First in the group - first in the mouth: risk assessment of leadership in self-organized groups of animals

A bunch of twigs is much more difficult to break than a single twig. This metaphor in its various variations tells us that it will be much more difficult to cope with something alone than in a group. And in many respects it is true, but not everything is so simple. In the wild, animals, carnivores and predators, are often gathered in groups, schools, schools, etc. But unlike the metaphorical twigs, all the members of the group are not equal, for there are leading and driven: someone zealously running forward, someone bothered in the center, and someone lagging behind. And then the question arises - what is the price of leadership and what are the advantages, especially if the type of animals that are someone's favorite food is considered.

Today we will get acquainted with a study in which scientists conducted a series of unusual experiments with predatory fish and artificial school of fish, which allowed us to determine the positive and negative aspects of the situation in the group under the prism of prey-predator communication. What place in the “queue” is fatal, what are the pros and cons of leadership, what does a school of fish look like through the eyes of a predator? We will find answers to all these questions in the scientists' report. Go.
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The basis of the study

As I said earlier, many species of living organisms on the planet gather in groups. It is easier to fight all the hardships of life in the wild: penguins can survive the terrible cold by gathering in tight groups and warming each other with the warmth of their bodies; Pride of Lions jointly hunts, tracking down, surrounding and intercepting prey, which increases the chances of success; the chaotic (but not indiscriminate) movement of flocks of small birds strongly prevents the birds of prey from hunting them, etc.


Emperor penguins withstanding terrible cold, wind and many months of night.

And, again, in many groups there is one individual that leads all the others, although not always consciously. In self-organized groups (birds or fish) such an individual is located at the head of the group, that is, in front. Such a situation allows the leader to be the first to receive information about the environment (availability of food, obstacles, hazards, etc.), therefore he can make decisions that can affect the actions of the whole group. The main benefit for the leading individual lies precisely in the mass character of the group she leads, which performs the necessary actions to maintain the life of the leader and the group simultaneously.

Scientists have always called predators the main drawback of leadership in moving groups of animals, because the frontal part of the pack / shoal can face the danger much more often, and predators are easier to navigate along the leading individuals during the hunt. However, no experiments have been conducted before in order to confirm or refute this theory.


Self-organized flock of starlings: a spectacle of incredible beauty.

The observations, which can confirm the theory of leadership, are very ambiguous. The researchers note that the definition of the leading and the slave in the self-organized groups of animals is due to the self-organization of the whole group, but the leadership and following (behind the leader) is due to other factors. Most often, the tendency to leadership is associated with dedication, driven by more information or greater need. That is, the position at the head of the group is most often occupied by larger, more hungry, less risky and less social individuals. Similar traits can be dangerous for individuals. In other words, it is difficult to assess what is more dangerous - leadership in a group or just the frontal part of a group.

The question also arises: if it is so dangerous to be a leader, then maybe it is worth leaving the group? Perhaps loneliness will be safer than living in a flock. Both in the first and in the second variant there are advantages and disadvantages that concern predators. However, in order to leave a group, an individual must completely change its priorities from social to targeted (ie, survival). On the other hand, if being a leader in a group is still a little safer than swimming by itself, then social behavior becomes a priority.

Many questions, few observations, as we have already understood. Therefore, the scientists decided to put everything in its place and understand that how it works in the social sphere of animal groups that are attacked by predators. For this, a special system of artificial virtual mining was developed. Having full control over the behavior of each of the units in this group, the scientists were able to understand the actions of the predator in relation to the leading and the led, as well as to the units separated from the group.

Preparation for the study

As live participants in the experiment were predatory fish stickleback caught in the Cary River (England). In the laboratory, they were placed in glass aquariums (40x70x35 cm) with a flow-through recirculation system and a temperature of 15-16 ° C. In each of the aquariums, there were approximately 40 individuals, which were regularly fed twice a day (except for the days when tests were conducted).

The prey was agent projection * on the front wall of the aquarium, which made it possible to fully observe the tests. As a basis for their behavioral model, scientists used an existing one, which they modified using Netlogo 5.0.5.

