GPS for dung beetle: multimodal orientation system

There are questions that we asked or tried to answer: why is the sky blue, how many stars are in the sky, who is stronger - a white shark or killer whale, etc. And there are questions that we did not ask, but the answer from this does not become less interesting. These questions include the following - what is so important that scientists from Lund (Sweden), Witwatersrand (South Africa), Stockholm (Sweden) and Würzburg (Germany) universities decided to study together? This is probably something very important, very complex and incredibly useful. Well, about this it’s hard to say for sure, but it’s definitely very entertaining, namely how dung beetles navigate in space. At first glance, everything is trivial here, but our world is full of things that are not as simple as dung beetles seem to confirm. So, What is so unique in the navigation system of a dung beetle, as scientists have verified this, and what does the competition have to do with it? We will find answers to these and other questions in the report of the research group. Go.

The main character

First of all, it is worth getting to know the main character of this study. He is strong, hardworking, persistent, beautiful and caring. He is a dung beetle from the superfamily of the scaraboid.



Dung beetles got their not very attractive name because of their gastronomic preferences. On the one hand, this is a little disgusting, but for a dung beetle it is an excellent source of nutrients, from which most species of this family do not need other sources of food and even water. The only exception is the species Deltochilum valgum, whose representatives like to eat centipedes.

The prevalence of dung beetles can be envied by most other living creatures, since they live on all continents except Antarctica. Habitat ranges from cool forests to sultry deserts. Obviously, a large accumulation of dung beetles is easier to find in the habitats of animals, which are the "factories" for the production of their food. Dung beetles prefer to stock up on food for the future.


A small video about dung beetles and the difficulties of their lifestyle (BBC, David Attenborough).

Different types of beetles have their own behavioral adaptation features. Some form balls from manure, which are rolled from the collection point and buried in a hole. Others dig tunnels underground, filling them with food. And the third, knowing the proverb about Mohammed and grief, simply live in heaps of manure.

Food stocks are important for the beetle, but not so much for reasons of self-preservation, but for reasons of concern for future offspring. The fact is that the larvae of dung beetles live in what their parent collected earlier. And the more manure, that is, food for the larvae, the greater the likelihood that they will survive.


Well, from the lyrics we go directly to the study itself.

As I mentioned earlier, some species of dung beetles form balls and roll them in a straight line, not paying attention to the quality and complexity of the chosen route, into a hole for storage. It is this behavior of these bugs that we are best acquainted with thanks to numerous documentaries. We also know that in addition to strength (some species can raise weight 1000 times their own), gastronomic preferences and caring for offspring, dung beetles are well oriented in space. Moreover, they are the only insects that are able to navigate the stars at night.

In South Africa (the place of observation), a dung beetle, finding "prey", forms a ball and begins to roll it in a straight line in a random direction, most importantly away from competitors who do not hesitate to take away their food. Therefore, for the escape to be effective, you need to move in the same direction all the time, without losing your course.

The sun is the main guideline, as we already know, but it is not the most reliable. The height of the sun changes during the day, from which the accuracy of orientation decreases. Why do not the bugs begin to wind circles, get confused in the direction and check the map every 2 minutes? It is logical to assume that the sun is not the only source of information for orientation in space. And then scientists suggested that the second guideline for beetles is the wind, or rather its direction. This is not a unique feature, as ants and even cockroaches are able to use the wind to find their way.

In their work, scientists decided to check how dung beetles use this multimodal sensory information when they prefer to navigate in the sun, and when in the direction of the wind, and whether they use both options at the same time. Observations and measurements were made in the natural habitat of the subjects, as well as in simulated controlled laboratory conditions.

Research results

In this study, the role of the main subject was played by a beetle of the species Scarabaeus lamarcki , and observations in the natural environment were carried out on the territory of the Stonehenge farm, near Johannesburg (South Africa).


Image No. 1: changes in wind speed during the day ( A ), changes in wind direction during the day ( B ).

Preliminary measurements of wind speed and direction were carried out. At night, the speed was the lowest (<0.5 m / s), but increased towards dawn, reaching a daily peak (3 m / s) from 11:00 to 13:00 (sun height ∼70 °).

Speed ​​indicators are noteworthy in that they exceed the threshold of 0.15 m / s, necessary for the menotactic orientation of dung beetles. In this case, the peak of wind speed coincides in time of day with the peak of activity of the Scarabaeus lamarcki beetles .

Beetles roll their prey in a straight line from the collection point to a fairly large distance. On average, the entire route takes 6.1 ± 3.8 minutes. Therefore, during this period of time, they must follow the route as accurately as possible.

If we talk about the direction of the wind, then during the period of maximum activity of beetles (from 06:30 to 18:30), the average change in the direction of the wind during a time period of 6 minutes is not more than 27.0 °.

Combining data on wind speed and direction during the day, scientists believe that such weather conditions are enough for multimodal orientation of beetles.


Image No. 2

Observation time has come. To check the possible influence of wind on the orientation characteristics of dung beetles in space, a round "arena" was created, in the center of which there was food. Beetles could freely roll the balls they formed in any direction from the center with a controlled stable air flow of 3 m / s. These tests were carried out on clear days when the height of the sun varied during the day as follows: ≥75 ° (high), 45–60 ° (medium) and 15–30 ° (low).

Changes in air flow and position of the sun can vary by 180 ° between two sets of the beetle ( 2A) It is worth considering the fact that beetles do not suffer from sclerosis, and therefore, after the first call, they remember the route they selected. Knowing this, scientists take into account changes in the angle of exit from the arena during the subsequent approach of the beetle as one of the indicators of successful orientation.

