PREDATOR AND PREY : Escape Responses of Noctuid Moths
Note : the figures below help you to understand easily, before you read.
Noctuid moths are a favorite prey of certain bats. Indeed, moths typically make up more than half of a bat’s diet. The bats capture their prey on the wing, locating these f lying insects by echolocation—that is, by emitting highfrequency sounds that bounce back to the bat from any structure in the environment. Here we will focus on how moths escape predation.
Kenneth Roeder (1967) has provided a fascinating account of how the relatively simple auditory apparatus of the moth is used to detect an approaching bat and how the moth then takes evasive action. When the bat’s ultrasonic echolocation pulses are soft, indicating that the bat is still at a distance, the moth turns and flies directly away.
However, loud ultrasonic pulses mean that the bat is very close, and emergency actions are needed—erratic, unpredictable looping and wingfolding to produce a free fall. Moths that hear a bat’s approach and take evasive action are about 40% less likely to be eaten. Roeder found that these moths have two ears, one on either side of the thorax (the insect’s midsection), and that each ear has only two auditory receptor cells.
The receptors are tuned to the frequencies of the echolocation calls of species of bats living in their vicinity, which is generally between 20 and 50 kHz. One receptor, called the A1 cell, is about ten times more sensitive than the other cell, the A2 cell. The A1 cell begins to respond when the sound is soft, indicating that the bat is still at a distance. The sensitivity of this cell is important because it will determine how much time the moth will have to take appropriate evasive action.
The A2 cell responds only to loud sounds, as would come from a nearby bat. The moth responds to bat sounds long before the bat can detect the moth. North American moths can detect a hunting big brown bat (Eptescicus fuscus) from a distance of nearly 100 ft, whereas the bat must be within about 15 ft to detect a moth-size target.
The A1 cell, then, warns the moth that there is a hunting bat in the vicinity, in much the same way that your car’s radar detector alerts you of a police radar trap. How does the moth’s nervous system analyze the available information and direct effective evasive maneuvers? The sensitive A1 cell responds to the sounds of a distant bat, and its input reveals the direction and distance of the bat.
If the bat, for example, is on the left side, the left A1 cell is exposed to louder sounds because the A1 cell on the right is somewhat shielded by the moth’s body. Therefore, the left receptor fires sooner and more frequently upon receiving each sound of the bat. When the bat is directly behind or in front of the moth, both neurons will fire simultaneously.
A slight turn of the moth’s body will then result in differences in the right and left receptors, which will reveal whether the bat is approaching from the front or rear. What about its altitude? If the bat is above the moth, the bat’s sounds are louder during the upward beat of the moth’s wings when the moth’s ears are uncovered than when the moth’s wings are down, covering the ears and muffling the bat’s cries.
However, if the bat is beneath the moth, the bat’s echolocation cries will reach the moth’s ears unimpeded regardless of the position of the moth’s wings. Therefore, the moth’s wingbeats will have no effect on the pattern of neural firing. The moth, then, is able to decode the incoming data, so that it detects both the presence and precise location of the bat.
How is this information processed to produce an appropriate escape pattern? If the bat is passing some distance away, the A1 cell begins to fire. Its firing rate will increase as the bat gets closer and its cries become louder. Up to a certain firing rate of the A1 cell, the distance between predator and prey is too great for the bat to detect the moth. Therefore, the most adaptive response of the moth would be to turn and fly directly away, thus decreasing the likelihood of detection by increasing the intervening distance and by exposing less surface area to the bat.
This escape pattern results when the moth turns its body until the A1 firing from each ear is equalized. When the bat changes direction, so does the moth. Bats fly faster than moths, though, and if the bat gets too close, then the moth’s evasive maneuver switches to an erratic flight pattern. The moth’s wings begin to beat in either peculiar, irregular patterns or not at all.
The insect itself probably has no way of knowing where it is going as it begins a series of loops, rolls, and dives. But it is also very difficult for the bat to pilot a course to intercept the moth. If the moth crashes into the ground, so much the better. It is safe here because the earth will mask its echoes. How does the moth determine whether the bat is gaining on it?
One clue is that the sound of an approaching bat grows louder. Recall that the A2 cell is less sensitive than the A1 cell and doesn’t begin to fire until the bat is close by. Based on these differences in threshold, Roeder suggested that the A1 cell functions as an “early warning” cell and the A2 cell as an “emergency” neuron that switches the moth’s evasive response to an erratic flight pattern.
As reasonable as the hypothesis seems, it is not consistent with the data. One would predict, for instance, that if the activity of the A2 cell was the switch that changes the evasive response from f lying directly away to erratic flight, then a moth with only one type of A cell would not switch to erratic flighis when the intensity of the bat’s call increased. Notodontid moths have a single type of A cell, but they display both types of evasive behavior. Thus in noctuid moths, which have two auditory cells, the A2 cell does begin firing when the bat is nearby, but this activity may not be responsible for the change to erratic flight.
Another clue to the bat’s proximity is provided by the type of echolocation sounds the bat produces because these change during the hunt. While the bat is searching for prey, its pulses are relatively long (about 10 ms) and are repeated slowly (about 10 per second). When prey has been detected, the bat switches to the approach phase of the hunt. The sound pulses get shorter (about 5 ms) and are repeated more rapidly (about 20 per second).
In the final approach, which begins when the bat is within a meter of its prey, the bat begins a feeding buzz, consisting of short pulses (0.5 to 2 ms) repeated rapidly (100 to 200 per second). The response of the A1 cell can follow the bat’s call rates at all phases of the hunt up to about 150 ms before the bat would capture the moth. Since the call rate changes as the bat gets closer to its prey, the output of the A1 cell provides information about the distance of the bat. The A1 cell sends this information directly to two interneurons, called 501 and 504. These interneurons respond differently to the same input from A1. The differences in interneuron responses somehow encode information about the distance of the bat and direct the appropriate escape response.
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