Blood color differs across the animal kingdom, with some animals producing blood that appears green (a group of lizards in New Guinea) or blue (horseshoe crabs).

Blood has evolved many times over hundreds of millions of years, resulting in different blood colors and compositions—and there are even some organisms that don't need blood at all.

Biofluorescent animals—those that absorb light, such as ultraviolet rays, and re-emit other types of light, such as neon green—include some flying squirrels, scorpions, sea turtles, and platypuses.

Water is naturally tinted with a slight blue, but sediments and organics can alter its color

The innate coloration comes from water absorbing red light in the visible spectrum more than green or blue, giving shallow waters in some parts of the Caribbean their turquoise hue. If sand is light, it can be churned up by turbulent water, clouding its appearance. Plankton and other organisms can color water by affecting how the rest of the spectrum is absorbed or reflected.

View a gallery of biofluorescent animals, which contain cells that emit various colors when exposed to ultraviolet light.

US labs drain an estimated 500,000 horseshoe crabs of blood each year to harvest a compound that screens vaccines and medical devices for bacterial toxins.

Snake venoms are repurposed proteins responsible for other bodily functions

Just as saliva aids digestion in humans, proteins responsible for bioactivities such as digestion and blood clotting were eventually secreted in the glands and fangs of snakes across evolutionary history. Organisms that could not withstand the effects of these molecules (e.g., excessive bloodstream clotting) became ideal prey for snakes.

The strange endurance of consuming blood for health

Blood's importance to humanity is best summarized this way: without it, we die. This importance naturally begets assumptions about its medicinal properties, many of which appear accessible through drinking or eating the fluid connective tissue. This article details the history of blood drinking, from ancient Roman gladiators to modern-day sanguinarians.

Starting with red, colorful objects will appear to lose their colors the deeper they are submerged because water scatters light, reducing the amount available to reflect off objects and be observed.

Seven things you might not know about blood

The red stuff currently pumping throughout your body is a type of connective tissue that is both a solid and a liquid. The liquid portion—plasma—acts as a carrier for the solid portion, which includes blood cells and their biological brethren. Your body is constantly refreshing its supply of blood, creating new cells to replace those that have expired or died. This article overviews some lesser-known facts about blood (and why it's not always red).

Birds, rodents, monkeys, lizards, bats, apes, and livestock can all get malaria—just like humans.

Without liver bile, human poop would be a pale silver color

Stomach acid and enzymes break down food into smaller particles, expelling a semi-fluid mass called chyme into the small intestine, where bile is added. As the intestines absorb nutrients and water, the chyme solidifies and changes color.

Each second, the human body produces 2 million to 3 million new red blood cells.

The economics of horseshoe crab blood

Horseshoe crabs are older than dinosaurs and have survived five mass extinctions that wiped most life from the planet. But it's only now that their future as a species is in danger. This blood, worth about $60,000 per gallon, is hyper-sensitized to certain bacteria that are fatal to humans. This 17-minute podcast dives into humanity's pursuit of horseshoe crab blood, a "somewhat magical" mixture heavily relied on by the pharmaceutical industry.

Chameleons change color by altering the spacing of nanocrystals under their skin, which changes what colors of light are absorbed and reflected.

Explore this blood type interactive to learn more about how the 8 most common blood types are defined.

Poison frogs get their toxins from eating insects and mice

Other animals, including royal ground snakes, are able to survive eating poison. Scientists study the creative antitoxin defenses in these animals to discover treatments for poisoning.

What is the rarest color in nature?

Plants, animals, or minerals found in nature bear almost every color imaginable. There are two factors that influence what hues you see in the wild: physics and evolution. So, which colors are you least likely to see in the natural world? Victoria Hwang explores one of nature's rarest spectacles.

Blacklights can illuminate certain biological materials, including blood, teeth, and fingernails, because they contain fluorophores—molecules that visibly glow immediately after exposure to higher-energy light.

Virtually all plants are biofluorescent because their photosynthesis is powered by chlorophyll, a pigment that glows red as a way to get rid of excess energy when exposed to ultraviolet light.

Bioluminescent organisms produce their own light through chemical reactions involving unstable, energetic molecules, while biofluorescent organisms almost immediately re-emit light shone on them, but at a different wavelength.

Explore the microscopic composition of blood.

The history of blood, as told by a hematologist

Since ancient times, blood has been seen as a semi-magical substance. Alchemists theorized it was a mixture of the four elements. Others proposed it was one of four bodily fluids whose balance dictated health. This 2019 essay by a hematologist reviews blood's reputation across cultures and centuries.

'Golden blood' is the rarest blood type in the world—fewer than 50 people are known to have it.

Why 75% of ocean animals glow

About 75% of ocean creatures can make their own light. It's called bioluminescence—and it all comes down to a special chemical reaction happening inside their bodies.

Mosquito saliva acts as an anticlotting agent

The bug's piercing mouthpart, the proboscis, is actually several tubes—one for sucking, one for spitting. But the chemicals in its spit actually help its blood meal to flow, combating its victim's ability to clot at the puncture site.

