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.
In the early days of the last US elections, Hillary Clinton’s campaign was accused of deliberately darkening Barack Obama’s skin in a TV ad. The implication was that by highlighting Obama’s “blackness”, Clinton’s camp was trying to exploit negative associations that voters might have with darker skin. But you don’t need editing software to do that – a fascinating new study suggest that people literally change the way they see a mixed-race politician, depending on whether the candidate represents their own political views.
Liberal American students tend to think that lighter photos of Barack Obama are more typical of him, while conservatives think he’s best represented by darker photos. You can see this effect even after adjusting for any racial prejudices, be they hidden or overt, and even with a person less famous than Obama. And regardless of political views, people who associated Obama with lightened photos were most likely to vote for him.
Eugene Caruso from the University of Chicago, who led the study, thinks that this effect is the result of two biases: the positive associations of white and lightness among some Western cultures; and the tendency to view people of the same group (political or otherwise) more favourably than those of another group. He says, “Group membership provides a lens through which people generate representations of reality.”
Caruso asked 221 students about their political ideologies and then showed them three photos of Obama and three of John McCain. On the grounds that some photos can capture the “true essence” of a politician better than others, the students were asked to rate how well each photo represented each man. But unbeknownst to them, two of each set of pictures had been altered with Photoshop, so that the subject’s skin tone was either lighter or darker.
When it came to McCain, the students’ political leanings had no bearing on their choice of photos. For Obama, it was a different matter – liberal students were more likely to pick the lightened photo as the one that represented him best. Conservative students were more than twice as likely to associate him with the darkened photo. These biases were reflected in the students’ votes. Whether liberal or conservative, the more people associated Obama with the lightened photo, the more they were likely to vote for him.
Of course, this effect could simply be down to racism – people who harbour prejudices against Blacks would be more likely to associate Obama with a darker photo and less likely to vote for him. But Caruso accounted for that – he repeated the experiment with 49 people a week before the last election and specifically evaluated the recruits’ attitudes on race.
Each of them filled in a questionnaire called the Attitudes Toward Blacks scale, which asked them whether they agreed with statements such as “Generally, Blacks are not as smart as Whites.” Obviously, people can lie on these questionnaires, so each volunteer also did an “implicit association test” (IAT), designed to reveal any hidden prejudices (try one here).
The same patterns emerged as before and this time, Caruso found that they remained even after adjusting for racial attitudes, both hidden and explicit. A week after the election, Caruso caught up with his recruits and confirmed that those who thought the lightened photos represented Obama were actually more likely to have voted for him. Those who linked him to the darkened photo were more likely to have voted for McCain. Amazingly, the photo effect turned out to be a better indicator of voting choice than the scores on either of the two prejudice tests.
Barack Obama’s fame is perhaps a bit of a distractor since people can judge him on his policies, personality and more than just his skin colour. But Caruso found the same effect using a non-celebrity. He asked 102 students about their views on important educational issues and then showed them three photos of a man allegedly running for a position in the US Department of Education. He told them that the mystery politician either agreed or disagreed with most of their stances.
But Caruso kept two important things from the recruits. First, the ‘politician’ was actually Jarome Iginla, a mixed-race ice hockey player whose father was a Black Nigerian and whose mother was a White American. None of the students twigged to this. Secondly, as before, some of the photos had been doctored so that Iginla’s skin tone was either lighter or darker.
The same trend emerged. Caruso found that students who were told that the politician supported their views were more than twice as likely to pick the lightened photo as the most representative one. Those who thought that Iginla disagreed with them were more likely to associate him with the darkened photo. And across the board, people who picked the lightened photo were most likely to vote for him.
Across all three experiments, the way that American students literally see a mixed-race politician depends on whether they agree with his views. If they felt aligned with a candidate, they tended to mentally lighten his skin, and Caruso suggests that this might reflect subconscious associations of white with good, and black with bad. Sadly, the study provides no information about the ethnicity of the students involved – it would be very interesting to see if the same bias in perception applies to viewers who are themselves Black.
Reference: PNAS doi:10.1073/pnas.0905362106
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