No one likes a mouthful of sand. Even that single speck of grit that crunches in your molars after a day at the beach is maddening. It turns out non-human animals aren’t fans of eating sand either. That’s why certain plants use sticky hairs to coat themselves in layers of grit. For keeping hungry animals away, it works like a charm.
Eric LoPresti, a graduate student in ecology at the University of California, Davis, and his advisor Richard Karban have listed over 200 species of plants that coat themselves in sand. (The trick is called “psammophory,” if you’re looking to expand your vocabulary in a mostly useless way.) These plants grow in deserts and dunes all around the world, where they trap sand that the wind blows at them.
Scientists have suggested various theories to explain the sandy plants. Maybe sand makes plants more difficult for animals to eat. Or maybe camouflage is the reason: a coating of sand makes these plants blend into their surroundings, disguising them to animals. “The conventional wisdom also says that it protects them from sandstorms,” LoPresti says, especially in Africa’s Namib Desert, where sandy plants were first described.
LoPresti and Karban tested a couple of these theories right in their neighborhood of Northern California. They looked at two different plants. One, the sand verbena Abronia latifolia, is common on dunes along the California coast. It has clustered yellow flowers and is “really sticky to touch,” LoPresti says. The second plant was Navarretia mellita, or the honeyscented pincushion plant. It surrounds its white-and-blue flowers with nasty, sticky prickles and is “unpleasant” to feel, LoPresti says.
Both plants have enemies in the animal kingdom. The sand verbena gets nibbled by snails, caterpillars, mice, and maybe deer. As for the pincushion plant, some indifferent animal (probably a jackrabbit) likes to bite off the entire flowering heads—spikes and all.
To test the idea that sand makes plants less palatable to these animals, the researchers did a few simple experiments. They chose a batch of A. latifolia growing in the wild and used a damp sponge to gently wipe half the stalks clean of sand. They returned a few times over the next two months to re-clean the plants and check on the damage they’d received so far.
The prickly, intricate N. mellita plants were harder to clean. So the scientists chose plants in the wild and added sand to half of them instead. These plants “received a gentle shower” of sand, they write, which clung to their sticky foliage. Again, they returned over the next two months to re-shower the plants and check on damage.
In both species, the results were dramatic. Among sand verbenas, clean plants had twice as much damage from animals. And among the extra-sandy pincushion plants, only 1 out of 19 got beheaded by a browsing rabbit—compared to 8 out of 18 untreated plants.
Was this difference all due to the unpleasantness of eating sand, or was camouflage a factor? To find out, the researchers did another experiment on the sand verbenas. This time, they cleaned some stems and dusted others with new sand. This sand was either the usual tan color, to match the plants’ background, or dyed green to match their stems. If sand normally protects plants by camouflaging them, then the plants coated in green sand should be fair game for animals.
But to the scientists’ surprise, the tan plants got eaten just as often as the green ones. Camouflage doesn’t seem to be the reason for A. latifolia‘s sand armor. Instead, the grossness of chewing on sand seems like the main thing protecting these plants.
“We were both a little bummed” not to prove the camouflage theory, LoPresti says, because they thought they’d found a perfect test for it. But he doesn’t think what they’ve seen so far is the whole story. Although the sand verbena may not use camouflage, LoPresti is confident that “some plant, somewhere” does get a benefit from blending in with sand.
And there are still other possible benefits of sand armor that no one has tested. Sand might protect plants from radiation, help them hold on to water, or protect them from sandstorms. The plants need to get some benefits to balance out the costs of coating themselves in sand (such as getting less sunlight and having to manufacture their own glue). Plants in different parts of the world may have different reasons for covering themselves in grit. But no matter the reason, a plant that looks bedraggled and half-buried to human eyes is really a well-protected fortress.
Images: Eric LoPresti
Eric F. LoPresti, & Richard Karban (2016). Chewing sandpaper: grit, plant apparency and plant defense in sand-entrapping plants. Ecology : 10.1890/15-1696