Agent modeling * is a simulation modeling method that studies the behavior of decentralized agents and the effect of such behavior on the behavior of the system as a whole.

The artificial mining group consisted of 1 leader, 4 slaves and 2 asocial units. The leader and the slaves were programmed so as not to react to the behavior of asocial units.

Each slave unit was tied to the leader and other slaves so that the distance between them was 6.5 cm. The slave units could also play the role of leader if that one was not near. Thus, the movement of the group has always been from head to tail, that is, there has always been a leader and 1-2 driven units.


Image number 1: A - sticklet attacks a projected point (artificial prey) on the wall of the aquarium; In - the position and trajectory of the movement of artificial production (color determines the type of unit).

A little explanation about the color distribution of the types of units in image 1B :

• violet is a solitary (asocial) unit (the absence of other units within the threshold distance);
• black - wrong direction (there are neighboring units within the threshold distance, but move along a trajectory that does not coincide with the black unit);
• blue - the leading (leading) unit;
• green - unit in the middle of the group;
• yellow - unit in the tail of the group.

Behavioral rules that governed the types of production units were applied, which allowed us to obtain different behaviors of the entire system. Thus, the predator (fish) had the opportunity to choose between a leader, a slave or an asocial unit. The initial position of all units was randomized. All units were represented on the screen as red dots (2.5 mm), resembling, according to scientists, Daphnia (plankton crustacean).

Daphnia

When the system was fully prepared, one fish was placed in the test aquarium (as we remember, hungry). The fact of the attack on the unit was the acceleration of the fish in its direction, the opening of the mouth and contact with the screen on which the unit was projected.

Experimental results

In total, 133 out of 201 fish attacked artificial prey at least once during test runs, each was 10 minutes long. In order to minimize the behavioral changes in the predator, associated with the inability to eat virtual prey, the scientists took into account in further calculations only one attack per test.

To determine which behavior of the virtual victim was decisive in the prey-predator issues, the scientists compared eight binomial generalized linear models that predicted which particular prey the predatory fish attacked depending on various explanatory variables.



Above is a table of explanatory variables, that is, a table of models using one or another variable in the calculations: position (position), number of neighboring units (number of near neighbors), whether the unit is isolated or not (whether solitary or nor), total group pray size), extraction with attraction (prey with attraction), type of extraction (prey type), extraction seems social (prey appears social) and zero model / no explanatory variables (null model / no explanatory variables).

AICc * is the Akaike information criterion, modified to fit a small sample size.

The analysis showed that the presence of at least some variable is naturally better than the absence of them at all. Models that used the division of prey types (slave, leaders and asocial) showed that leaders and "hermits" are subject to more frequent attacks by predators. But the model in which the leader and the slaves were in the same category was not confirmed by real observations, that is, the risk from predators varies from the type of unit in the group (the leader, in the middle of the group or in the tail).

The model that took into account the behavior of the victim (model unit) during the attack, together with the division of production by type, was the most indicative. In other words, each unit was initially programmed for a specific behavior, that is, a type (to be in front, follow the leader, or stay away from the whole group). However, the actual behavior during the test itself was much more weight.

Increasing the number of groups reduced the risk of predator attacks. At the same time, a model that takes into account not only the number, but also the position of an individual unit relative to others, turned out to be more likely. Also likely was a model in which an important role was played not so much by the size of the group, as by its density, that is, the distance between individual units.

After analyzing all possible models, the scientists came to the conclusion that it is necessary to take into account all possible factors, no matter how strange it may sound. Most attention needs to be paid to behavioral variables, rather than the number and distance between the predator and the prey.


Image number 2: the degree of risk of attack depending on the position of production (purple - asocial; black - in the group, but not in the same direction; blue - leading (leaders); green - in the middle of the group and yellow - in the tail of the group).

The greatest risk of being attacked was asocial units (the purple circle in the chart above). This indicator confirms the theory that leadership is still safer than complete isolation from the group. Even a unit that does not move like the whole group is safer than an asocial one.

