The predator-prey relationship has produced all sorts of novel defenses, from animals that load themselves with toxins to others that have evolved the appearance of a toxic species but actually don't produce toxins themselves. In today's issue of Science, a paper describes a form of camouflage that specifically targets the ability of bats to hunt via echolocation: some species of tiger moths can apparently emit ultrasonic chirps that jam the bat's ability to zero in on it.

But before the moths could use the chirps for jamming bat signals, they first evolved them for other purposes. There are a number of species of tiger moths, and many have an organ called a tymbal that allows them to emit noises in the ultrasonic range used by many species of bat to locate potential prey. In at least some cases, the noises act in a similar manner to the bright coloration adopted by many species that produce toxins: they warn predators off. Bright colors won't work to warn an animal that hunts in the dark without relying on vision, so moths that produced toxins clearly needed an alternate way of alerting bats that they wouldn't make a good meal. Producing noises in a frequency range that the bats were clearly paying attention to provides an obvious solution.

But the authors of the new paper note that past research has shown that the use of tymbals is widespread among tiger moth species, the production of toxins less so. Two alternate explanations had been offered: the chirps from non-toxic species might startle the bats, or it could serve to jam their navigation.

The authors note that the three explanations—warning, startling, and jamming—should produce different patterns of predation. Bats raised in captivity should eat a few toxic moths before avoiding them, producing a downwward curve of predation. Since animals habituate to being startled, that should produce an ascending predation curve. Jamming, in contrast, should stay at a steady, low state, since it would presumably take a while for bats to learn to cope with it.

The researchers chose a tiger moth species (Bertholdia trigona) that was thought not to be toxic, and an unrelated species known to be eaten by bats. They used captive big brown bats, an insectivore that overlaps with these species in their range in North and Central America. The moths were tethered, and the bats let in to the room to hunt on nine consecutive nights.

For unknown reasons, about a quarter of the bats never bothered with the tiger moths, even when the tymbal was damaged, and so were excluded from the study. The rest happily ate them at rates similar to the control species. The rates of feeding remained constant throughout the experiment, which is contrary to the prediction of either the warning or the startling models.

But the authors also found some positive evidence for the jamming model. Rates of successful hunts of moths with intact tymbals were only about a quarter of the rates seen when the tymbal was damaged. Tracking the animal's hunting patterns in the room shows that, as animals approached a jamming tiger moth, they frequently wound up resetting their approach-track-terminal hunting pattern, going back from tracking a moth to approaching it, or from the terminal attack to tracking.

The authors suggest that the tymbal was probably adapted for warning the bats off a toxic species before becoming specialized for jamming the bats' echolocation. We see that sort of pattern in many cases when it comes to visual signals exchanged among predators and prey. Still, it's fascinating to that what works visually can also function just as effectively in the dark.

Science, 2009. DOI: 10.1126/science.1174096

Listing image by Nickolay Hristov