The color red

Every winter I notice that blueberry twigs have turned red. That got me thinking about colors and that color in particular. We see reds in other plants—lots of berries, dwarf dogwood leaves, columbine flowers, and so on.

Red and reddish colors can be produced in many ways, by a variety of pigment molecules. In many cases, the degree of reddishness is determined by the chemistry (e.g., acidity) of the cellular fluid around the pigment molecules, so sometimes the color (as we see it) is orange or pinkish-red or purplish-red. And often the pigments are present without contributing to color, as they have many other functions. Here are some pigments that can produce red color:

Animals carry a red pigment called hemoglobin, which (in vertebrates) ferries oxygen around the body, and a similar one called myoglobin in red muscles. The redness of blood under the skin accounts for blushes, the red faces of some monkeys, and the red marks on the face of mandrill apes. There is turnover in the population of hemoglobin molecules, new ones replacing old ones, and as the old ones break down, they form another pigment, a ‘bilin’ called bilirubin, yielding the yellow color of urine and jaundice.

Bilirubin also occurs in some taxonomically diverse plants. However, it seems that this pigment only contributes to color in the genus Strelitzia, native to southern Africa. These so-called bird-of-paradise plants have seeds bearing nutrient-rich appendages called arils, which are bright orange, attracting seed-dispersing birds.

Animals also have pigments called melanin. One type of melanin (eumelanin) produces browns and blacks; in vertebrates, they color hair and skin and protect skin from UV rays, while in insects they have many other roles. But another one, phaeomelanin, produces red and yellows; in combination with a brown eumelanin, it accounts for the color of red fox fur and for red hair in humans.

Other pigments called betalains produce colors grading from red to yellow or red to violet. Betalains take their name from the Latin name of beets (Beta). They occur in plants of most taxonomic families of the order Caryophyllales and in some fungi. Examples include not only beets, but amaranths, cacti, and sundews. Apparently, betalains function as antioxidants and disarm free radicals (unstable atoms or molecules with an unpaired electron that can link up in some unexpected places, causing damage, although at low concentrations they also have proper metabolic functions).

Anthocyanins produce red color in acidic conditions (but blue in nonacid or alkaline ones). They act as a sort of sun-screen in plants, protecting the photosynthetic machinery from excessively strong light, scavenging free radicals and reactive forms of oxygen, protecting photo-sensitive defense compounds, and helping the plants tolerate stress. They sometimes appear in very young leaves of some trees, where they may protect from strong light or even deter herbivory in some way. We see them in the showy fall color of some maple leaves, the flowers of wild columbine, and red cabbage.

Carotenoids are widely distributed in plants, where they help transfer light energy to chlorophyll for photosynthesis. They are also antioxidants and scavenge free radicals. They contribute colors ranging from yellow to red in autumn leaves and many flowers, the orange of carrots, and the red of ripe tomatoes. Animals can’t synthesize these pigments and must obtain them from food. They function there as antioxidants and controllers of excess free radicals, as precursors to vitamin A, and enhance immunity; they also function in mate choice and rivalries. The bright red of male red-winged blackbird epaulets and of male cardinals is produced by carotenoids, as is the red bill on zebra finches, the brightness depending on the availability of the pigments in the diet, and females prefer bright males. They contribute to color in fishes, amphibians, and reptiles, and even in certain bats that have yellow skin on ears, nose, and wings.

Pterins produce colors ranging from yellow to red, but unlike carotenoids, they can be synthesized by animals. They participate in many fundamental physiological processes. They also modify UV reflectance in males of some butterflies, producing sexual signals contributing to mate choice. They often interact with carotenoids, for instance producing the orange spots that male guppies use to court females.

Unique to parrots are special kinds of red-orange-yellow pigments that are synthesized by the birds, not obtained from the diet. The turacos of Africa have some red feathers produced by a pigment known to occur almost nowhere else.

