Crickets Learn to Fear Spiders Before Birth

No one would accuse female crickets of being overprotective mothers; they consider their maternal duties done once they’ve laid their eggs, and they don’t even wait around until the eggs hatch. Yet researchers now think that cricket mothers still somehow manage to pass on a few pointers to their offspring.

Although no one is sure yet how it happens, mother crickets — and perhaps other species, too — appear to be able to forewarn their unborn offspring about possible predators even if the mothers never see their brood, according to the authors of a new study. Published in the March 2010 issue of The American Naturalist, the study is the first evidence that parents can change their progeny’s reaction to predators in species where the two generations never interact.

The authors based their conclusions on experiments involving crickets and one of their predators, the wolf spider. While all crickets are naturally timid around spiders, those whose mothers had previously been exposed to spiders were more cautious when they encountered evidence that a spider might be near, and more likely to survive when faced with the spider itself. “The reaction does have an innate component to it, but it’s stronger if your mom was freaked out by a spider,” says Jonathan Storm, a biologist at the University of South Carolina Upstate in Spartanburg who is the paper’s lead author.

In order to test how a mother’s experience affects her offspring’s behavior, the researchers kept some lab-reared female field crickets in a completely spider-free environment. They put others in a terrarium with a wolf spider and the bits of silk and excrement the spider leaves behind as chemical cues.

In the wild, wolf spiders eat field crickets — and, left to their own devices, they would have done the same in the lab. So Storm put wax caps on the spiders’ fangs. “That way the female [cricket] can sense the chemical cues of the spider, the visual cues of the spider, and that the spider could potentially stalk [her], but it couldn’t actually kill [her],” he says.

Storm found that the offspring of spider-exposed mothers were much more cautious when they encountered a spider’s chemical cues. They spent 27 percent more time keeping entirely still, a common defense when a spider might be near, than the offspring of mothers who had never been in contact with a spider. The crickets whose mothers had been around a spider also spent more than twice as much time hiding, helping them to survive significantly longer in a terrarium with a hungry spider.

Most field crickets, as their name implies, live out in the field, and Storm decided that was where his research should go as well. “We wanted to see if this phenomenon was present in the natural world,” he says. The research team captured field crickets from some sites where spiders were common, and some where they were rare. After the wild females laid their eggs, Storm tested the offspring’s behavior. What he found confirmed his earlier results. Cricket broods from sites with lots of spiders were significantly more responsive to the predators’ chemical cues.

Other researchers are impressed by the findings. “I thought it was a beautiful study,” says Ann Hedrick, a University of California, Davis, biologist who studies cricket behavior. “They’ve really covered a lot of bases,” controlling for other explanations and testing their theory in the field as well as the lab, she says.

While this study didn’t provide evidence for any particular way mother crickets could pass on their knowledge of predators to offspring they’ve never met, Storm guesses it might be due to a hormone the mother crickets release in response to spiders, or due to DNA methylation — a chemical change that alters gene expression and occurs in rats as a result of maternal influences early in life.

Hedrick’s hypothesis is similar. “We know that [the hormone] octopamine is released with stress, even in crickets, so that would be one obvious thing to check,” she says. But both Hedrick and Storm stress that more research is needed to determine the mechanism.

Storm thinks that some other species probably communicate with their young in similar ways, forewarning them about possible risks even before birth. He expects that it’s more common in animals “where there’s an intimate relationship between mom and offspring” — particularly mammals, which spend more time in the womb than insects do, so there’s more time for the mother’s experience to have an effect. Frogs, salamanders, and other species that lay their eggs in water are likely candidates as well, Storm says, since chemical cues could travel through the water from mom to egg.

Hedrick agrees that the phenomenon is probably seen in other parts of the animal kingdom. “I can start looking for it now,” she said.