episode had set up, to whether Reylo is inevitable or a problematic perpetuation of the romanticization of male toxicity in film. These discussions left little room for the brief, dialogue-free scene in which Luke Skywalker is shown milking a giant semi-aquatic mammal then drinking straight from the tap.

Well, I am here to remedy that situation. And not unlike the scientists at the open-source science journal PLOS, who hilariously peer reviewed The Last Jedi, I’m doing it with SCIENCE.

Rey and her new friend

The scene to which I refer involves Luke milking a creature who lives on the island and procuring a vividly green-colored milk that he messily drinks as Rey looks on. Articles that have covered this scene have consistently called it “weird,” “gross,” and “bizarre,” nevermind the fact that 1.) We are a species that evolved to drink milk from a breast, and yet 2.) we think that is gross and, instead, choose to consume the milk of another species despite the fact that that it is nowhere near anything like our milk in composition, and 65% of us can’t even digest the stuff. But, I digress.

Today we’re gonna talk about straight-up nerd science. What I’m interested in, here, is not just the question of why the milk is green—We will get to that, but there is SO much more, here! In this brief, dialogue-free scene we are given several “clues” about this animal: various characteristics about its milk—not just the color, but also it’s viscosity, and opacity, and some very unique (and quite memorable) mammary anatomy. We also get some clues about it’s environment and behavior. But for the sake of brevity, I will be focusing mostly on the creature’s very memorable mammaries. If we take these clues and apply some of the known mechanisms (rules) of evolution from our own planet, we can make some guesses as to the evolution and social behavior of this creature. Let’s science the bantha fodder out of this!

The Last Jedi Visual Dictionary describes the Thala-siren as a “large, flippered marine mammal,” so we will presume that it is a mammal in that it 1. gives live birth, and 2. Produces milk for it’s young. (And also has hair and 3 middle-hear bones, technically.)

Image: Lucasfilm


Mammalian milk is a living substance whose composition varies depending on the needs of the infants of a species—and even from hour to hour based on the needs of the infant, in response to either environmental stimuli or information communicated to the mother by the infant. For example, a mother may produce milk with a higher water content on a hot day, or produce antibodies to a specific pathogen within hours of coming into contact with it. 

But one of the things that is most fascinating about milk is that its composition can–generally speaking—predict the caregiving behavior of that species (Tilden & Oftedal, 1997). The milk of a species like deer, for example, which may leave its fawn for up to 12 hours at a time to go forage, is high in protein and fat to provide sufficient energy to sustain the fawn through long periods without suckling. In total, the fawn may only eat 2 or 3 times a day.By contrast, primates (including us), have among the lowest protein and fat content and highest sugar content of the mammals. This is consistent with a species that carries its young everywhere, feeds with high frequency, and remains in constant contact with the infant 24-7. As parents and caregivers will note, human infants eat with pretty high frequency! Although, as primatologist Sarah Hrdy (Not a typo — pronounced “HER-dy“) notes, relying on others for care or finding other ways to minimize the energy costs of caring for calorically expensive infants is not at all unusual from a cross-species or evolutionary perspective (Hrdy, 2009).

So with this information in mind, what can we deduce about the composition of the milk of the Thala-siren? And what might that tell us about its parenting behavior?

Does a body good.

The way that the milk splashes on Luke’s beard was the first clue that stuck out for me. If the milk were low in fat and protein, it would run clear or translucent. But it doesn’t. If you pay close attention to that scene, you will see that it appears to be pretty opaque and frothy as it clings to Luke’s grizzled beard-hairs. Therefore, we can guess — in our imaginations, of course — that this creature has pretty decent proportions of fat and protein content in its milk, and is able to leave its babies alone for some period of time.

But probably a better hint comes from the way the animal reclines on the rocks, making booming walrus-like or whale-like sounds. Indeed, The Last Jedi Visual Dictionary describes the Thala-siren as a “large, flippered marine mammal often found sunning themselves one the coastal rocks of the island.” Clearly, the inspiration for these creatures was largely taken from pinnipeds — walruses, seals, and the like.