That’s just a sample of pigments producing reddish color; they are clearly diverse and widespread. But blue, green, and white pigments seem to be far less common. Here is a sample of those:

Pterins can produce white pigments, which absorb so much UV that they appear white to us; this occurs in the wings of cabbage white butterflies. In general, however, white colors are usually due to the absence of colored pigments.

Blue is usually made structurally; the blue of bluebird and bluejay feathers, for instance, is created by air pockets and molecules that reflect blue light. The showy blue of morpho butterflies comes from overlapping, ridged scales that reflect light. Anthocyanins can look blue or purple (e.g., blue gentians, wild iris, blueberries) if in nonacidic cellular fluid. But conversely, Hydrangea flowers turn from pink to blue when grown in acidic soils with aluminum ions. I found one blue pigment, a ‘bilin’, that is synthesized by olive-wing butterflies and found in some swallowtails too.

Greens are produced in several ways. Turacos also have a unique green pigment, but most green or greenish feathers (e.g., on the backs of many warblers) are produced by blue structural colors overlying yellow pigment. The plant world offers several chlorophylls, involved in photosynthesis. Some ‘bilins’ produce green (the green huntsman spider) or the green bones of garfish.

So now the usual question: Why are red-making pigments apparently more diverse and common than other pigments?

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A green world

Five decades ago, some well-known ecologists looked around and noted that their terrestrial world was very green. Why didn’t herbivores demolish the greenery? The observers suggested that predators limited herbivore populations to low enough levels that the vegetation continued to thrive. That suggestion led, of course, to years of counter-suggestions and dispute.

Although such discussions seldom end altogether, some answers have emerged. Do predators limit their vegetation-eating prey populations? Sometimes. But another kind of answer has greater explanatory power: the plants defend themselves against herbivore assaults—so the terrestrial world is still green. The idea of plant defense took hold and now the information about those defenses would fill an encyclopedia of many volumes.

The space for my essays is far less than that (thank goodness!), so here I can only present some overview and examples.

–Some plants enlist the aid of protective animals. Acacias and peonies offer nectar rewards and, in the case of acacias, housing to certain ants, which patrol the plant, attacking browsing mammals (goats, giraffes). Mammal browsing in some acacias induces greater development of hollow thorns that house protective ants.

Other plants emit airborne molecules that attract predators of herbivorous insects; for example, aphid damage stimulates soybean plants to emit certain volatile chemicals that attract syrphid flies, which lay eggs on the plant and their larvae eat aphids.

–Many grasses just keep growing if they’ve been cropped short by sheep (or a lawn mower), replacing tissues that were lost.

–Thorns, spines, prickles, and hair-like trichomes on green plant parts deter some herbivores; e.g. thin, spiky thorns on some acacias reduce browsing by large mammals and spines on some nightshade plants impede the movement and feeding of the tobacco hornworm. Nettles have trichomes, which can prevent insect eggs from sticking, impede insect movement, and deter browsing by mammals; one species adds a little sting when a trichome is broken. On cacti, spines may deter most mammals from munching, but dense spines on some cacti have additional functions of reducing evaporative water loss and providing shade and winter insulation. On stems, spiky protrusions may protect against some climbing leaf-eaters. Porcupines don’t seem to be deterred by raspberry thorns, but what about those wicked spines on devil’s club? Maybe some invertebrates (slugs, for instance) are deterred?

–Hard, tough, leathery leaves can be difficult to handle by chewing insects (although there can also be other evolutionary advantages, such as reducing water loss).

–Conifers and many broad-leaved trees produce resins, sticky material loaded with chemicals, providing both a physical and a chemical barrier. They commonly function in defense against insects such as bark beetles, deterring the initial attack, preventing entry of pathogens, and helping heal the wounds. High levels of resin in ponderosa pines also appear to reduce foraging by Abert’s squirrel, whose winter diet is mainly the inner bark from particular trees with low levels of resin. Ponderosa pine resin may also reduce infections of dwarf mistletoe.

