What Does it Take to be Indestructible?

The indestructible insect: Velvet ants from across the United States avoid predation by representatives from all major tetrapod clades (2018), Gall et al., Ecology & Evolution. https://doi.org/10.1002/ece3.4123

Image credit: Adam Hasik, image cropped

The Crux

Predation is a selective force that everyone is familiar with. One organism (the predator) kills and consumes another (the prey), and there is usually little nuance to the outcome of this interaction. The prey either escapes and survives, or it is killed and eaten. Due to this extreme pressure, prey organisms have evolved a remarkable array of defensive abilities and behaviors to attempt to reduce predation. Some colorful examples include the pufferfish and its ability to greatly increases its size, the octopus and its ink, or the hilarious (yet effective) behavior whereby the killdeer (a small bird here in North America) will make a lot of noise and fly a short distance before pretending its wing is broken in order to distract a predator from its offspring.

One animal that possesses a suite of such defensive abilities is the velvet ant (Dasymutilla spp.). Despite their name, velvet ants are a group of parasitoid wasps covered in a fine layer of setae (the velvet) where the females are wingless and look like ants. Because these females spend most of their time searching for ground-nesting insects to lay their eggs on/in and cannot fly, one might expect that these insects are particularly vulnerable to predators. But what’s really cool about these insects is just how many defenses that they have to ward off predators. First and foremost, they are brightly colored (just LOOK at that thing, nothing about that insect says “eat me”), which is usually enough of a warning in the natural world. Beyond their coloration, females also possess a venomous sting that is reputed to be one of the most painful stings in the world (see Did You Know?). I mean, that velvet ant in the featured image is colloquially known as the “cow killer” because of its painful sting. Velvet ants also possess a remarkably thick exoskeleton that is difficult to crush, and because it is rounded bites and stings tend to glance off of the abdomen. Today’s authors sought to understand just how effective all of these defenses were for reducing predation.

Did You Know: Schmidt Sting Pain Index

Justin O. Schmidt is an American entomologist known for his creation of the Schmidt Sting Pain Index, a comprehensive list describing how painful many insect stings. I admire the dedication and courage of this man, who took it upon himself to create this list by getting stung by every animal he could. The index goes from 1 to 4, with 4 being the most painful. Most small bees and the fire ant are considered to be of Pain Level 1, while the cow killer (featured image) causes “intense agony and pain” and is rated at Pain Level 3.

What They Did

Because only one previous study has investigated interactions between a velvet ant species and a natural predator, the authors chose to focus on a number of potential predators that may consume velvet ants in nature. These predators included birds, moles, shrews, toads, and lizards. The authors used a number of different tests to understand if and how often a predator consumed velvet ants relative to other prey items (crickets). Predators were either offered live velvet ants or live crickets, mealworms painted the same colors as velvet ants vs. mealworms painted with less “dangerous” colors, or dead velvet ants vs. dead mealworms.

What They Found

Predators tended to avoid velvet ants, yet had no such reservations about eating the other prey items presented (crickets or mealworms). Despite using a number of trials with multiple predator species, only one instance of successful velvet ant predation was recorded. A toad swallowed a velvet ant and did not regurgitate it, though the toad appeared to be in a significant amount of distress for quite a long time, so much so that the authors briefly thought it had died. Some predators struck at/swallowed a velvet ant, but those that did avoided the velvet ants afterwards or (for those that swallowed an ant) quickly regurgitated them. All predators that had interactions with velvet ants, including the single toad that consumed a velvet ant, avoided all interactions with velvet ants upon a secondary exposure.

This blister beetle (Epicauta conferta) is another insect that uses bright coloration and chemical weaponry to warn off would-be predators from trying to make a meal of it (Image credit: Adam Hasik).

Problems?

Though they used a large number of tests, the authors used predators that “might” be natural predators of velvet ants. Because the predators used in this study were not definitive predators of velvet ants, these results have to be interpreted with some caution. The results make it clear that the velvet ants tend to avoid predation by every predator used, but there is a possibility that there are predators that have evolved adaptations that make them well suited to consume velvet ants, and as such the findings of this study that velvet ants avoid predation may not hold for other predator species.

So What?

The velvet ant is well known for its myriad defensive abilities, and today’s study showed that these abilities make the velvet ant a very difficult meal for most predators. While no organism is truly indestructible or invulnerable, the authors demonstrated that the velvet ant may be the closest thing we have to it in the natural world. Evolution is an amazing process, and the fact that the velvet ant has evolved such an effective suite of defensive abilities goes to show what a strong and pervasive effect predation has on prey evolution.


Dr. Adam Hasik is an evolutionary ecologist interested in the ecological and evolutionary dynamics of host-parasite interactions and is a BIG fan of the velvet ant. You can read more about his research and his work for Ecology for the Masses here, see his personal website here, or follow him on Twitter here.

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