Sea creature
The Thala-siren perched on the side of a cliff in its ecological niche. In this piece created before the film premiered, fan artist Eli Hyder depicts the Thala-siren as a pale, somewhat distant ghost-like creature that almost seems part of the ocean spray, evoking the will-o’-the-wisps of the Celtic region where this scene was filmed. Eli’s work is amazing. Find him on Instagram, Twitter, or YouTube.

Pinnipeds, which live in a similar environment (marine—and judging by Luke and Rey’s attire, pretty cold), have extra high levels of fat for thermoregulation (maintaining optimal body temperature). The milk of the hooded seal, for example, is fattier than the richest ice cream! (However, pinniped milk also has very low–or even no–sugar, so it probably tastes more like butter.) Pinnipeds will spend a few days tanking 
their pups up with energy in the form of fat, then leave them for up to 2 weeks at a time (!!) to hunt. And you don’t want them burning that fat for energy, so their phenotype (the thing their genes tell their bodies to do) might actually prioritize the burning of milk sugars and storage of the milk fats.

Judging by the immense size of the Thala-siren, we can presume the infants spend a long time in a state of rapid growth, so protein and micronutrients such as calcium would also need to be high to support this rapid development. Rapid growth is calorically expensive, and since pinnipeds are known to have have little to no lactose in their milk, and since they likely need to store the fat to keep warm, Thala-sirens infants might also be relying on high levels of protein and amino acids to meet their energy needs, like the baby deer.  (Whales also have a very high fat & protein/low sugar composition, similar to the pinnipeds.)

In the next section, we’ll encounter some evidence that the mother may indeed be ingesting foods rich in protein and other important nutrients.


Why would a creature’s milk be green? More specifically, what chemical characteristics would make it green? And what of the blue milk that Luke drank on Tatooine? Which leads to the question — Why is the milk on Earth white, anyway?

Green human milk.

The protein casein is largely responsible for Earth milk’s distinctly white color, though the content of milk can vary from mother to mother as well as change by the day and even the hour. Humans’ use of breast pumps makes it easier for us to observe subtle (and usually temporary) color variations in the milk of our own species as it is expressed. Mothers have thus reported seeing streams of milk tinted blue, yellow, green, pink, and orange. It is usually a result of characteristics of food the mother has eaten, and is generally considered normal and harmless. Like sunlight, these colored compounds diffuse into the rest, ultimately appearing white. 

As for green specifically, low concentrations of casein and/or lactose (the latter being known to be low in pinnipeds) has been known bring out the greenish tint of the riboflavin present in milk. Riboflavin has also been implicated in milk that appears to have a blueish hue, and indeed if you look at a carton of skim milk, which has had much of the fat removed, it will appear bluer than your kids’ whole milk. However, such milk is also more translucent, and Thala-siren milk, as discussed earlier, is anything but. Therefore, this might not be the explanation we are looking for.

Maybe some kind of blue compound is combining with a yellow compound to make green? The “golden” color of colostrum (the milk that mothers synthesize in the days immediately following birth) is thought to be due to high levels of carotenoids, a class of antioxidants that help newborns cope with oxidative stress. We also know that milk can have a bit more of a yellowish tint if it contains higher levels of fat, and indeed we have thus far had several reasons to suspect that Thala-siren milk has a lot of fat. But what would cause blue that we’d need to mix with yellow to get green? Many will recall that this is not the only time we have seen Luke drinking colored milk—In A New Hope, he is seen pouring himself a glass of blue milk of a local elephant-like beast of burden known as the bantha. We also see mama Padmé serving Anakin a glass of bright blue milk in Attack of the Clones

This one is tricky because, on Earth, the only known causes of blue-tinted milk are low fat levels, but that results in a very pale blue color, not bright blue. Another widely-reported culprit is blue Gatorade. Somehow I don’t think the Thala-siren is much of an athlete, but the bantha, being a very hairy beast of burden on a desert planet, might find it helpful!

