In the wild world, hunters and prey are constantly playing catch-up, both literally and figuratively. As soon as the hunter develops new skills through evolution or other methods, how the prey adapts to them so as not to be eaten. This is an endless game of poker with a constant increase in the stakes, the winner of which receives the most valuable prize - life. We have recently considered , which is based on the generation of ultrasonic noise. Among the insects that are a delicacy for winged sonar, masking one's ultrasonic signal is a vital skill. However, bats do not want to remain hungry, therefore they have a skill in their arsenal that allows them to see prey despite disguise. How exactly do bats cosplay Sauron, how effective are their hunting tactics and how do plant leaves help them in this? We learn about this from the report of the research group. Go.
Research basis
Bats have always evoked a whole range of feelings in people: from curiosity and reverence to outright fear and disgust. And this is quite understandable, because on the one hand, these creatures are excellent hunters, who actually use only their hearing during the hunt, and on the other hand, they are terrible night creatures that climb into the hair and strive to bite everyone (these, of course, are myths generated by human fears) . It is difficult to love an animal that in popular culture is associated with Dracula and Chupacabra.
Hey, I'm not at all scary.
But scientists are impartial people, they don't give a damn about how you look or what you eat. Whether you are a fluffy rabbit or a bat, they will be happy to perform a couple of experiments on you, and then another dissection of your brain to complete the picture. Okay, let's leave the black humor (with some truth) aside and get down to business.
As we already know, the main tool of bats during hunting is their hearing. Mice are active at night due to fewer competitors/dangers and more prey. By emitting ultrasonic waves, bats pick up all the return signals that bounce off objects around them, including possible prey.
Itβs cool to emit masking ultrasonic noise, of course, but not all applicants for the position of dinner for bats have such a talent. But mediocre insects can hide their location. To do this, they need to merge with the environment, but not like the Predator from the film of the same name, because we are talking about sound. The night forest is full of sounds from a variety of sources, some of which is background noise. If the insect sits, say, on a leaf motionless, then there is a high probability of getting lost in this background noise and surviving until morning.
Given this, many scientists believed that such prey for bats was simply out of reach, but this is not entirely true. Some species of bats still managed to solve the mystery of "invisible" insects and successfully catch them. The question remains - how? To answer this question, scientists at the Smithsonian Tropical Research Institute used a biomimetic sensor that records any fluctuations in echoes from insects sitting quietly on leaves (i.e. hiding). Next, the scientists calculated ideal attack paths, i.e. flight paths and angle of capture of prey for bats, which can help bypass camouflage. After that, they tested their calculations and theories in practice by observing bats attacking camouflaged prey. It is curious that the leaves on which the insects sat so carelessly served as a tool for their capture.
Isn't she a beauty?
The role of the subjects in this study was played by 4 males of the species Micronycteris microtis (common big-eared bat), which were caught in their natural habitat on the island of Barro Colorado (Panama). During the experiments, a special cage (1.40 Γ 1.00 Γ 0.80 m) was used, located in the forest on the island. Scientists have recorded data on the flights of individuals placed in this cage. The night after the capture, the actual experiments began. 1 individual was placed in the cage, which was supposed to find and catch the βdisguised preyβ. No more than 16 hours of experiments were carried out with one individual (2 nights for 8 hours) in order to minimize the effect of spatial memory and stress for the animal. After the experiments, all bats were released in the same place where they were caught.
Researchers had two main theories to explain how bats hunt camouflaged prey: the acoustic shadow theory and the acoustic mirror theory.
The "acoustic shadow" effect occurs when an object on the surface of the sheet scatters the echo energy, thereby reducing the strength of the echo from the surface of the sheet. To maximize the acoustic shadow of an object, the bat should approach directly from the front in a direction perpendicular to the background surface (1A).
Image #1
In the case of an acoustic mirror, wood bats act like their trawl relatives, which capture prey from the surface of a reservoir. Echolocation signals emitted at a low angle to the surface of the water bounce off the hunting bat. But the echo from possible prey is reflected back to the bat (1B).
The researchers suggested that the leaves act like the surface of the water, i.e. act as a signal reflector (1S). But for the full effect of the mirror, a certain angle of attack is needed.
According to the acoustic shadow theory, bats should attack prey from a frontal direction, so to speak, in the forehead, because in this case the shading will be the strongest. If an acoustic mirror is used, then the attack must occur at a limiting angle. In order to determine which angle of attack could be optimal, the scientists conducted acoustic measurements at different angles relative to the leaf.
After completing the calculations and testing the theories, behavioral tests were carried out with the participation of live bats and comparison of the observational results with the results of theoretical modeling.