At a sun height of ≥75 ° (high), azimuth changes in response to a 180 ° change in wind direction between the first and second sets were grouped around 180 ° (P <0.001, V-test) with an average change of 166.9 ± 79.3 ° ( 2B ). In this case, a 180 ° change in the position of the sun (a mirror was used) caused a subtle reaction of 13.7 ± 89.1 ° (lower circle by 2B ).

It is interesting that at medium and low elevations of the sun the beetles kept their routes, despite changes in the direction of the wind — average altitude: -15.9 ± 40.2 °; P <0.001; low altitude: 7.1 ± 37.6 °, P <0.001 ( 2C and 2D ). But a change in the direction of the sun's rays by 180 ° had the opposite reaction, that is, a radical change in the direction of the beetle’s route - the average height: 153.9 ± 83.3 °; low height: −162 ± 69.4 °; P <0.001 (lower circles on 2A , 2C and 2D ).

Perhaps the orientation is affected not by the wind itself, but by smells. To test this, the distal antennae segments responsible for the sense of smell were removed from the second group of test beetles. Route changes in response to a 180 ° change in wind direction demonstrated by these bugs were still significantly grouped around 180 °. In other words, there is virtually no difference in orientation between beetles with and without smell.

An intermediate conclusion is that dung beetles use the sun and wind in their orientation. Moreover, under controlled laboratory conditions, it was found that the wind compass prevails over the solar in the case of high altitudes of the sun, but the situation begins to change when the sun approaches the horizon.

This observation indicates that there is a dynamic multimodal compass system in which the interaction between the two modalities changes in accordance with sensory information. That is, the beetle is guided at any time of the day, relying on the most reliable source of information at this particular moment (the sun is low - the landmark of the sun; the sun is high - the landmark of the wind).

Next, scientists decided to check whether the wind helps in orienting beetles or not. For this, an arena with a diameter of 1 m was prepared with food in the center. In total, the beetles made 20 sets with a high position of the sun: 10 with the wind and 10 without the wind ( 2F ).

As expected, the presence of wind increased the accuracy of orientation of beetles. It is noted that in early observations of the accuracy of the solar compass, the change in azimuth between two successive sets is doubled at a high position of the sun (> 75 °) compared with a lower position (<60 °).

So, we realized that the wind plays an important role in orienting dung beetles, compensating for the inaccuracies of the solar compass. But how does a bug collect information about wind speed and direction? Of course, the most obvious thing is that this happens through the antennae. To verify this, scientists conducted tests in a room with a constant air flow (3 m / s) with the participation of two groups of beetles - with antennae and without them ( 3A ).


Image No. 3

The main criterion for the accuracy of orientation was the change in azimuth between two approaches when changing the direction of air flow by 180 °.

A change in the direction of movement of beetles with antennae was grouped around 180 °, in contrast to beetles without antennae. In addition, the average absolute change in azimuth in beetles without antennae was 104.4 ± 36.0 °, which is very different from the absolute change in beetles with antennae - 141.0 ± 45.0 ° ( 3V graph ). That is, beetles without antennae could not normally navigate in the wind. However, they were still well oriented in the sun.

On image 3AThe test setup is shown to test the ability of beetles to combine information from various sensory modalities to adjust their route. For this, both landmarks (wind + sun) were present in the test during the first sunset or only one landmark (sun or wind) during the second. Thus, multimodality and unimodality were compared.

Observations showed that changes in the direction of movement of beetles after switching from a multi- to unimodal reference point were concentrated around 0 °: only wind: −8.2 ± 64.3 °; only the sun: 16.5 ± 51.6 ° (graphs in the center and right at 3C ).

This orientation characteristic did not differ from that which was obtained in the presence of two (sun + wind) landmarks (graph on the left at 3C ).

This suggests that under controlled conditions the beetle can use one landmark, if the second does not provide sufficient information, that is, compensate for the inaccuracy of one landmark with the second.

If you think that scientists have stopped at this, then this is not so. Next, it was necessary to check how well the bugs store information about one of the landmarks, and whether they use it in the future as a supplement. For this, 4 calls were made: in the first there was 1 landmark (sun), in the second and third air flow was added, and during the fourth there was only air flow. A test was also conducted where the landmarks were in the reverse order: wind, sun + wind, sun + wind, sun.

The preliminary theory is that if beetles can store information about both landmarks in the same area of ​​spatial memory in the brain, then they should keep the same direction in the first and fourth approaches, i.e. changes in direction of travel should be grouped around 0 °.


Image No. 4

The collected data on the change in azimuth during the first and fourth approaches confirmed the above assumption (4A), which was additionally confirmed by modeling, the results of which are shown in graph 4C (left).

As an additional check, tests were carried out where the air flow was replaced by an ultraviolet spot (4B and 4C on the right). The results were almost identical to the results of tests using the sun and air flow.

For a more detailed acquaintance with the nuances of the study, I recommend that you look into the report of scientists and additional materials to it.

Epilogue

The set of experimental results both in the natural environment and in the controlled one showed that in dung beetles visual and mechanosensory information converge in a common neural network and are saved as a multimodal compass image. A comparison of the effectiveness of using either the sun or wind as a guideline showed that beetles use the guideline that provides them with more information. The second is used as a spare or complementary.

This may seem like a very common thing for us, but do not forget that our brain is much larger than that of a small bug. But, as we understood, even the smallest creatures are capable of complex mental processes, because in the wild, your survival depends on either strength or the mind, and most often on a combination of both.


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