So far, in my previous blog posts I’ve talked about individual colours and how they were created and used in Ancient Egypt (see the beginning of the series here). But let us now explore a fascinating property which brings them all together – iridescence. It’s a phenomenon whereby surface colour appears to change with the angle of viewing or illumination and is caused by an optical effect rather than pigmentation. The word itself derives from the Greek goddess of the rainbow – Iris, while the Latin suffix ‘-escent’ means having a tendency towards something. A perhaps less glamorous term for iridescence, goniochromism, can also be traced back to Greek words ‘gonia’ meaning angle, and ‘chroma’ meaning colour.

Iris Carrying the Water of the River Styx to Olympus for the Gods to Swear By, Guy Head, c. 1793 – Nelson-Atkins Museum of Art (Photo: Daderot).

Iridescence is a type of structural colouration and occurs in the natural world (e.g. insects, birds) as well as in man-made materials (glass, soap bubbles, playing surface of a CD).

Blue Morpho butterfly showing off its glorious colour (Photo: Derkarts).

A brilliant example of the use of iridescence in nature can be found in the Blue Morpho butterfly (Morpho menelaus) whose upper wings appear to be bright blue. It is one of the largest butterflies in the world and can be found in South American rainforests. Those beautiful and rare butterflies use iridescence to evade predators by becoming briefly invisible! As they fly, the colour of their wings shifts between brilliant blue and brown, so against the background of the forest and sky they seem to disappear for a flash just to reappear a little further away, confusing anyone who might be trying to catch them.

Perhaps a more familiar example of iridescent colouring is mother-of-pearl, or nacre, which has long been admired and used for many decorative purposes, from jewellery to furniture, artwork to cutlery. Some specimens can even be found in the Petrie Museum collection. In nature, nacre occurs on the inner shell of some molluscs (such as abalone sea snails) or on the surface of pearls. Its purpose is once again defensive as the molluscs secrete layers of nacre on the inner surface of their shells to protect the soft layers beneath from parasites and debris. As a material, nacre is made up of tiny hexagonal platelets of aragonite, a form of calcium carbonate. The thickness of the platelets (between 300 and 1500 nm) allows them  to interfere with different wavelengths of visible light at various viewing angles, creating an iridescent effect. However, studies using Scanning Electron Microscopy (SEM) have shown that the effect is also partially caused by diffraction resulting from a high groove density of the surface.

Inside of an abalone shell (Photo: Marac).

Some plants have also evolved to use thin layers of photosynthetic structures, called iridoplasts, to bend and absorb more light in dark environments such as the lower levels of tropical forests. This causes the surface of their leaves to appear iridescent and almost glowing in the dark. For instance, peacock begonia (Begonia pavonina) from South East Asia shows a beautifully intense metallic blue as it amplifies  the small amount of visible light it receives. The iridoplasts bend the light repeatedly thus making very efficient use of long red and green wavelengths while reflecting the blue ones.

Peacock begonia (Photo: Shyamal).

Many more examples of iridescence exist in nature and this blog post could easily become a very long article if I attempted to include them all. I guess it’s very easy to assume that this phenomenon is mainly decorative and meant to create attraction, like peacock’s feathers for example. But, as we can see, there are plenty of instances where the effect serves a purpose very different to what we might originally have imagined or is an almost accidental by-product of a completely unrelated function . In my next post I will explore how one man managed to replicate natural iridescence for purely ornamental purposes, so stay tuned for Part 2!

This is the third segment in a series on incest; you can go back and read the previous segments on incest in ancient Egypt and incest in the Hapsburg family.

Firstly, did you know that despite the earliest forms of life emerging around 3.8 billion years ago, sex has only existed for 1.2 billion years? Before that, asexual reproduction was the only form of reproduction to evolve. When you think about it, this is the most extreme type of incest, reproducing yourself with …yourself, cloning yourself. Nowadays, most mammals tend to not engage in inbreeding. If they do, we have seen that incest can lead to depression inbreeding with offspring experiencing health problems. For this reason, scientists used to think that Nature might have weeded out incestuous behaviour through natural selection.

However, recent studies have actually shown that incestuous behaviour has not completely disappeared and that it is more common than generally thought. Some species are asexual or still breed with themselves in situations where there is no advantage to sex; others commit incest where there is no penalty to inbreeding. And guess where those incestuous species are mostly found? Islands and mountaintops. In these isolated places, it is difficult to find someone who does not fit somewhere in your family tree.

Incestuous species

• Mongoose

Mongoose live in close-knit groups with a median size of 18 adults. Each group has both male and female dominant members, who do most of the breeding and reproducing—those on the periphery only reproduce occasionally. Most group members remain with their group for their entire lives. This close-knit living arrangement has led to a high incidence of incest. A study has found that 64% of newborn pups were the result of mating between members of the same natal group (Nichols, 2014). Father/daughter incest was documented eight times over the course of the study run over 16 years; no mating attempts between mother/son were reported. The researchers point out that females tend to have short lives and generally die before their sons are old enough to mate with them.

Yellow mongoose, Cynictis penicillata

• Whiptail lizards

This one is with no doubt my favourite.

Some women might have dreamed of a world with no men. Whiptail lizards have done it. Females whiptail lizards are able to clone themselves. And this is not the only species with this capability. There are actually quite a few, 80 groups to be precise, which include amphibians, reptiles, and even fish. But the specificity of these female lizards is that though they don’t need to have sex to survive, they still display mating behaviours, meaning that females sometimes mount other females. Scientists think this behaviour is hormonally driven; high progesterone levels may cause females to mount others. But they probably don’t just bump cloacal regions for fun. Studies have shown that females who are mounted by another female are more fertile than those who go it alone, likely because the mounting behaviour promotes ovulation (Wade, 2013).

Mating behaviour among whiptail lizards: female lizard mounting another female.

• Spotted salamanders

Among spotted salamanders, DNA analysis shows inbreeding at the level of first cousins, on average. Despite having hundreds of possible mates to choose from, females tended to fertilize their eggs with sperm from related males.

Spotted salamander

Interestingly, in some cases, the natural selection mentioned earlier seems to contradict other studies showing that for some animal or insects, inbreeding within first cousins or brother/sister gives better chance of survival to the offspring. Inbred ambrosia beetles, for example, fared no worse than outbred insects, and the eggs produced by brother-sister pairs are likelier to hatch than the eggs of unrelated pairs (Andersen 2012). Similarly, another study has found that for at least one fish species, fathers from brother-sister couples spent more time, on average, defending their caves and that both parents tended to pay more attention to their kids than unrelated couples.” How to explain this? The ecologist who supervised the study reports, “Couples which are full siblings are more cooperative in brood care. … [T]he males and females stay with the offspring for several weeks and guard them—they defend them—and there’s less aggression between full siblings.”

Stay tuned for next and final segment in a series on incest. We will talking about the practice of incest in modern societies: Modernization or cultural maintenance?

 References

Andersen, H., Jordal, B., Kambestad, M., & Kirkendall, L. (2012). Improbable but true: The invasive inbreeding ambrosia beetle Xylosandrus morigerus has generalist genotypes. Ecology and Evolution, 2(1), 247-257.

Nichols, H., Cant, M., Hoffman, J., & Sanderson, J. (2014). Evidence for frequent incest in a cooperatively breeding mammal. Biology Letters, 10(12), 20140898.

Wade, J., Huang, J., & Crewst, D. (1993). Hormonal Control of Sex Differences in the Brain, Behavior and Accessory Sex Structures of Whiptail Lizards ( Cnemidophorus Species. Journal of Neuroendocrinology, 5(1), 81-93.