Results of calculations and observations
Image #2
First of all, an acoustic model (dome) of a leaf with and without prey was created by combining all the echoes at different angles of attack into one picture. As a result, 541 positions were obtained on 9 semicircular trajectories around the sheet (2Π).
For each point, we calculated power spectral density* ΠΈ acoustic dimension* (TS - target strength) of the target (i.e., echo intensity) for 5 different frequency ranges, which roughly correspond to the harmonic components of the outgoing bat signal (2V).
Power Spectral Density* is the signal power distribution function depending on the frequency.
Acoustic dimension* (or target acoustic strength) is a measure of the area of ββan object in terms of the acoustic response.
On the image 2S shows the results of the derived angles of attack, which are the angles between the normal relative to the leaf surface at the center of production and the position of the signal source, i.e. bat.
Image #3
Observations have shown that both types of sheets (with and without prey) in all frequency ranges demonstrate the largest acoustic size at angles < 30Β° (central parts of the graphs 3A ΠΈ 3B) and smaller acoustic dimension at angles β₯ 30Β° (outer part of graphs on 3A ΠΈ 3B).
Picture 3Π confirms that the sheet actually acts as an acoustic mirror, i.e. at angles < 30Β° a strong specular echo is generated, and at β₯ 30Β° the echo is reflected from the sound source.
Comparing a leaf with prey on it (3Π) and without prey (3V) showed that the presence of prey increases the acoustic size of the target at angles β₯ 30Β°. At the same time, the echo-acoustic effect of production on the sheet is best seen when plotting the production-induced TS, i.e. difference in TS between leaf with and without prey (3S).
It is also worth noting that an increase in the acoustic size of the target at angles β₯ 30Β° is observed only in the case of high frequencies, at low frequencies there is no additional effect at all.
The above calculations made it possible to theoretically determine the range of angles of attack in the case of implementing the theory of specular reflection - 42Β° ... 78Β°. In this range, the same increase in the acoustic size of the target from 6 to 10 dB at higher frequencies (> 87 kHz) was observed, which is consistent with the acoustic data of M. microtis bats.
Such a hunting method (at an angle, so to speak) allows the predator to very quickly determine the presence / absence of prey on the leaf: a weak and low-frequency echo - the leaf is empty, a strong and broadband echo - there is a tasty treat on the leaf.
If we consider the theory of acoustic shadow, then the angle of attack should be less than 30. In this case, according to the calculations, the interference between the leaf and prey echoes is maximum, which leads to a decrease in TS compared to the leaf echo without prey, i.e. this results in an acoustic shadow.
With the calculations finished, let's move on to the observations.
During observations, various insects from the diet of bats located on an artificial leaf were used as prey. Two high-speed cameras and an ultrasonic microphone were used to record the behavior of bats as they approached their prey. From the resulting recordings, 33 flight paths of bats approaching and landing on prey were reconstructed.
The bat attacks its prey.
The flight paths were based on the position of the bats' nostrils during each frame as they transmitted their signal.
As expected, observations showed that the bats approached prey at an angle.
Image #4
On the image 4Π a XNUMXD map of prey attack trajectories is shown. It has also been found that the distribution of angles of attack follows acoustic size curves for higher frequencies (4V).
All subjects attacked the target at angles < 30Β° and clearly avoided more frontal directions. Of all the attack angles observed during the experiments, 79,9% were in the predicted optimal range of 42Β°β¦78Β°. More precisely, 44,5% of all angles were in the range of 60Β°β¦72Β°.
Attack of production at an angle and spectrograms of the emitted acoustic signal.
Another observation is the fact that bats have never attacked prey from above, as other researchers have suggested.
For more detailed information about the nuances of the study, I recommend looking at ΠΈ to him.
Finale
The use of echolocation as the main, and sometimes the only, hunting tool is already a very unique and amazing phenomenon. However, bats never cease to amaze, demonstrating much more sophisticated attack tactics than previously thought. Finding and capturing prey that does not hide is not difficult, but finding and capturing an insect that is trying to hide in acoustic background noise requires a different approach. In bats, this approach is called acoustic shadow and acoustic mirror. Flying up to the leaf at a certain angle, the bat instantly determines the presence or absence of probable prey. And if there is one, then dinner is guaranteed.
This study, according to its authors, can push the scientific community to new discoveries in acoustics and echolocation, both in general and among the animal world. In any case, learning something new about the world that surrounds you, and about the creatures that live in it, has never been a bad thing.
Friday off-top:
To survive, sometimes it is not enough to be an excellent hunter. When it is incredibly cold around, and there is no food at all, the only thing left is to sleep.
Off-top 2.0:
Someone uses speed, someone uses strength, and someone just needs to be quiet, like a shadow.
Thanks for watching, stay curious and have a great weekend everyone! π
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Source: habr.com