There are questions we’ve asked or tried to answer: why is the sky blue, how many stars are in the sky, who is stronger — the great white shark or the killer whale, etc. Then there are questions we haven’t asked but which are still fascinating. One such question is — what significant aspect did researchers from Lund University (Sweden), University of the Witwatersrand (South Africa), Stockholm University (Sweden), and University of Würzburg (Germany) come together to explore? It must be something very important, very complex, and incredibly useful. It's hard to say for sure, but it’s certainly intriguing — specifically, how dung beetles navigate in space. At first glance, it seems trivial, but our world is full of things that are not as simple as they appear, and dung beetles are a testament to that. So, what’s unique about the navigation system of the dung beetle, how did the scientists verify this, and what does competition have to do with it? We will find answers to these and other questions in the research group's report. Let's dive in.
The Main Character
First and foremost, let's get acquainted with the main character of this study. He is strong, hardworking, persistent, handsome, and caring. He is a dung beetle from the Scarabaeidae superfamily.
Dung beetles got their rather unattractive name due to their gastronomic preferences. On one hand, it's a bit gross, but for the dung beetle, it's an excellent source of nutrients, meaning that most species in this family don’t require other food sources or even water. The only exception is the species Deltochilum valgum, whose members enjoy snacking on millipedes.
Dung beetles are more widespread than most other living beings, as they inhabit every continent except Antarctica. Their habitat ranges from cool forests to scorching deserts. It’s clear that dung beetles are more easily found in areas populated by animals that serve as 'factories' producing their food. Dung beetles prefer to stockpile food for the future.
A short video about dung beetles and the complexities of their lifestyle (BBC, David Attenborough).
Different types of beetles have their own behavioral adaptations. Some form balls out of dung, rolling them away from the collection site and burying them in burrows. Others dig tunnels underground, filling them with food. And some, who know the saying about Muhammad and the mountain, simply live in piles of dung.
Food supplies are important for the beetle, but not so much for self-preservation as for caring for future offspring. The thing is, the larvae of dung beetles live in what their parent collected earlier. And the more dung, which is food for the larvae, the greater the chance that they will survive.
I came across such wording while gathering information, and it sounds a bit off, especially the last part:…Males fight for females by bracing their legs against the walls of the tunnel, pushing their opponent with horn-like protrusions…Some males lack horns and therefore do not engage in combat, but they have larger reproductive glands and guard the female in a neighboring tunnel…
Well, let’s move from the lyrical portion directly to the study itself.
As I mentioned earlier, some species of dung beetles form balls and roll them in a straight line, regardless of the quality and complexity of the chosen route, into a burrow for storage. We are most familiar with this behavior of these beetles thanks to numerous documentaries. It is also known that, in addition to strength (some species can lift weights 1,000 times their own), dietary preferences, and care for their offspring, dung beetles are excellent at navigating space. Moreover, they are the only insects capable of navigating at night by the stars.
In South Africa (the observation site), a dung beetle, upon finding 'prey,' forms a ball and begins to roll it in a straight line in a random direction, primarily away from competitors who will not hesitate to take the food he has gathered. Thus, for the escape to be effective, it is necessary to move in the same direction at all times, staying on course.
The sun is the primary point of reference, as we know, but it is not the most reliable. The sun's height changes throughout the day, reducing directional accuracy. Why then do beetles not start to wander in circles, get confused about direction, and check a map every two minutes? It is logical to assume that the sun is not the only source of information for navigating space. Here, scientists proposed that a second reference for beetles is the wind, specifically its direction. This is not a unique feature, as ants and even cockroaches can use the wind to find their way.
In their work, scientists aimed to examine how dung beetles use this multimodal sensory information, when they prefer to navigate by the sun and when by the wind direction, and whether they use both options simultaneously. Observations and measurements were conducted in the natural habitat of the subjects, as well as in controlled laboratory conditions.
Research Results
In this study, the main subject was a beetle of the species Scarabaeus lamarcki, and observations in the natural environment were carried out at the "Stonehenge" farm, which is near Johannesburg (South Africa).
Image No. 1: changes in wind speed over the course of the day (A), changes in wind direction over the course of the day (In).
Preliminary measurements of wind speed and direction were conducted. At night, the speed was at its lowest (<0.5 m/s), but increased closer to dawn, reaching a daily peak (3 m/s) between 11:00 and 13:00 (sun height ∼70°).
The speed readings are noteworthy because they exceed the threshold of 0.15 m/s needed for menotactic orientation in dung beetles. Moreover, the peak wind speed coincides with the peak activity of the beetles. Scarabaeus lamarcki.
Beetles roll their dung over a straight line from the collection point over a considerable distance. On average, the entire journey takes 6.1 ± 3.8 minutes. Therefore, during this time, they must adhere to the route as precisely as possible.
Speaking of wind direction, during the peak activity period of beetles (from 06:30 to 18:30), the average change in wind direction over a time interval of 6 minutes does not exceed 27.0°.
By combining data on wind speed and direction throughout the day, scientists believe that these weather conditions are sufficient for the multimodal orientation of beetles.
Image #2
It's time for observations. To check the potential influence of wind on the orientation characteristics of dung beetles in space, a circular 'arena' was created with food in the center. The beetles could freely roll the balls they formed in any direction from the center in the presence of a controlled stable airflow at a speed of 3 m/s. These tests were conducted on clear days when the sun's height varied throughout the day as follows: ≥75° (high), 45–60° (medium), and 15–30° (low).