When processing light, the brain can perceive colors with no natural wavelength

While some animals are sensitive to many forms of light, humans possess receptors that are sensitive to red, green, and blue wavelengths. These receptors send signals to the brain, which are processed and mixed to produce the array of colors we observe, including magenta, pink, and "supercolors" that lack a corresponding natural wavelength in the visible spectrum.

Reindeers are the only known mammals whose eyes change color with the seasons, going from golden yellow in the summer to deep blue in the winter.

Flamingos get their pink coloring from carotenoids—pigments of yellow, orange, and red—in their food, but they cannot take on other colorations because their bodies cannot process other pigments.

Vampire bats are the only mammal to feed exclusively on blood

Scientists believe their ancestors may have eaten parasites and insects off larger animals until vampire bats started feeding directly from those animals. Colonies work together to stay fed, as a vampire bat can die without just two days of eating.

Bioluminescence became common in the ocean to support survival in dark environments

Despite seeming like a rare ability in land animals and unseen in plants, bioluminescence has evolved at least 29 times across marine species, indicating its necessity for survival where hiding spots are rare and highlighting how environments provide evolutionary pressure.

Without the option of modern blood transfusions, some doctors in the mid-1800s tried to help save cholera patients by injecting them with cow's milk, hoping it would boost their nutrition and hydration.

Mosquitoes drink a maximum of 0.01 milliliters of blood in a single meal.

Coral reefs get their colors from zooxanthellae—photosynthetic algae that line corals' digestive tracts and provide them with sugars in exchange for a home within this protective tissue.

Cholera was called the 'Blue Death'

In the mid-1800s—when John Snow identified cholera in London—the disease was known simply as the "Blue Death." That's because blood would thicken in patients' veins when they were struggling with extreme dehydration. As a result, their skin would turn a blueish hue as their blood was starved of oxygen.

Cold-blooded reptiles seek heat, and that behavior may help combat fungal infections

This Radiolab podcast episode takes on the idea that though our human bodies are too warm for the majority of fungal species to effectively replicate inside our bodies, that's not the case for reptiles. Yet when those species warm their bodies by basking in the sun on rocks, they can soak up heat and may also help keep fungal infections in check.

An interactive explanation of blood transfusion

The antigens and Rh factors of blood cells (which determines your blood type) must be compatible with your immune system—which is why blood transfusions between patients must have matching types. In a mismatch, the immune system will attack and destroy donor blood cells, triggering a potentially fatal reaction.

View examples of luciferin, the light-emitting molecule behind bioluminescence

Bioluminescent marine organisms rely on various unique luciferins, including Cypridina-type luciferin in seed shrimp and chlorophyll-based dinoflagellate luciferin, to react with oxygen to produce light. These chemicals require unique catalysts to help them bind with oxygen.

Bioluminescence provides organisms with a method of hunting, defense, or mating

Unlike glow sticks, bioluminescent reactions use renewable resources like oxygen and ATP. This ability allows fireflies to mate and deter predators, anglerfish to confuse and attract prey, and deep-sea shrimp to defend themselves by vomiting glowing slush.

Bone marrow generates roughly 500 billion new blood cells each day.

During the fall, shorter days trigger cells in leaves to produce less chlorophyll, a green pigment that is typically present in sufficient concentrations to mask other pigments, such as yellow and orange.

View a gallery of bioluminescent organisms, including jellyfish and mushrooms

Some species produce a quick flash of light, while others glow steadily with an eerie glow, often to seek attention or in response to being disturbed. Snails, worms, fungi, and other organisms highlight that this ability evolved independently, rather than from one common ancestor.

How the pigments that absorb light for photosynthesis contribute to plants' green color

Although light energy is captured to power the reactions that produce the building blocks of starch, cellulose, glucose, and other sugars, chlorophyll a and b are most sensitive to violet, blue, orange, and red light, leaving green light mostly unabsorbed. This green light is instead reflected, giving most plants this color appearance.

A brief history of human blood groups

The blood group that determines whether you have type A, B, AB, or O blood was discovered near the turn of the 20th century. This short academic article dives into three main theories about the evolution and emergence of blood groups, the exact nature of which remains unclear.

Jaundice, which can make the skin and eyes appear yellow, is a buildup of bilirubin, a waste product that forms when old red blood cells break down.

From glands beneath their skin, hippos produce their own sunscreen in the form of a fluid containing hipposudoric and norhipposudoric acids, which absorb sunlight in the ultraviolet and visible range.

Genetic analysis suggests Anopheles mosquitoes fed on humans some 1.8 million years ago.

See this graphic of the complex malaria transmission lifecycle.

The proteins and sugars on the surface of your blood cells determine your blood type.

Electrogenic animals generate their own electricity within an electric organ by changing the pattern of electrical charges in disc-shaped cells called electrocytes.

Because it is observed in organisms that are not closely related, biofluorescence is thought to have emerged independently many times in nature—over 100 in fish alone.