In the Lord of the Rings, Gandalf rides upon a magnificent white stallion called Shadowfax. White horses have been greatly prized in human societies as a sign of wealth and dignity, largely because their bright coats are both pretty and rare. There are reasons for that. In the wild, the same conspicuousness that inspires legendary tales also makes white horses vulnerable to predators and sensitive to skin cancer. But they have an unexpected benefit – they make horses less attractive to horseflies.
Anyone who has been bitten by a horsefly (formally, a tabanid) knows that they’re much more irritating than your average midge or mosquito. Rather than puncturing skin, their mandibles are designed to rip and shear. As a result, their bites hurt and they can drive grazing animals to distraction. They can also transfer serious diseases, including Equine Infectious Anaemia, parasitic worms, and even anthrax.
Now, Gabor Horvath from Eotvos University, Hungary, has found that white coats are more horsefly-proof than darker ones. They reflect very little polarised light – light vibrating on a single plane – and it’s this light that horseflies use to track down their next blood meal.
On a sunny June day, Horvath watched two horses – one brown and one white – as they grazed in a local field. Both were almost continuously attacked by horseflies and had to defend themselves by tail-swishing, kicking, shuddering, head-swinging, biting, licking and even rolling on the group. But the white horse had the better time of it – photographs revealed that, on average, the brown horse had 3.7 times more horseflies on or near it. Eventually, the attacks were so irritating that the horses were driven into a nearby shady forest, where they gained a temporary respite. Again, the brown horse was always the first to cave and spent longer in the shade.
In the White Sands National Park of New Mexico, there are three species of small lizard that all share white complexions. In the dark soil of the surrounding landscapes, all three lizards wear coloured coats with an array of hues, stripes and spots. Colours would make them stand out like a beacon among the white sands so natural selection has bleached their skins. Within the last few thousand years, the lesser earless lizard, the eastern fence lizard and the little striped whiptail have all evolved white forms that camouflage beautifully among the white dunes.
Erica Bree Rosenblum from the University of Idaho has found that their white coats are the result of changes to the same gene, Mc1r. All of these adaptations arose independently of one another and all of them reduce the amount of the dark pigment, melanin, in the lizards’ skin. It’s a wonderful example of convergent evolution, where the same environmental demands push different species along the same evolutionary paths. But Rosenblum has also found that there are many ways to break a gene.
Each of the three lizards has a different mutation in their Mc1r gene, that has crippled it in diverse ways. These differences may seem slight, but they affect how dominant and widespread the white varieties are, and how likely they are to branch off into new species of their own. Even when different species converge on the same results – in this case, whitened skin – and even when the same gene is responsible, their evolutionary paths can still be very different.
The Mc1r gene encodes a protein called the melanocortin 1 receptor (MC1R). It’s a messenger that sits astride the cell’s membrane and transmits messages across it. It triggers a sequence of events that stimulates the production of the dark pigment melanin. In this way, it affects the skin colour of many animals and faulty copies of the gene tend to result in lighter colours. In humans, for example, around 80% of redheads owe their hair colour to common faulty variant of Mc1r.
In each of the White Sands lizards, just one of the MC1R protein’s many amino acids has been swapped (red circles above), and it’s a different one in each species. All three amino acids lie within the part of the protein that straddles the cell membrane. These regions are important for keeping the protein together, and for channelling signals from one side of the membrane to another.