–Latex is sticky and can coagulate around a wound, inhibiting pathogen entry; it also carries defensive chemicals. It’s common in the milkweeds.

–Chemical defenses are numerous and diverse–many different kinds of chemicals are used in defense but, for simplicity, let’s not deal here with the chemical details but rather focus on the biological effects. Tannins can bind proteins; some reduce digestibility of vegetative material, with negative effects on health and reproduction for vertebrates. However, habitual eaters of tannin-containing food, such as moose, have evolved means of counteracting the tannins. In insects, tannins apparently don’t bind protein but have other effects, such as deterrence or toxicity.

Some chemicals are toxic to consumers. For example, wintercress and its relatives contain types of toxin that defend against rust fungi or (depending on wintercress genotype) deter feeding by some insects. Geranium petals contain a certain amino acid that paralyzes Japanese beetles, making them vulnerable to their predators for several hours. Some plants produce juvenile hormones, which inhibit metamorphosis in insects, so they can’t mature. Nicotine in tobacco plants is toxic to mammals and insects that feed on those plants, causing digestive upsets, convulsions, difficulty walking, and other negative effects in mammals, and it can be lethal to insects. Particular chemicals in milkweed plants are toxic to some herbivorous insects. But in other milkweed-feeders, the toxins are sequestered in the body; they are distasteful (at least) and potentially toxic to the herbivorous insects’ predators; these herbivores bear warning coloration, which their predators learn to avoid.

Monkshood plants are toxic to mammals but many types of insects feed on them. Photo by Kerry Howard

Bog laurel is toxic to mammals too; and pollinating bees that eat the pollen make toxic honey. Photo by Bob Armstrong

Adding even more complexity: some plants can warn their neighbors, transmitting signals through root contact or even mycorrhizal fungal connections. Sometimes the warnings are airborne: The leaves of bullhorn acacia have distasteful chemicals; if nevertheless damaged by a browsing herbivore, the leaves send a volatile signal to others, sometimes miles away, inducing production of the distasteful chemical there.

Plants have been very inventive, evolving an immense array of defenses. Some things to remember: Some chemical defenses and some structural defenses are not built-in but induced by the activity of herbivores. Characteristics related to defense against herbivores may also have other functions. Bear in mind that no defense is likely to be perfect and there are likely to be some consumers that are not affected. In fact, there are some good stories of co-evolution in these interactions: if some insect is resistant to a plant’s defense and feeds on it, that sets up natural selection for better defense, and that in turn leads to selection for better resistance, and thence to still better defenses and then better resistance and onward…

Nitrogen

Nitrogen is essential for growth and development of living things. It’s involved with almost all aspects of life: in amino acids that constitute protein, in DNA and RNA that control genetic inheritance, in ATP molecules that provide energy for cellular metabolism involved in muscle contraction and nerve impulse propagation, in lots of enzymes that control physiological processes, and in some vitamins (e.g., B complex), and so on. It’s abundant in earth’s atmosphere, comprising almost eighty percent of all atmospheric gases. But it’s not in a form usable by many organisms, including all the ones we can actually see without a microscope.

This essential element is provided in small quantities by lightning bolts, which separate nitrogen atoms from each other and lets them bond with oxygen in the air, to be picked up by water droplets and dropped as nitrates to the earth in rain. However, most of the nitrogen used by living things is retrieved from the atmosphere by bacteria, many species of them. They have the machinery to ‘fix’ nitrogen into forms usable by other organisms by oxidizing it to nitrates (nitrogen with three oxygen atoms).) These so-useful bacteria live in all kinds of places: in the soil and decomposing litter, in root nodules of legumes (e.g., lupine, beans) and alders, inside conifer needles, certain ferns, fungi, and lichens, in water, on leaf surfaces in the crowns of trees and shrubs, on tree bark, and many other places.

