Woolly Bear Biology: Part 1

Wow, you folks were fast to break 100 records.

As promised, here is a short discussion of sound production in the woolly bear, Pyrrharctia isabella.

Plenty of tiger moths (Erebidae: Arctiinae) produce sound in various contexts. And with >11,000 species of tiger moths world-wide, that's potentially a lot of sound being made in the skies at night! These sounds resemble clicks, because they are produced in discrete "packets" of sound which are quickly damped (more on this in a future journal entry). The most common contexts in which these moths produce sound are in response to physical restraint or bat echolocation. How do we know this? That too will be future journal entry.

As we currently understand it, there are 3 major reasons tiger moths produce sound. The first is to warn predators that they are chemically defended (also the subject of a future journal entry); we call this "acoustic aposematism" in the biz. The second reason is to mimic the warning signals of other tiger moth species; we call this "acoustic mimicry", and often the mimics can be dishonest cheaters looking to present themselves as being more dangerous than they really are to bats or birds (or curious biologists). The third reason is to disrupt the echolocation signals of bats; we call this "sonar jamming", but this requires a certain level of complexity in the moth sounds. The acoustic characteristics which generate a "sonar-jamming effect" are currently a hot topic of investigation in my work and the work of other researchers. An additional complication is that all three of these functions could be working at the same time; we say these are "non-mutually exclusive functions".

Pyrrharctia isabella is one of the many tiger moth species which produce sound. They click in response to physical restraint and in response to bat echolocation. You would never know this by grabbing one of these moths from your sheets and listening. Why? Because even the lowest pitch they produce is well above 20,000 Hz, the oft-cited frequency threshold for humans. At 29 years old, my hearing threshold is only 16,500 Hz, so typically humans do much worse than 20,000 Hz. Instead of listening for the sounds, you can look for the physical action of the sound-producing organ; we call this organ the "tymbal". I'll have a special post about this in a future journal entry.

And they produce a lot of sound! I won't go into the details of how we quantify these signals (maybe another journal entry?), but I personally believe that the woolly bear is capable of jamming the echolocation of its bat predators. That's a big deal because that behavior has really only been confirmed for one tiger moth species, Bertholdia trigona. I'm currently investigating this possibility in conjunction with other researchers. Your contributions to this project are really important to push this work forward!

I promised there would be recordings, so I've uploaded one to YouTube. You can check that out at the link below. I said before that you wouldn't be able to hear these sounds, and that is true. But I've digitally manipulated the signals to make them audible to you and me. So, keep in mind that in reality these sounds would be happening about 10 times faster and the pitch would be about 10 times higher. I present to you the first public recording of the acoustic responses of Pyrrharctia isabella in response to the bat, Eptesicus fuscus (the big brown bat). The chirps are the bat cries and the crunchy clicks are the moth sounds!

Video: https://youtu.be/EXgio0fBdgM

I have mentioned two contexts in which tiger moths produce sound. But there is a third! Some produce sounds during sexual courtship. This behavior is extremely under-studied, but we do have some information. Pyrrharctia isabella is one species in which we know sound plays a role in courtship. I'll discuss this further in the future too!

But, before I say more, GO BREAK 200 OBSERVATIONS! I know you can do it. I'll discuss more of these topics once we get there! :) Please comment below to let me know if you want me to cover a particular topic sooner than another.

That's all for now.

Thanks for the support!

Posted on April 19, 2018 01:36 PM by njdowdy