Whether we’re eating prawns or fish, chicken or sheep, we tend to remove the guts of animals before eating their meat. There’s another predator that shares our culinary practice: the Chinese mantis.
The mantis, a finger-sized animal found in the eastern US, is one of the few hunters that successfully eats the toxic caterpillars of the monarch butterfly. These larvae are poisonous enough to ward off ants and birds, but the mantis has a special trick for dealing with them—it guts them. It removes their intestines before eating the rest of their bodies in safety.
Monarch caterpillars take in toxic chemicals called cardenolides from the milkweed plants they eat. Rather than succumbing to these poisons, the caterpillars store and repurpose them for their own defence. And they advertise their chemical payload with warning colours—bright stripes of yellow, black and white, running down their flanks.
Some predators can get around this. Birds like orioles and grosbeaks will sometimes eat the innards of the adult butterflies, avoiding the outer layers that are richest in toxins. Ants and ladybirds eat monarch eggs, or very young hatchlings that haven’t had a chance to build up their cardenolide stockpile. But the older caterpillars—the ones that have had a lifetime of storing cardenolides—are safer. Assassin bugs and hungry wasps will tackle them, but not much else.
That is, except the Chinese mantis (Tenodera sinensis). While doing unrelated experiments in a local field, Jamie Rafter from the University of Rhode Island noticed that mantises would gut monarch caterpillars before eating them. It’s not a delicate process. After grabbing a victim, the mantis starts nibbling at it, chews open a hole, and lets the guts fall away. Around 40 percent of the caterpillar goes to waste.
This isn’t typical behaviour for the mantis. Rafter showed that when they captured the non-toxic caterpillars of the greater wax moth or the European corn borer moth, they ate everything, guts and all. They only left 14 percent of these meals, and even then, only because bits of blood would messily dribble away when they ate.
The obvious explanation is that the mantis is trying to avoid the poisonous bits of the caterpillar’s body, but things aren’t that simple. Rafter found that the caterpillar’s guts have exactly the same level of cardenolides as the rest of its body, so the mantis isn’t actually avoiding the poisonous tissues. But then, why gut the monarchs, while eating the palatable moths whole? What’s going on?
There are two possibilities. The mantis might only be vulnerable to some cardenolides and not others. The caterpillar’s body has around three times as many types of these chemicals as its guts, but at lower concentrations. This suggests it’s processing or breaking down the cardenolides that it gets from the milkweed before storing them in its other tissues. Maybe the mantis can tolerate these processed forms, but is trying to avoid the originals in the guts.
Alternatively, the mantis might just find the guts distasteful, or not worth the effort. These organs tend are usually filled with chewed-up plant matter and contain 58 percent less nitrogen than other tissues. So perhaps the mantis is just feasting on the richest tissues and discarding the nutrient-poor ones. And by happy coincidence, that reduces the total amount of poison that it consumes.
Rafter noticed one pattern that supports this second idea: she watched their mantises eating 21 caterpillars, and they only gutted 18 of them. The other three were all harbouring parasites! Two of them contained the larvae of a tachinid fly, which were slowly devouring them from the inside out. The third was heavily infested by a fungus. As a result of these body-snatchers, the caterpillars had barely eaten any milkweed. When the mantises broke into their bodies, they found no plant matter in their guts. So, they just ate the lot.
Either or both explanations might be correct. Regardless, Rafter’s discovery might explain why the Chinese mantis has done so well in the eastern US, since its introduction from China. Its culinary antics might have given it access to a source of food that other predators left alone.
Hat tip to Jeramia Ory for telling me about this paper
Reference: Rafter, Agrawal & Preisser. 2013. Chinese mantids gut toxic monarch caterpillars: avoidance of prey defence? Ecological Entomology http://dx.doi.org/10.1111/j.1365-2311.2012.01408.x