By: Daniel Bender, Thomas Dickey, Laryssa Kalbfus, and Emily Langmeyer (Stonehill College, BIO323: Evolution, Spring 2019)
The American Naturalist article, Testing Darwin’s Hypothesis about the Wonderful Venus Flytrap: Marginal Spikes Form a “Horrid Prison” for Moderate-Sized Insect Prey, by Davis et al. (2019) is a set of field observations and lab experiments in support of the hypothesis that marginal spikes increase the success rate of prey capture for medium-sized insects in carnivorous plants. Marginal spikes are the spikes along the edge of a Venus flytrap. As a Venus flytrap closes on its prey, these spikes create an enclosed cage-like structure securing it in place. Darwin theorized that very small insects can slip through the spaces between spikes for escape and very large insects are able to overpower the plant’s snap trap and break free, but moderate-sized insects will be trapped in a “horrid prison” should they find themselves captured. Davis et al. were able to test their hypothesis by measuring prey capture efficiency on plants with marginal spikes and with the marginal spikes removed for different sized crickets. They were able to collect these necessary data sets by performing field data collection of wild plants, a controlled laboratory experiment, and a semi-natural experiment.
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Wild field data collection was initially performed at North Carolina’s Green Swamp Preserve, where the researchers estimated prey capture rates of 14 individual plants using a flashlight to illuminate the trap from behind, making anything inside the trap visible as a silhouette. Any empty closed plant traps were recorded as 0 for unsuccessful capture, whereas closed traps containing prey were recorded as 1 for successful capture. Davis et al. began their laboratory and semi-natural experiments by purchasing Venus flytraps and creating optimal environments for them to survive in, as well as a variety of crickets to fill the role of prey. In the laboratory, prey capture experiments first were made fora single plants with marginal spikes intact, one cricket, cricket food, and a small ramp to allow the cricket to approach the plants. Each plant then had its marginal spikes removed and was given 1 week to recover after the initial laboratory experiment was performed. The size of the trap, the mass of the cricket, and the closing speed were all measured. The plants were checked for closed traps after 3 days and 1 week. After the laboratory study was performed, Davis et al. performed a semi-natural experiment where 22 Venus flytraps were planted in a botanical garden with half having intact marginal spikes and the other half having them removed. The catching data and the trap length data was recorded in the same fashion as the laboratory study and wild field data collection.
Shortly after completing these field, laboratory, and semi-natural experiments, Davis et al. were prepared to determine prey capture success rates of their Venus flytraps. They concluded that even with the spikes intact, there was a very small percentage of prey getting caught in each trap. In the field experiment, only 24% of the traps contained prey, and in the lab, the prey capture rate was even lower at 16.5%. Since other carnivorous plants have similarly low rates of success, it is theorized that the poor efficiency is actually beneficial because it makes it less likely that insects would learn or evolve to avoid the plants. In the field experiment, there was also a 73% decrease in prey capture rate with every 0.1-gram increase in prey mass. For every one-centimeter increase in trap length, the prey capture rate increased by a factor of 2.9.
The researchers discovered that marginal spikes increased the likelihood of prey capture in each experiment. In the laboratory experiment, traps without spikes are 90% less likely to catch prey overall, and the semi-natural experiment showed a change in rate success from a 13.3% capture rate to 9.2% capture rate with the spikes removed. However, marginal spikes do not seem to make much of a difference with larger traps or large prey. This might be because some of the larger crickets were seen climbing on the marginal spikes on the larger traps and using them to pry themselves free. This supports Darwin’s hypothesis that marginal spikes aid in capturing medium-sized prey, but not large or small prey. More experiments should be performed with other types of prey to find out if there is a decline in prey capture rate for the extremely small prey and to find out if other large insects besides crickets can escape the same way. In conclusion Davis et al. were able to demonstrate that the novel marginal spikes forming a “horrid prison” are an adaptation for prey capture with nonlinear effects on larger prey and trap sizes.