So I think what we’re looking for, here, is some kind of bright green compound originating from the animal’s diet. A major known cause of green milk is high iron intake. But perhaps most plausibly, it may be a simple color-staining from a high consumption of green or blue-green algae or kelp from the surrounding aquatic environment. This conveniently fits with the depiction of these creatures as domesticated grazers—like a cow of the sea.

Perhaps they even use their long snouts to forage around on the ocean floor for grasses and algae that grow down there. Blue-green algae on Earth is a rich source of proteins of the sort that the Thala-siren would need to grow their ginormous babies, as well as many crucial micronutrients.

Thala-siren filter mouth
Artist: Jake Lunt Davies / Owned by: Lucasfilm

And indeed, with a little bit of digging I discovered that, according to Jake Lunt Davies, concept designer for Lucasfilm on the Star Wars sequels, the art team imagined the Thala-siren as a filter feeder, sporting a tubular baleen-like filtration system in it’s mouth instead of teeth. Interestingly, it also uses it’s prehensile mouth to grab shellfish and smash them on rocks to open them. Shellfish on earth are a rich source of iron, which might also add to the green color.

So I think we found our culprit!



First, some terminology — The part that is large and round like a ball is called an“udder” when it’s located on the abdominal or inguinal (groin) region, and a “breast” if it’s located on the thoracic (chest) region. Same thing, different names. The smaller part that the milk comes out of is called a “teat” if it’s on the abdominal or inguinal region, and a “nipple” if it’s on the thoracic region. Breasts/udders may or may not always be present. Most animals just have a teat or a nipple.

The thing I found most curious about the Thala-siren was its very pronounced udders, somewhat like a cow. This sort of thing is, as far as I know, rare in wild mammals. What IS totally unheard of, outside of humans, is the er, shape. Even amongst humans, a breast that round and protruding against the force of gravity is generally considered to be a creation of medical technology, not nature. Human breasts come in a variety of shapes and sizes—all of which have at least *some* amount of “sag.”

Therefore, it is quite curious indeed that TLJ director Rian Johnson and his art team chose to evoke the full, round Western ideal of a human breast such that one can’t help but wonder how the scene got approved by Disney execs. (Not that I think breasts should be censored, anyway. Way to be progressive, Disney! #freethenipple etc. But mostly I’m glad because otherwise I wouldn’t get to write this ridiculously fun piece.) I struggle to think of any Earth animal that reclines in an upright or semi-upright position whose breasts or udders protrude in quite this manner.

Despite what you see in the media, there exists a wide variation in human breast shape.

Nevertheless, human breasts are indeed generally more pronounced than any other non-domesticated mammal. Therefore, examining our own mammary evolution might be instructive, here. Have you ever wondered why humans have protruding breasts, anyway? Most people assume that the purpose of prominent, fatty breasts is to advertise fertility and attract males. However, contrary to popular belief, attraction to breasts is not actually a human universal. I have met anthropologists who tell me that the cultures they visited tell them “your men are like babies!” because of our sexualization of the female breast. This fact calls these theories into question.

“Dairy Princess”: A Slave Leia Thala-siren miniature sculpture by Mike Strick.

There are a number of theories proposed to explain the evolution of the protruding human breast, from advertising sexual maturity to thermoregulation of the milk. Sexual selection doesn’t hold up super well since sexual attraction to breasts is not a human universal. Thermoregulation hasn’t found a lot of support, especially since there is so much variation between humans in the amount of fat storage in a breast. 

The prevailing (but not unchallenged) theory at the moment is that, as prognathism (a protruding face or snout) in humans reduced over the course of hominid evolution, protuberance of the breasts increased to make it easier for infants to latch, and latch is really important for efficient milk extraction as well as minimizing pain to the mother (which would make her aversive to breastfeeding). The Thala-sirens, though, have reeeeally prognathic faces, so that wouldn’t help them. (For our purposes, we will assume that their babies have snouts that are long as well, but as with most baby mammals are shorter than the adult snouts.) Thus, latch assistance makes is an unsatisfactory explanation for the protruding udder of the Thala-siren.