Changes in airflow and the position of the sun can vary by 180° between two approaches of a beetle (2A). It is important to note that beetles do not suffer from sclerosis, and therefore after the first approach, they remember the route they took. Knowing this, scientists take into account the 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), changes in azimuth in response to a change in wind direction of 180° between the first and second approaches were clustered around 180° (P <0.001, V-test) with an average change of 166.9 ± 79.3° (2B). At the same time, a change in the sun's position (a mirror was used) by 180° evoked a negligible response of 13.7 ± 89.1° (the lower circle at 2B).
). Interestingly, at medium and low sun heights, beetles maintained their routes despite changes in wind direction — medium height: -15.9 ± 40.2°; P <0.001; low height: 7.1 ± 37.6°, P <0.001 (2C and 2D). However, changes in the direction of sunlight by 180° had an opposite reaction, causing a radical change in the beetle's route — medium height: 153.9 ± 83.3°; low height: −162 ± 69.4°; P <0.001 (the lower circles at 2A, 2C and 2D).
It is possible that orientation is influenced not by the wind itself, but by the scents. To test this, the distally segments of the antennae responsible for olfaction were removed from the second group of beetles. The changes in the route in response to a change in wind direction of 180°, demonstrated by these beetles, were still significantly clustered around 180°. In other words, there is no difference in the degree of orientation between beetles with and without olfaction.
The preliminary conclusion is that dung beetles use the sun and wind for their navigation. In controlled laboratory conditions, it was found that the wind compass predominates over the solar one when the sun is at high positions, but the situation begins to change as the sun approaches the horizon.
This observation suggests that there is a dynamic multimodal compass system, where the interaction between the two modalities varies according to sensory information. This means that the beetle navigates at any time of day by relying on the most reliable source of information at that specific moment (sun low — sun as a reference; sun high — wind as a reference).
Next, scientists decided to test whether the wind aids beetle navigation or not. For this purpose, an arena with a diameter of 1 m was prepared with food in the center. A total of 20 approaches were made by the beetles at high sun positions: 10 with wind and 10 without wind (2F).
As expected, the presence of wind increased the accuracy of beetle navigation. It is noted that in early observations of solar compass accuracy, the change in azimuth between two successive approaches increased twofold at high sun positions (> 75°) compared to lower positions (<60°).
So, we understood that wind plays an important role in dung beetle navigation, compensating for the inaccuracies of the solar compass. But how does the beetle gather information about wind speed and direction? Of course, the most obvious way is through their antennae. To verify this, scientists conducted tests in a controlled environment with a constant airflow (3 m/s) involving two groups of beetles — with antennae and without them (3A).
Image #3
The main criterion for navigation accuracy was the change in azimuth between two approaches when the direction of airflow was changed by 180°.
The change in movement direction of beetles with antennae was clustered around 180°, in contrast to beetles without antennae. Additionally, the average absolute change in azimuth for beetles without antennae was 104.4 ± 36.0°, which is significantly different from the absolute change observed in beetles with antennae — 141.0 ± 45.0° (chart on 3BThis means that the bugs without antennae could not navigate properly by the wind. However, they still oriented themselves well by the sun.
In the image 3A This shows the test setup to check the bugs' ability to combine information from various sensory modalities to adjust their route. In this test, both cues (wind + sun) were present during the first approach, while only one cue (sun or wind) was available during the second. Thus, multimodality and unimodality were compared.
Observations showed that the changes in the direction of the bugs' movement after switching from multi- to unimodal cues were concentrated around 0°: only wind: −8.2 ± 64.3°; only sun: 16.5 ± 51.6° (graphs in the center and right at 3C).
This characteristic of orientation was not different from that obtained with two cues (sun + wind) present (graph on the left at 3C).
This indicates that under controlled conditions, a bug can rely on one cue if the second does not provide sufficient information, thus compensating for the inaccuracy of one cue with the other.
If you think that the scientists stopped at this point, they did not. Next, it was necessary to check how well the bugs retain information about one of the cues and whether they use it in the future as a supplement. For this, four trials were conducted: in the first trial, there was one cue (sun), in the second and third, airflow was added, and in the fourth, there was only airflow. A test was also conducted where the cues were in reverse order: wind, sun+wind, sun+wind, sun.
The preliminary theory is that if the bugs can store information about both cues in the same area of spatial memory in the brain, they should maintain the same direction in the first and fourth trials, meaning that changes in the direction of movement should cluster near 0°.
Image No. 4
The collected data on the change of azimuth during the first and fourth trials confirmed the aforementioned hypothesis (4A), which was further validated by modeling, the results of which are depicted in graph 4C (on the left).
As an additional check, tests were conducted where the airflow was replaced with a spot of ultraviolet light (4B and 4C on the right). The results were virtually identical to those obtained with sunlight and airflow.
For a more detailed understanding of the nuances of the research, I recommend checking out the and related to it.
Epilogue
The cumulative results of experiments in both natural and controlled environments showed that the dung beetles integrate visual and mechanosensory information into a common neural network, preserving it as a snapshot of a multimodal compass. The comparison of the effectiveness of using either sunlight or wind as a reference showed that the beetles prefer the cue that provides them with more information, while the second serves as a backup or supplement.
This may seem quite ordinary for us, but it’s worth noting that our brains are much larger than that of a small beetle. However, as we understood, even the tiniest creatures are capable of complex cognitive processes, as in the wild, your survival depends on either strength, intelligence, or most often, a combination of both.
Friday off-topic:
Even beetles fight over food. And it doesn’t matter that the prey is a ball of dung.
(BBC Earth, David Attenborough)
Thank you for your attention, stay curious and have a great weekend, everyone! 🙂
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Source: habr.com
