A definitive explanation for why plants evolved spines remains elusive, and human biases compound the problem.
Quanta Magazine said:Why do roses have thorns? The answer seems self-evident: Thorns, spines and prickles are plants’ defensive weaponry, making their most precious parts unpalatable — even untouchable — to big plant-eaters, like deer and other mammals. For decades, this has been the working assumption of scientists who study ecological interactions between plants and animals. The overwhelming bulk of the scientific literature on the ecological and evolutionary purpose of thorniness (or, to use biologists’ preferred terminology, spinescence) has focused on the hypothesis that mammalian herbivores are the main target.
That may have been a mistake. Over the years, studies of how well sharp deterrents discourage hungry mammals have returned mixed results. Moreover, some paleontological work offers evidence that spines evolved before the first plant-eating terrestrial animals did. Now, a report in Biology Letters suggests that, at least for some plant species, spines play an important role in defending against insects — a hypothesis that has long been ignored if not dismissed entirely. The counterintuitive results show the pitfalls of trusting human experience and intuition too much when trying to understand the evolutionary dynamics of other species in a prehistoric world.
Rupesh Kariyat, an ecologist studying plant-insect interactions in the biocommunication group at the Swiss Federal Institute of Technology Zurich (ETH Zurich), didn’t set out to upend the conventional wisdom about plant spines. As a doctoral student at Pennsylvania State University, he was working with Mark Mescher (also now at ETH Zurich) on the genetic consequences of inbreeding and herbivory on Carolina horsenettle plants (Solanum carolinense) when the pair noticed something strange. After tobacco hornworm caterpillars (Manduca sexta) fed on the plants, new shoots sported more spines. “That made us think that these spines could also have [an] additional function than what we have been taught,” Kariyat said.
Researchers had documented other plant species that grew extra thorns in response to predation by mammals. But seeing caterpillars have the same effect raised a flag for Kariyat and Mescher. From an ecological perspective, it’s wasteful for a plant to expend energy growing a structure like a thorn unless it thwarts the attacker at hand. Surely the tiny caterpillars could simply climb around or over the horsenettle’s spines.
To test whether that was so, Mescher explained, the team designed a set of experiments that could explore the question three ways. First, using the same inbred S. carolinense variety — which has fewer spines than genetically normal plants do — they examined the effects of spine density on caterpillar movement. After removing all of a plant’s leaves save the freshest, tastiest one at the top, they placed caterpillars on the soil next to the plants’ stems and clocked how long it took the bugs to reach the tasty morsel above, comparing inbred plants to normal ones. Then they went a step further by manipulating the spine density, slicing away spines with a razor blade so that all the plants had the same number, and repeated the experiment. Lastly, they expanded the research to include two other species in the same genus: S. aethiopicum, the Ethiopian nightshade, and S. atropurpureum, the purple devil. The three species vary in spine density: Ethiopian nightshade has the fewest, while the purple devil has the most. Again they compared plants with their spines intact and removed, but this time they measured how long it took caterpillars to remove all of a plant’s leaves rather than speed to the leaves at the top. If caterpillars fell off the plants during the process, the researchers put them back on an unfinished leaf and allowed them to continue feeding.
Kariyat, Mescher and their colleagues found the same pattern across all three trials: Spines were always an impediment to the hungry larvae. With the prickles cut, caterpillars more quickly reached the leaves they desired. For intact plants, the species with the most spines — the purple devil — proved most difficult for the caterpillars to defoliate. Moreover, the spines not only slowed the caterpillars but also interfered with their footing and forced them into more acrobatic maneuvers. As a result, the caterpillars frequently fell off while traveling between leafy shoots. Sometimes they even impaled themselves on the larger spines. According to Kariyat, these kinds of movement restrictions “have a huge effect on their fitness in the long term” — and thus, the authors concluded that the spines of Solanum plants are effective caterpillar deterrents.
...