Large udders may indicate that these are domesticated animals. The cow was bred by humans to have larger udders than their ancient ancestors, the aurochs, in order to achieve the right combination of ease in milking and reduction in bacterial growth in the area.Perhaps these animals have been modified beyond their naturally-selected form by the island’s sentient caretakers, known as the Lanais. Access to a stable source of fat would be an advantage for a semi-aquatic species, especially if there are cold winters. The ability to store dairy products in the form of cheese would help the male Lanais (known as “Visitors”) survive their long, cold hunting expeditions at sea. In fact, The Last Jedi Visual Dictionary describes the milk as “nutritious,” lending support to the idea that this milk is a staple in the diet of the Lanais.

The fact that the creature does not try to run or attack when Luke approaches is certainly supportive of the idea that this is a domesticated creature who is quite used to being milked. And indeed, the Visual Dictionary supports this as well, referring to the Thala-sirens as “docile” and notes that it is not hunted, and therefore not fearful of the intelligent island natives.

There is also a more obvious factor that supports the idea that this is a domesticated creature: Here we have an animal that is clearly in a state of active lactation, while an infant is nowhere to be seen. It would be a waste of resources to continue milk synthesis beyond weaning, and we know that nature is generally not wasteful. Aquatic mammals do leave their babies for long periods of time, but it’s to go hunting—not to lounge on the beach.

What is even more unusual (by Earth standards, anyway) is the Thala-siren’s 1 teat per udder ratio—as opposed to cows, which only have 1 udder. (It often looks like 2 but really what you’re seeing is the ligament that holds the whole thing up), and 2 pairs of teats (totaling 4 teats.) This is a little unusual in nature as well, but not unheard of. One of the coolest mammals you’ve never heard of, the pangolin, has breasts with a 1:1 udder-teat ratio. As do elephants (BET YOU DIDN’T KNOW THAT!) Most Earth mammals with four teats have only 1-2 babies at a time. Pinnipeds usually have singletons. Twins are rare and have high mortality rates. So there’s another possible fun fact we just learned about the Thala-siren: It probably gives birth to singletons.

File under implausible: The force of the milk ejection reflex may be an indication that the animal feeds underwater like whales, which basically shoot the milk into their baby’s mouths, allowing it to feed on the go. However, this adaptation evolved to allow for a more streamlined body shape, which precludes any kind of protrusion like a breast/udder, so it is unlikely to be the case here. It is probably just a forceful letdown as a result of very full udders or very active hormones. Just ask any mom who has accidentally shot milk across a room! 

Ice-T shoots Jimmy Fallon in the face with a Super Soaker
I basically only wrote this paragraph so I could include this gif.

…But this brings to mind another fact we can probably conclude: The Thala-siren, while it does have flippers, is probably not a particularly fast swimmer.

Putting it All Together

So here is what we can speculate about the Thala-siren, based on this scene: It is a large, semi-aquatic mammal that gives birth to one (or rarely two) babies at a time. It tanks it’s babies up on milk with energy-dense fat and protein, then leaves it perched on the rocks for long periods of time to protect it from underwater predators while it goes off to forage for food—Though it is not a very fast swimmer, so it probably only goes away for hours, not days, and probably only goes as far as the shallow waters of nearby islands. It eats nutrient-dense blue-green algae and plankton-like food, which it filters through its baleen-like mouth, and iron-rich shellfish, which it grabs with it’s prehensile mouth and cracks open on the rocks with its powerful neck. Baby Thala-sirens are probably adorable (#headcanon), and likely have shorter snouts than the adults and maybe even whiskers on their snouts to help them find food on the murky sea floor. Somebody should definitely draw one. Thala-sirens are probably modestly social for a large mammal. They live in large groups scattered about the rocks, and probably teach the babies to forage so they can fuel their rapid growth. They are relaxed, docile, and probably domesticated by the locals, who use their energy-dense milk–which probably has a buttery, fishy taste–to make cheeses and yogurts that keep for a long time, sustain them through cold winters and long expeditions at sea.

