Friday, August 14, 2009

The bat and the moth, and the ant and the butterfly

What better way to resume blogging after a break than with a couple of fascinating stories from the natural world, about predator and prey, defensive arms races and survival cues?
Bats invoke a variety of emotions from people, ranging from disgust and (unfounded) fear to “they’re cool”. At least the Batman sometimes did some good in helping kids get rid of their fear or paranoia of bats. But while bats might look like silly rats with wings, they are indeed supremely efficient hunting machines. While some bats eat fruit, a majority of them hunt flying insects in the dark, and eat vast quantities of moths, locusts, flies, mosquitoes and any other bug that flies. In order to do this efficiently in the dark, they have a fabulously developed system of “echolocation”, a better sonar system than most battleships. This is great for the bat, and allows them to locate flying insects with pin-point accuracy. But what about the insects? Obviously, they have a pressing need to survive and need to escape bats. Therefore many insects have evolved remarkable ways to evade their hunters.

Some insects have developed evasive flying maneuvers, others just taste bad and the bats learn to avoid them, while others yet have evolved a neuronal auditory system that can detect the bat sonar frequency and allow them to escape. So there is this constant fight between bat and insect in evolving better sonar or ways to evade it. But, taking a cue from standard defense technology, do any insects actually jam or disrupt bat sonar? It appears that a certain species of tiger moth can do precisely this.

The tiger moth is a perfectly edible snack for the echolocating bat. But some tiger moths emit specific ultrasonic clicks in the presence of attacking bats. These clicks could potentially serve as a warning sound, or perhaps be used to startle bats (thus giving the moth time to escape), or perhaps affect the bat sonar. A group of researchers decided to investigate this phenomenon in a tiger moth species called B. trigona, and used an ingenious test to determine what role these ultrasonic clicks were playing. They pitted moths against bats in a closed chamber and precisely observed what the bats did. If the click was a warning sound (for say a poisonous or distasteful insect), the bat would at first attack the insect, but drop it or spit it out, and then learn to avoid the insect. If it was a startling sound, the bat would at first be startled, but would learn to avoid it. If the click was indeed a sonar jamming sound, the bats would continue to be confused by the clicking over time. In their experiments, the researchers used a bunch of juvenile or adult bats and presented them with either the clicking moths, or other moths of the same size that didn’t click, or just a different type of edible, non-clicking moth. What they found was fascinating. The bats indeed did eat the clicking moths. However, the bats were 400% more likely to eat a non-clicking moth than the clicking B. trigona. But what if these clicking moths just tasted worse? To make sure that this wasn’t the case, the researchers disrupted the clicking mechanisms of these moths, and then let them out with the bats. This time, the bats hunted them down as well as the other non-clicking moths. It became very apparent that the moths used the clicking sound in order to disrupt the bat sonar.

As far as the moths go, the evolutionary race for survival is pretty simple. Out in the wild, they don’t need to develop a fantastic sonar jamming device to completely disrupt the bat sonar. All they need to do to get a huge survival edge is to be able to disrupt the bat echolocator just a little bit (but more than any other insect around), so that they can get away and the other insect gets eaten. To do this, they only needed to develop a simple tymbal structure, and this structure is now widespread amongst some tiger moth species. And by doing that, they haven’t evolved to escape all bats, but have just enough to gain that much needed survival edge over other bat prey.

(Original reference: Corcoran, A., Barber, J., & Conner, W. (2009). Tiger Moth Jams Bat Sonar Science, 325 (5938), 325-327 DOI: 10.1126/science.1174096)


This next story is just as fascinating, though more incomplete and raising more questions. Plants obviously are under constant risk of being eaten by some herbivore or the other. So some plants have a very effective defense strategy. They secrete sugars onto their stem or leaves so that they can attract ants. These ants then stay on the plant, and serve as a nice, natural defense against other plant eating insects or animals. Butterflies, on the other hand, are insects that plants share a love-hate relationship with. On one hand, the butterfly pollinates the flowers, allowing the transfer of genetic material from one plant to the other, thus enabling reproduction. On the other hand, butterflies lay their eggs on plant leaves and the caterpillars then devour the leaves. And for the butterfly itself, the last place it wants to lay eggs on is a leaf full of predatory ants which would eat up the eggs or caterpillars.

But can a butterfly, a mere non-thinking insect, know not to lay eggs on leaves with ants? The answer, surprisingly, is yes. In this little paper in The American Naturalist, some researchers devised ingenious experiments to see if butterflies would distinguish between leaves that had ants, or didn’t have ants on them, in order to decide which leaves to lay their eggs on. In their experiment, the researchers took dead specimens of three species of ants, two of which were predatory (and would eat the eggs/caterpillars) and one of which was a bug of a similar size and shape, but a harmless herbivores. Then, they pinned these ants on different leaves, and let the butterflies decide where they laid their eggs. What they saw was surprising, to say the least. The butterflies not only avoided the leaves with the predatory ants, but also didn’t mind laying eggs on the leaves which had the harmless bug on them. So it wasn’t as if the butterfly was just laying eggs on leaves with no ants on them, but actually seemed to know that laying eggs on leaves with the herbivorous bug wouldn’t hurt their eggs and so ignored the innocuous bug. Clearly, it appears that butterflies can use visual clues and decide where to lay their eggs.

What is particularly fascinating to me though is not the fact that butterflies can distinguish between predatory and non-predatory ants, but the fact that they know how to do so without any prior “training”. After all, butterflies are far away from animals or birds which care for their young and potentially teach them about predators or food. Butterflies aren’t even social insects, to have groups to collectively “learn” from. So what is the internal wiring they are born with that tells them some ants are dangerous, while others aren’t? What neuronal and signaling pathways do visual cues of predatory ants activate, while those of innocuous bugs do not? And how does that happen? Do other things, like smell, also influence the butterflies? There is a whole world of questions out there, waiting to be answered.

Original reference: Sendoya, S., Freitas, A., & Oliveira, P. (2009). Egg‐Laying Butterflies Distinguish Predaceous Ants by Sight The American Naturalist, 174 (1), 134-140 DOI: 10.1086/599302)