Seeing Red

The different meanings of the color red/orange as used by plants and animals as a means of communication are fascinating. What woman does not have “a little red dress” to be worn on the right occasion? Who is not aware of the use of a red cape to attract the attention of the bull in classic bull fights- yet of course the bull does not actually see the color red and only notices the movements of the cape. Indeed we are fortunate that as primates we are capable of color vision within a certain spectrum; it is hard for us to imagine life without color. Yet other species may be color-blind or have perception outside of our spectral range within the ultraviolet (kestrels and some insects), polarized light (honeybees), and infrared (pit vipers and pythons). Presumably the need for color vision of our primitive primate ancestors was related to the difficulties of discriminating among green and ripe fruits, and in distinguishing small differences among leaves that were toxic or not. Field studies of gorillas and chimps have shown that they eat hundreds of different foods and must learn subtle discrimination among kinds and qualities of potential foods.

So although it is clear that red is the color which signals “look at this and pay attention,” it does not unequivocally indicate that this thing is either good to eat or not, or poisonous or not. So how do animals tell the difference? How does a bird judge whether a red fruit is ripe and ready to eat, or poisonous or simply unripe? A few examples here will illustrate the conundrum. I can only surmise that animals in the course of their daily lives have learned, or in some cases developed innate knowledge over the course of evolution, of how to interpret the meaning of red color in each specific case.

We are all familiar with beautiful red birds such as the classic male cardinal. I have shown a less familiar bird, a red male summer tanager which I photographed in the Yucatan as it was migrating northwards. Obviously this use of the color red is for signaling to the females of the species and other males that here we have a virile male of the species. What one wonders is how such a bright color might expose the males to increased risk of predation. Male scarlet tanagers lose their red breeding plumage by molt, but this does not happen with summer tanagers. The cost of increased predation is apparently out-weighed by the advantage to a given male in fathering progeny.

The striking orange of the queen butterfly sends exactly the opposite message to potential predators- that it is toxic. the queen is one of a group of “milkweed insects” including the monarch, queen, soldier, and several red bugs and beetles which gain toxicity from eating the poisonous milkweed, and are thus protected. The Gulf fritillary butterfly is similarly colored but gains its protection from eating passionvines as a caterpillar and is likely involved in a Muellerian mimicry complex with the milkweed butterflies. The red eft, a juvenile newt, is bright red/orange advertising its toxicity due to tetrodotoxins that it places in its skin.

The color red in fruits classically indicates ripeness and readiness for consumption by potential seed dispersers. I have shown an example of rose hips but there are many others. But there are quite a few examples of the opposite circumstance, where unripe red fruits turn dark when they are ripe. The firebush which is used so widely in warm climates to attract butterflies and hummingbirds to its bright orange/red flowers, illustrates the red to dark transformation. A very interesting variation on this theme is for plants to have dark fruits but a part of the plant nearby turns red in an apparent means of advertising the dark, hard to see fruits. The photo I show of sassafras illustrates this well where the dark fruit sits up on a red base. Pokeweed also has dark fruits but the stems of the plant turn bright red; Virginia creeper and black gum similarly advertise dark fruits with red leaves. Such “fruit flagging” is actually rather common once you become aware of it.

So enjoy your primate color vision, go out and look at colors, revel in the riot of natural color in plants and animals, and be awed by the subtle and diverse ways in which color is used to signal various messages.

Bill Dunson, Englewood, FL & Galax, VA
wdunson@comcast.net