Remarkable similarity between fluffy ant and fruit is not due to mimicry

Although they look alike, the white coloration of desert thistledown velvet ants and the fruits of the creosote bush is actually not due to the ants mimicking the plants.

Thistle-down velvet ant. © Joseph S. Wilson

Sometimes, the most obvious answer to a question isn’t the right one. Take the uncanny similarity between the fruits of the creosote bush and the thistledown velvet ants that share their North American desert habitat. New research overturns a widespread consensus that it’s a straightforward case of mimicry.

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Don’t let those soft, fluffy exteriors fool you. Both the velvet ants and the creosote fruits pack a formidable punch. The plants are rammed with an arsenal of pesticidal toxins, while the insects (which are wingless wasps rather than true ants) are armed with fearsome stings.

It has long been assumed that their striking resemblance comes down to mimicry. The story goes that the ants are harder to spot amongst the fallen fruits. And perhaps each gains a degree of protection from predators who are deterred by the other’s defences.

A thistledown velvet ant (left) and the fruit of a creosote bush (right). © Joseph S. Wilson
A thistledown velvet ant (left) and the fruit of a creosote bush (right). © Joseph S. Wilson

But when Joseph Wilson of Utah State University started taking an interest in the evolutionary history of North American deserts, he realised that all was not how it seemed.

“The creosote bush did not evolve in North America, rather it evolved in South America and only relatively recently arrived in the north – probably during the last ice age, based on fossil evidence,” says Wilson. By that time, he says, the velvet ants had already developed their white bristles.

What’s more, Wilson and his colleagues have shown that the fruit and ant reflect light rather differently – to the extent that they may look similar only to human eyes and not to those of the predators that eat them.

“Basically, we realised there was a disconnect in the common narrative people used to explain the white coloration,” says Wilson.

So is it all just an unlikely coincidence? It seems not. Wilson’s experiments suggest that, by reflecting heat, the white bristles provide protection from the intense desert heat. In which case, the fruit and the insect look alike because they have come up with similar solutions to a common problem.

Read the paper in Biology Letters.


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Main image: Thistledown velvet ant. © Joseph S. Wilson