"Enriched" mice have less complex social interactions than mice raised in a "modest" environment

Scientists at the Weizmann Institute have developed an automatic system for long-term monitoring of a group of mice, under quasi-natural conditions; and unique mathematical tools that make it possible to analyze the information, and draw from it highly sensitive conclusions about their social behavior.

It is commonly assumed that the modern environment, rich in stimuli, encourages individual behavior (not to mention shuts down), while people living in more modest environments gain a developed community life. A study by scientists at the Weizmann Institute of Science reveals that, as far as mice are concerned, this explanation is correct: "enriched" mice raised in a highly stimulating environment, in their adulthood, have less complex social interactions than mice raised in "spartan" conditions. The discovery was made possible thanks to a double development: an automatic system for long-term monitoring of a group of mice, under quasi-natural conditions; and unique mathematical tools that make it possible to analyze the information, and draw from it highly sensitive conclusions about their social behavior.

The study, published today (Tuesday) in the scientific journal eLife, is the result of a collaboration between two research groups from the Department of Neurobiology at the Weizmann Institute of Science, from two opposite ends of the research approach: Prof. Alon Chen is an experimentalist, who focuses on behavioral and molecular studies, and Dr. Elad Schneidman is a theorist who specializes in the characterization of complex biological systems. The experimental system created by the team of researchers, which included Dr. Yair Shemesh, Dr. Oren Furkosh, Dr. Hezi Steinberg and Tamar Shalfoversky, was composed of a large "arena" with focal points of interest such as nests for sleeping, feeding and playing facilities, and more. Groups of four mice were introduced into the arena, whose hair was dyed with different shades of punk hair dye, which glowed under ultraviolet light. The use of ultraviolet light, which is invisible to the mice, allowed them to carry out their regular activity routine (mainly in the dark) without interruption. The activity of the mice was recorded during four consecutive nights, with the highest resolution.

The tracking data showed that the mice spend most of their time in ten points of interest in the arena. Although the number of possible combinations of four mice in ten locations is 10,000, the scientists discovered that the interactions between the mice limit the possible group situations, because some of them are "not acceptable" from a social point of view. A theoretical calculation based solely on the individual preferences of each mouse predicted the existence of 4,000 combinations, but only half of them actually existed.

Later it became clear that in addition to the accepted statement that "a group is more than the collection of individuals that make it up", it is also possible to say that "a group is more than the collection of possible pairs within it". The scientists tested whether information about the location of one mouse makes it possible to determine the location of another mouse. The data analysis of pairs of mice showed that the chance of correctly determining the location of a certain mouse based on information about the location of another mouse is 5%. When trying to determine the location of a mouse based on the locations of all three other mice, the chance increases fivefold. A very large part of the information about the location of the mouse - 75% - does not depend on social interactions; This is the degree of freedom of the individual to decide where he wants to be.

To determine the extent of the influence of personal traits against social dependence, the scientists used a mathematical-physical model, called the "Maximum Entropy Model", which provides an explanation for a collection of observations, while relying minimally on prior assumptions. Using the model, all possible degrees of dependence between mice were tested - starting from the behavior of individual mice (lack of dependence), through pairwise dependence, to third-order and fourth-order dependence - that is, between all members of the group. The model, which is used in a variety of biological and physical studies dealing with interactions in complex systems, yielded surprising conclusions: unlike all the systems that have been tested using it so far (such as networks of nerve cells in the brain or genetic networks), where the decisive contribution to the system's behavior came from interaction in pairs, the social behavior of mice relies, To a much larger extent, on a triangular structure. This finding is also surprising in light of the fact that a large part of social activities are based on couple interactions, such as chases, courtship and fights. Why does social activity require such complex interactions, even more than those of the nerve cells in the brain? The scientists believe that the reason for this lies in the importance of social information for survival, which led to the development of highly sophisticated mechanisms for its analysis.

Are there differences in the social structure of groups of mice raised in different environments? And is the maximum entropy model able to detect such differences? To answer these questions, the scientists used a model to compare the social structure of an "enriched" group of mice, raised in a large cage with a variety of facilities and accessories, with that of a group raised under standard conditions. It turned out that an enriched environment during the period of childhood and adolescence (four to ten weeks of mouse age) significantly affects the social structure of the group of mice: these mice showed less coordinated social behavior, and pair interactions explained a larger part of the group's behavior. In contrast, the second group demonstrated complex interrelationships, and even demonstrated a gradual progress in the degree of their "sociability" during the days of the experiment - progress that was not seen in the "enriched" group. The scientists hypothesize that the difference is due to the fact that a rich society allows more individualism and freedom, and reduces the mouse's dependence on the group. Another possibility is that an enriched environment encourages aggression, as well as causes a stronger influence of the dominant mouse - two characteristics related to marital relationships.

The development of a sensitive research tool, which makes it possible to examine social structures and compare them, in combination with an automatic system for tracking groups of mice, and methods from the field of molecular biology, opens countless research possibilities. How do mutations in different genes affect social behavior? How will mice that produce an excess amount of oxytocin ("the love hormone") or testosterone behave? What are the unique social characteristics in cases of autism or schizophrenia? The current study paves the way for answers to these questions, and many other questions.