All that from a 25-second, dialogue-free scene.

The Thala-siren puppet on location at Skellig MIchael Island
The Thala-siren puppet on location at Skellig Michael Island. It was created on a soundstage, flown in by helicopter, and operated on location by three puppeteers. Photo: Jake Lunt Davies/Lucasfilm

I’ll close with a piece of fun trivia — In researching the Thala-siren, I learned that the creature 
featured in this scene was (true to Star Wars tradition) a practical effect. It was a huge puppet, not unlike the triceratops in Jurassic Park. First, the Skellig Michael island location off the West coast of Ireland was digitally mapped. Using this model, Lucasfilm created a replica of the seashore at Pinewood Studios in London, which allowed them to build the creature in such a way that it would look very natural in its surroundings. Once finished, the giant Thala-siren puppet was flown in by helicopter to be filmed on location. It was operated by 3 puppeteers — one to control the head, and two inside the creature itself to control the flippers and the “milk.” (I bet their job was a real “letdown”….haha thank you, I’ll be here all week!) I get nauseated imagining whatever it cost to produce this 25-second scene.

But however much it was, it was 100% worth it.

Further Reading and Fun Stuff:

Smithsonian Magazine: The evolution of lactation

The Brain Scoop (video): The Breast Story Ever Told – Human breastfeeding and evolution

March Mammal Madness (a mammal tournament happening right now) and Dr. Katie Hinde’s Mammals Suck Milk blog The Geology of Crait (the planet with the red dirt)

PLOS: Scientists peer-review The Last Jedi “Thala-siren Milkshake” recipe Ridiculously fascinating article about human milk 

What human milk looks like when you remove all the fat and protein 


NIH: 65% of human adults are lactose intolerant

Thala-siren Wookiepedia page

Tilden & Oftedal 1997. “Milk composition reflects pattern of maternal care in prosimian primates.” American Journal of Primatology 41 (3): 195-211

Blaffer Hrdy, Sarah. Mothers and Others: The Evolutionary Origins of Mutual Understanding Harvard University Press, 2009.  – I just love the cover image on this book!

Peaker, M. “The Mammary Gland in Mammalian Evolution: A Brief Commentary on Some of the Concepts.” Journal of Mammary Gland Biology and Neoplasia 7(3):347-53 · August 200 – Great overview of how the development of breastfeeding as an adaptation lead to a more intensive caregiving style and more complex, cooperative social development among mammals (especially primates).

Hildago, Pablo. The Last Jedi Visual Dictionary. DK Children, 2017.

Penn State College of Agricultural Sciences: Baby deer eat 2-3x/day

Anderson, Philip. “Unusual Milk Colors.” Breastfeeding Medicine, Vol. 13 No. 3. 2017. – notes various causes of green milk.

Lawrence, R.A. Breastfeeding: A Guide for the Medical Profession, 7th ed. Elsevier, 2010. – Beta carotene as a known cause of yellow tinting in human milk.

Kankofer, M. and Albera, E. “Postpartum relationship of beta carotene and vitamin A between placenta, blood and colostrum in cows and their newborns.” Experimental and Clinical Endocrinology & Diabetes. 2008 Jul;116(7):409-12 – Carotenoids help newborns deal with the oxidative stress associated with transitioning from womb to world.

Long, Charles A.  “The Origin and Evolution of Mammary Glands.” BioScience. Vol. 19, No. 6 (Jun., 1969), pp. 519-523 — “Breasts are basically glorified apocrine sweat glands!” -My anatomy professor, years ago.

Jake Lunt Davies, character artist for the Thala-sirens, on conceiving it’s feeding system

Caro, T.M. “Human breasts: Unsupported hypotheses reviewed.” Human EvolutionVolume 2, Issue 3, pp 271–282

A table of milk composition by species — assembled from course notes by Robert D. Bremel, University of Wisconsin and from The Handbook of Milk Composition, by R. G. Jensen, Academic Press, 1995.

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