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Transmission of Danger Information Past Physical Barriers by Ants Transmission of danger information past physical barriers by ants A.Reyes1, M.Curbelo2, F.Tejera3, A.Rivera4, G. Simon5, O.Ramos5, M.S.Turner7, and E.Altshuler1,* 1Group of Complex Systems and Statistical Physics, University of Havana, 10400 Habana, Cuba 2Biology Faculty, University of Havana, 10400 Havana, Cuba 3The Rockefeller University, New York, USA 4Zeolites Engineering Lab and Group of Complex Systems and Statistical Physics, Physics Faculty, University of Havana, 10400 Havana, Cuba 5Institut Lumiere` Matiere,` Universite´ Claude Bernard Lyon 1, France 7Physics Department & Centre for Complexity Science, University of Warwick, Coventry, CV4 7AL, UK *ealtshuler@fisica.uh.cu ABSTRACT Can insects transmit information about a threat through a physical barrier they cannot cross? While the emission and detection of alarm pheromones is the universally accepted mechanism of panic communication, there are still open questions about the specific pheromones and type of ant-to-ant contacts involved in specific scenarios, and how relevant is the role of non-chemical communication. Here we report experiments suggesting that “panicked” ants can transmit danger information through a physical barrier to nestmates located in a safe arena when danger is simulated by using insect repellent. As ants in the safe arena get danger information from nestmates trapped in an area contaminated with a chemical, they retreat from the barrier in average. Visual clues and pure mutual antennation seem to play a minor role as mechanisms of danger information transmission through the obstacle. A detailed quantification of the ant behavior reveals that examination of the legs of ants in the danger arena by nestmates in the safe one may be relevant for danger information transmission. Introduction One of the first attempts to demonstrate that ants are able to transmit danger information to other ants was reported in 1881 by Sir John Lubbock. To investigate the matter, he used state of the art technology of his day: the telephone. He “disturbed” a nest of Lasius niger having a telephone held close over the nest. A second telephone was placed at a nearby nest of the same species, assuming that, if any audible danger signal was transmitted through the telephone line, “the ants in the other nest ought to have been thrown in confusion”. Unfortunately, the author did not find any evidence of it1. More than 130 years later, the subject of how ants communicate danger is still not fully understood. Ants are known to use a wide variety of mechanisms for communication2–4 like acoustic signals5, tapping6, oral transfer of substances (trophalaxis)7 and even “simple" physical contact8. However, there is consensus that the emission and detection of specific chemicals called pheromones is their main communication mechanism9, 10. In fact, ants use different pheromones arXiv:1904.03236v1 [q-bio.PE] 5 Apr 2019 to transmit information related to scenarios spanning from foraging to danger. Communication mechanisms are further complicated by the fact that the effects of pheromones are caste, age, and context-dependent11, 12. In particular, the reaction to alarm pheromones may be either “Flight" or “Fight": in the first, the receivers escape from the source of danger, while in the second they are attracted to, and eventually attack, the threat13, 14. However, which of the two is triggered by the pheromone depends on many factors, such as the pheromone specific chemical composition and concentration, the spatial context where the emission and detection takes place, and the age of the participating ants2, 15, 16. The eventual touching between the antenna of two ants, known as mutual antennation, is believed to allow recognition of members of the same nest2 —a process involving both sensing and emission of chemical signals17, 18. Other uses of mutual antennation are the subject of debate, like the possibility of communicating nestmates the correct (and incorrect!) foraging paths4,8, 19 . One unanswered question, for example, is whether ants can transmit any danger information by mutual antennation. Our previous research on foraging ants submitted to a localized perturbation suggests that danger information is not transmitted by mutual antennation over large distances20: a short-range study is necessary. Here we investigate the transmission of information from a group of ants located into a first physical space were they are experiencing danger associated to an insect repellent, to ants located into a different physical space. Our experiments reveal that “panicked” ants of the species Atta insularis20–22 locked into an arena where the repellent is injected, are able to transmit danger information to ants in a safe arena, located on the