If we consider the ideal group, when all units are aligned in one direction, then the scientists saw a clear gradation of the degree of risk (from major to minor) - at the beginning of the group, in the tail and in the middle. Scientists had expected that the degree of risk for units in the middle of the group and in the tail would be significantly different, but this was not the case.

It can be assumed that being in the tail is no less dangerous than at the head of a group. Not for nothing, in many horror films, a monster often attacks a group of people from the tail. However, these studies suggest that in the tail and in the middle of the group the risk of being eaten is about the same, although significantly less than at the beginning.

It is worth noting that in the extraction simulation used, the behavior of the units was stochastic and changed over time when the unit left or joined the group. Thus, during the predator attack simulations there were many variations of prey. To account for all these variations, scientists have developed a special test in which a random predator was simulated, choosing prey during each simulation to an equal degree of probability. Interestingly, given such a randomization, the mining model based on its position in / outside the group turned out to be the most acceptable compared to others.

Next, scientists needed to find out the mechanism of prey selection from a predator. To do this, the risk of attack was calculated depending on the distance to the target of the attack, the difference of movement trajectories and bearing * to the nearest neighboring production unit in front (from all production with bearing <90) and from behind (from all production with bearing> 90).

Bearing * - horizontal angle between the northern part of the meridian of the observer and the direction from the observation point to the object.

Among all the variables, the distance to the nearest unit before the prey (the target of the attack) had the greatest explanatory power in predicting the risk of attack, followed by the importance factor of the distance to the next unit after the prey and the difference in the direction of movement with the next unit ahead.


Video of observations of several variants of predator attacks on certain prey.

The risk of being the victim of a predator's attack was minimal if the unit was in the middle of the group, the distance between it and the neighboring units in front and behind was small enough, and the direction of movement of all units was the same. The bearing of adjacent units had no particular effect on the risk of attack. In other words, it is enough to be between two extraction units to increase your chances of not being eaten.

As a result, according to the simulation data, the predator attacks a certain prey not by chance, and the risk for prey is due to its position in the group, difference in the direction of movement and bearing of its neighbors (little influence factor).

In addition, predators can choose whom to attack, that is, not to attack immediately. The choice of the victim will depend not only on the available information about the group at the moment, but also when taking into account possible changes in its configuration in the future. That is, the predator can wait for the right moment to attack, based on the pattern of behavior of the group or individual prey.

To test this theory, scientists analyzed the results of simulations involving live predators and artificial prey. It was necessary to calculate which prey behavior patterns were most often used by the predator for an attack or he attacked in a random order.

Data analysis (more than 19,000 samples) showed that the predator chooses a certain moment to attack when there are more single units around it.


Image number 3

The graph above shows the proportions of attacks depending on the position of production. Circles are indicators of attacks during experimental observations. The colored areas on the graph show the distribution of the expected share of attacks to each prey position: if the predator attacked the prey randomly in the same cases as in the observed attacks (darker areas) or in selected simulation samples (lighter areas).

If the observed attacks (circles) are outside the color area, then predator targeting a specific victim was selective and non-random.

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

Epilogue

This study is the first real proof of a theory that claims that an individual at the head of a group is at greater risk of being attacked by predators. In this case, the risk can be reduced if the leader is located at a sufficiently small distance from the speculums that follow him. Also, scientists have confirmed the fact that being in a group (even in a leadership position) is much safer than moving individually.

Thus, the leader of the group not only has to take care of achieving the goal (for example, searching for food), but also monitor the integrity of the group, that is, its construction, which is extremely important not so much for the individuals it leads, but for its own survival. In the future, the researchers intend to find out exactly what actions the group leader should take to achieve the above objectives.

This work, of course, can not be called a revelation, but he gave real data confirming what was previously only a theory, untested in practice. Being a leader means knowing what the whole group needs. Being a leader and profitable, and dangerous at the same time - you can get to the first food, and you can become the first food. Despite the risks accompanying leadership in the wild, animals continue to form groups where there are both slaves and leaders. Perhaps at some genetic level, they understand that in a group, the chances of survival are greater than separate, even if you are a leader.



Thank you for your attention, stay curious and have a great weekend, guys. :)

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