Nevertheless, the team does not insist that their results prove insects were the primary reason that plants developed spines, prickles and thorns. To the contrary, Kariyat said, We think that spines evolved against mammalian herbivores. But they suspect that at some point in history, the horsenettles and other plants found an even more effective weapon toxic alkaloids in their tissues and mammals stopped eating them regularly. The caterpillars, which were largely unaffected by the alkaloids, became specialists at preying on the plants. Evolution may then have co-opted the plants spines for a new defensive purpose, a phenomenon known as exaptation. So over time, these spines have started to have an additional benefit, helping the plant win the arms race against the insects, he said.

The unexpected results were a bit hard for others in the field to swallow. When I first read this paper, my overwhelming response was: Oh, thats just [nonsense] there are so many problems! said Angela Moles, a research professor at the University of New South Wales in Australia, who studies the ecological strategies of plants. Then the more I read it, the more I was, like, Actually, its right. Her sentiments were echoed by Mick Hanley, an associate professor at Plymouth University in the United Kingdom, who was lead author on a 2007 review paper about plants structural defenses. I looked at it atfirstand Ithought,hmm. Then I read it again, and I saw that it all sort of hangs together, he said.

Others are less convinced. Im not sure that we can conclude from their results that spinescenceis an adaptation against insect herbivory, said Tristan Charles-Dominique, a plant-evolution specialist at the Xishuangbanna Tropical Botanical Garden in China. He and William Bond, emeritus professor at the University of Cape Town in South Africa, used phylogenetic methods to show in 2016 that the diversity of spiny plants in Africa coincided with an uptick in bovid mammals, such as wildebeests and gazelles. That finding supports the traditional view that spines defend against large mammals.

I think they did a good job at showing that the feeding rate of caterpillars is indeed slowed down as their movement is disrupted, said Charles-Dominique, but I think that there is quite a lot of information to be gathered before being able to test the potential coevolution between spiny plants and insects.

Kariyat does not disagree. One of the things we want to see is whether this effect is just on the caterpillars, he said. He explained that they also want to better quantify how insects are affected by impaired movement: How much does it affect their growth and development and pupation, and how does that affect them long term?

But Kariyat and Meschers findings arent the first to suggest a potential role for spinescence in deterring insects. Moles noted that the results are consistent with paleontological evidence that spines evolved before large herbivores. She pointed to a 1970 review paper by the late British paleobotanist William Gilbert Chaloner, in which he noted that a number of plants from more than 400 million years ago showed small apparently non-vascularized appendages on the stem, distributed more or less randomly and variously termed spines, emergences, teeth or enations.

So weve got a lot of species with these funny prickles that we dont know what they do, Moles explained, and its, I dont know, at least 10 or 20 million years before the first terrestrial vertebrate herbivores start appearing.

The results highlight a tricky issue in the study of evolution and adaptation: Because of exaptation, understanding the current function of a trait is very different from inferring the evolutionary pressures under which it initially evolved. Its impossible to track why a defense that works now might have evolved millions of years ago, Hanley said. Those spines could have evolved for a completely different reason that has nothing to do with herbivory.

Indeed, hypotheses about the original purpose of plant spines range far beyond the deterrence of herbivores of various sizes. Its been theorized that the spines might have boosted plants surface area to enhance photosynthesis, or that they helped to direct water to the plants roots. Or that they first helped plants to sprawl and climb. Other than being able to replay the evolutionary tape over millions of years, Hanley said, weve got no real way of knowing.

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The Thorny Truth About Spine Evolution - Quanta Magazine