opposite side of a separating barrier. Instead of being “thrown in confusion” (or trying to help their endangered nestmates) the ants in the safe area react by moving away from the barrier. Experiments performed with different types of barriers suggest that visual clues and mutual antennation are not the ways danger information is transmitted: “body search” seems to be needed –with emphasis in legs. Results A system consisting in two-cells separated by a barrier was designed to probe the possible transmission of danger information by ants in one cell to ants in the other cell through an obstacle –see Fig.1A. In a typical experiment, ants are deposited in both cells, which are filmed during 5 minutes. Then, an insect repellent is added at the center of the DANGER cell, and another 10 minutes of film is taken. In a CONTROL experiment, the same procedure is followed, except for the fact that no ants are introduced into the DANGER arena. We hypothesize that, if ants in the SAFE arena move away from the barrier after the addition of the repellent a significantly larger distance than they do during a CONTROL experiment, DANGER information has been transmitted by the “panicked" ants in the DANGER cell to the ants in the SAFE cell, beyond any “passive" diffusion of the repellent through the barrier. In an attempt to understand the details of the possible mechanisms of danger information transmission, we use different kinds of barriers (see section “Experimental details"). The GLASS barrier is a glass wall, the THICK barrier is made in such a way that ants from both sides of the barrier can touch their respective antennas, but cannot antennate the bodies of individuals in the opposite arena (Fig.1B), while the THIN barrier does allow it (Fig.1C, D). A SAFE SAFE DANGERDANGER D D B C Figure 1. Danger transmission experiments. (A) General view of the two cell arena for 2D experiments (less ants than in real experiments are sketched). (B) Top view of Atta insularis ants near a THICK barrier (C) Same, near a THIN barrier. (D) Lateral view of an ant in the SAFE cell near a THIN barrier. As a reference, the length of the ants bodies is around 1 cm. The experiments are performed in workers of the ant species Atta insularis, called bibijagua in Cuba. Collective behavior: response of the center of mass After converting the video images to binary format, we track the Center of Mass (CM) of the black pixels (assumed to be proportional to the presence of ants) in both cells for each protocol. As illustrated in Fig.2, A to C, for typical experiments in the 2D Arena, the motion of the CM at the DANGER cell is always near its geometrical center, independently from the 2/11 protocol used. A similar situation holds for the SAFE cell, with the conspicuous exception of THIN, where the CM of mass tends to move away from the barrier starting a few seconds after the repellent is added to the left cell. Fig.2, E to H shows the time evolution of the x-position of the CM at the SAFE cell, corresponding to the average between 10 repetitions of each type of experiment (the plotted data is tabulated in table TableA1 as Additional Information, where xi and x f are the initial and final potisions of the CM, respectively). DANGER SAFE A E 1 0 -1 B 1 F 0 Glass I 10 -1 Thick 10 C G 1 Thin 10 0 Control 10 -1 -0.4 0.0 0.4 0.8 1.2 D 1 H Net displacement of the CM (cm) 0 -1 x 0 2 4 6 8 10 12 14 Time (min) Figure 2. Collective behavior: motion of the ants “center of mass”. The upper left panel shows snapshots of experiments (A) GLASS (B) THICK (C) THIN and (D) CONTROL (see text for details). We indicate the motion of the ants center of mass by green, blue and red traces corresponding to consecutive 5-minutes time windows from the start of an experiment (the snapshots were taken at the end of each experiment). From (E) to (H) we indicate how the center of mass (averaged over 10 repetitions) evolves in time along the horizontal direction inside the right (SAFE) cell for each protocol (x = 0 is taken at the center of the cell). Light-colored shadows indicate the standard deviation from the mean. Notice that a repellent is added to the left (DANGER) cell at t = 5 minutes into the repellent reservoir seen as a white spot in (D). (I) Net horizontal motion of the ants “center of mass”, based on 10 repetitions of each type of experiment. The data was analyzed based on two k-sample t tests described in detail in the Statistical Analysis section. After each experiment is replicated 10 times, we quantify the motion of the CM in the SAFE cell by plotting its net displacement, calculated as the difference between its final and its initial positions averaged over the 5th and 15th minutes of the experiment, respectively (averaging over the first and the last 5 minutes yields qualitatively similar output).
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