It’s common to see lions or wolves working together in a nature documentary to pull down an unfortunate ungulate. Kazuki Tagawa and Mikio Watanabe have found that some carnivorous plants can hunt in packs too. The sundew Drosera makinoi (a synonym of Drosera indica) uses help from neighbouring sundews to pull in prey that would otherwise be too big to catch. Tagawa and Watanabe found that almost half the time that a sundew caught something large, it had help from another sundew.
Drosera makinoi grows by the town of Kawaminami, on the eastern shore of Kyushu Island, Japan. It’s a sundew with long slender leaves that reach into the air like fingers. The leaves are covered in sticky glue droplets. When a plant catches an insect or other animal on the glue, they struggle, and this motion activates the leaves. The leaves curl around the victim, trapping it further, and the plant releases digestive juices to pull in nutrients from its prey. But if the prey is big enough, then it is possible to escape from one of these leaves.
An animal that gets caught on two or more of these leaves has more of a problem.
Kazuki Tagawa noticed that rather than finding their own patch to hunt, the sundews grew closely together. So closely that often a large animal would be trapped in multiple leaves from different plants. Because of this, Tagawa wondered if the plants were cooperating to pull in prey or competing.
Tagawa and Watanabe decided to conduct some field observations. First, they identified some sundews with five leaves, the maximum for this plant. They then removed all the trapped carcasses. After twenty-four hours, they returned to see what the plants had caught.
“The results of our study showed that almost all D. makinoi plants (99.1%, 114/115) trapped one or more prey individuals, and that each D. makinoi plant trapped 6.27 (mean, standard deviation [SD] = 7.64, n = 115) prey individuals in 24 h,” Tagawa and Watanabe write in Plant Species Biology. “On the other hand, 40.0% (46/115) trapped one or more larger prey individuals (≥3 mm) and one D. makinoi plant trapped 0.526 (mean, SD = 0.74, n = 115) larger prey individuals in 24 h. Meanwhile, 43.4% (20/46) were caught by two trap leaves that belonged to two distinct D. makinoi plants. The major orders of larger prey were Diptera, Hemiptera and Lepidoptera.”
Putting the results together, the researchers were able to spot some correlations. The number of prey individuals dropped when more sundews surrounded a plant. This drop would appear to be bad news for plants with neighbours. However, the number of large prey animals increased as sundew density increased. This increase also led to sundews catching more biomass as neighbourhood density increased. The result was hunting next to a lot of neighbours made a plant more efficient as a hunter.
Quite how it helps to have so many neighbours isn’t entirely clear.
One reason could be that, together, the plants make a more attractive display for prey. But the authors note that Thum found that increasing density of D. rotundifolia didn’t help the plants hunt. So the display theory seems unlikely. Instead, Tagawa and Watanabe point out that D. makinoi grows long, slender leaves, while D. rotundifolia is better suited for catching small prey. Tagawa and Watanabe argue that when a D. makinoi snags an insect, having so many leaves around means pushing away from the trap is quite likely to cause it to fall onto another trap. In this way, the plants can work together to catch food.
It’s an interesting result. It’s known that working together helps plants attract pollinators, so it’s reasonable to ask if it should be true for attracting prey. So far, when people have investigated, they have found this hasn’t been the case, but Tagawa and Watanbe’s argument that the type of plant matters makes sense. Not all predatory animals hunt in packs either.