Nitrates are water-soluble, so they can leach out of bacterial cells, fall with rain from leaves to the ground, ooze in and out of root nodules and roots, get picked up by fungal networks and so transferred to plants. Plants are the basis of most food chains, providing food for herbivores that are food for carnivores. Soils differ in nitrogen content and nitrogen is often a limiting factor for plant growth (witness the use of commercial fertilizers).

Vertebrae muscle tissues usually have about fifteen to thirty percent protein, by wet (raw) weight. Insects seem to have more protein, on average (but it’s highly variable), although some of that protein is in chitin (e.g., in the exoskeleton of insects), and apparently only some birds and mammals can digest chitin well. So, with some exceptions, animals that eat other animals seldom have problems with nitrogen supplies; the nitrogen in animal tissues is generally in a form usable by consumers.

However, in general, plant tissues have lower levels of protein (usually less than twenty percent by dry weight, often much less). Plant-eating animals (herbivores) consume fresh green plant material and the wet weight varies with the water content, which is highly variable, depending on growing conditions, among other things. For comparison purposes, try fifty percent water, giving less than ten percent protein in plants. Plants and plant parts differ a lot in nitrogen content, availability, and digestibility; for instance, young needles of spruces have more than twice as much protein as old needles—but still a relatively low value. Pollen and seeds tend to have more protein than the green parts.

Ptarmigan. Photo by Bob Armstrong

Herbivores feed on green vegetation that often has relatively little nitrogen, so how do they obtain enough nitrogen? Some forage selectively; for example, the giant panda eats mostly bamboo but it favors young bamboo shoots, which have more nitrogen than older shoots. Another way is to eat large volumes of vegetation, which is most possible for large animals with large guts (elephants, horses, cows). Or an herbivore may pass small volumes of vegetation through the guts rapidly (panda); aphids and other sap-sucking insects ingest lots of fluid in order to get enough nitrogen, excreting the excess water and sugar as honeydew.

Herbivores have a variety of specializations of the digestive tract. But fundamentally, all vertebrates (and invertebrates too) depend on vast populations of microbes to process food. Some plant-eaters do a lot of food-breakdown in the stomach, which is generally divided into compartments; examples are camels, kangaroos, hippos, sloths, and the ‘ruminants’ (deer, cows, sheep),which can burp up a meal from the stomach to be reprocessed by the mouth. The first stomach compartment (the rumen) holds populations of microbes that do the work of digestion. Other herbivorous mammals (horse, elephant, manatees, gorillas) have enlarged colons for microbial digestion. There are billions of these microbes, which have many digestive uses, but they have very short individual lives. When they die, their multitudinous bodies can be digested by still other microbes, which produce amino acids usable by the herbivore.

Deer eating alder. Photo by Bob Armstrong

Many herbivores have special physiological adaptations that allow them to recycle urea (a metabolic product of nitrogen digestion, which humans excrete in urine) back into the digestive tract where it is used by gut microbes. Herbivorous birds such as grouse and ptarmigan can reflux urine and digestive material back ‘upstream’ from the cloaca where it is collected into the large intestine to be reprocessed. Or those things can be refluxed into ceca (sacs attached to the intestine in which further microbial digestion occurs). Lots of small mammals and some birds have ceca, which can retain fluids and small particles that can be digested more easily than large particles in a second round of processing.

Still another means of recycling is ingestion of feces (coprophagy). Many small mammals (lemmings, rabbits) with ceca produce two kinds of feces: ordinary ones that have only passed through the intestine and cecal ones that have been reprocessed by the ceca before being excreted. Cecal feces, reprocessed, have higher nitrogen content than ordinary feces, and are consumed by these herbivores. Much of that nitrogen results from microbial activity.

Hoary marmot. Photo by Jos Bakker

Vertebrate herbivores have solved the problem of nitrogen acquisition in many ways! I’ve emphasized vertebrates here, in hopes of keeping this subject tractable and within my usual space limitations.