Neurons in the wild

Anyone who’s thought about science knows the promise and perils of simplification. People joke about physicists who begin working on an applied problem in agriculture by saying, “Assume the cow is a perfect sphere...”

Working on an animal in a lab is a little like assuming a cow is a perfect sphere. You can get a pretty long way by simplifying the situation. But as I’ve talked about before, you often get many unexpected and delightful findings when you let an animal be an animal, in the environment and setting that its ancestors have navigated more or less successfully for millions of years.

A new paper by Kostarakos and Römer looks how neurons work in a natural setting, using a favourite critter for this blog: crickets. Although females of many crickets species can fly, they normally walk along the ground towards a calling male. There can be a lot of “stuff” in the way of that song: grass, rocks, trees, and more. And just being close to the ground is acoustically “bad” (that’s the technical term).

Kostarakos and Römer decided to look into all this by taking their recording equipment out doors.

One of the things this comes out from such experiments, although the authors don’t emphasize it, is how sensitive cricket ears to cricket sounds. It would be easy to miss this little aside, to which I’ve added some emphasis:

(A)t greater distances, the AN1 neuron still responded to the chirps even when the microphone recordings revealed no signal.

The AN1 neurons they mention appears to be critical for the cricket to recognize and find the song of another cricket. The AN1 neurons are so key that their firing patterns are probably very good indicators of the behaviour of the intact animal. A cricket has two AN1 cells: a right and a left one, thus obeying the law of bilateral symmetry.

From recording the AN1 neurons, we find out that these little guys can pick up songs from another cricket on the order of 10 meters away. Considering that the animal is only a few centimeters long, that’s a substantial distance.

One of the surprises, though, is that it seems that every cricket has some “dull spots,” where as the animal gets closer to the signal, the neural response drops a bit. (The authors call these “silent spots,” although the AN1 response probably rarely goes to zero.) Some crickets will hear the song better at 10 meters away than they will at 8 meters away, though it picks up again when the sound gets to 6 meters away. But it was very variable from case to case, and it’s not clear if that is due to the individuals, or some peculiarities of the individual recording session. Remember, this is all out in the field, with breezes, grasses, all kinds of things happening.

Although the animal can hear the song at 10 meters away, another bit of a surprise is that at that distance, it can’t tell where the sound is coming from, that is. In a lab situation, it is laughably easy for a cricket to locate a sound 30° off center. In the field, though, with a speaker placed 30° to one side or the other, but there’s no difference in spiking activity of the left and right AN1 neurons when the sound is about 5 meters away.

Again, though, there’s a lot of variation from individual to individual. And it even fluctuates over the course of several minutes. And it fluctuates with temperature. Neurons are very temperature sensitive; you can double the firing rate of AN1 by making it about 10°C warmer.

All of which suggests that crickets are performing the important job of finding a mate with a hearing system that is a little bit wonky and unreliable. There’s dull spots, you can hear a sound but not be sure of the direction, and what you hear depends on the temperature.

One thing that might be interesting would be to use a real calling male, rather than using a “canned” song played through a speaker. Might the caller somehow tweak the song to to maximize the physiological changes in the receiver?

Some people have to go and do some more field experiments to watch the song finding behaviour of females very closely. I think I might know just the people to call.

Given this, and some of the talks I saw at the International Congress of Neuroethology at the start of the month, recording out of doors could yield lots of new findings to work with. The science of ethology always prided itself on its emphasis of animals in their natural setting; I’m looking forward to neuroethologists reclaiming a little more of that ground truth you get from taking things into the field.

This is real life, baby.



Reference

Kostarakos K & Römer H. 2010. Sound transmission and directional hearing in field crickets: neurophysiological studies outdoors Journal of Comparative Physiology A 196(9): 669-681. DOI: 10.1007/s00359-010-0557-x

Picture by Gilles San Martin on Flickr, used under a Creative Commons license.

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