Decisions take time if information gradually accumulates to a response threshold, but the neural mechanisms of integration and thresholding are unknown. We characterized a decision process in Drosophila that bears the behavioral signature of evidence accumulation. As stimulus contrast in trained odor discriminations decreased, reaction times increased and perceptual accuracy declined, in quantitative agreement with a drift-diffusion model. FoxP mutants took longer than wild-type flies to form decisions of similar or reduced accuracy, especially in difficult, low-contrast tasks. RNAi knock-down of FoxP in αβ core Kenyon cells, or the overexpression of a potassium conductance in these neurons, recapitulated the FoxP mutant phenotype. A mushroom body subdomain whose development or function require the transcription factor FoxP thus supports the progression of a decision towards commitment.

Living in a group allows individuals to decrease their defenses, enabling other beneficial behaviors such as foraging. The detection of a threat through social cues is widely reported, however, the safety cues that guide animals to break away from a defensive behavior and resume alternate activities remain elusive. Here we show that fruit flies display a graded decrease in freezing behavior, triggered by an inescapable threat, with increasing group sizes. Furthermore, flies use the cessation of movement of other flies as a cue of threat and its resumption as a cue of safety. Finally, we find that lobula columnar neurons, LC11, mediate the propensity for freezing flies to resume moving in response to the movement of others. By identifying visual motion cues, and the neurons involved in their processing, as the basis of a social safety cue this study brings new insights into the neuronal basis of safety in numbers.

13Living in a group allows individuals to decrease their defenses enabling other beneficial 14behaviors such as foraging. The detection of a threat through social cues is widely reported, 15however the safety cues that guide animals to break away from a defensive behavior and 16resume alternate activities remain elusive. Here we show that fruit flies displayed a graded 17 decrease in freezing behavior, triggered by an inescapable threat, with increasing group 18 sizes. Furthermore, flies used the cessation of movement of other flies as a cue of threat and 19 its resumption as a cue of safety. Finally, we found that lobula columnar neurons, LC11, 20 mediate the propensity for freezing flies to resume moving in response to the movement of 21 others. By identifying visual motion cues, and the neurons involved in their processing, as 22the basis of a social safety cue this study brings new insights into the neuronal basis of 23 safety in numbers. 24 25

No abstract

Animals in groups integrate social information with that directly-gathered about the environment to guide decisions regarding reproduction, foraging and defence againts predatory threats. In the context of predation, usage of social information has acute fitness benefits, aiding the detection of predators, the mounting of concerted defensive responses, or allowing the inference of safety, permitting other beneficial behaviours such as foraging for food. Individual and group defence responses to predatory threats can vary in modality and vigour depending on the perceived threat level. Moreover, predation level has been shown to modulate the use of social cues about foraging sites. Whether and how different threat levels affect the use of social cues to guide defence responses, is currently unknow. We previously showed that Drosophila melanogaster display a graded decrease in freezing behaviour, triggered by an inescapable visual threat, with increasing group sizes. Crucially, we identified the movement of others as a cue of safety and its cessation a cue of threat and found the group responses to be primarily guided by the safety cues, resulting in a net social buffering effect. Here, we investigated how threat level impacts the use of social cues by exposing flies individually and in groups to two threat imminences using looms of different speeds. We show that freezing responses are stronger to the faster looms regardless of social condition. However, social buffering was stronger for groups exposed to the fast looms, such that the increase in freezing caused by the higher threat was less prominent in flies tested in groups than those tested individually. Through artificial control of behaviour, we created different group compositions, titrating the motion cues that were maintained across threat levels. We, found that the same level of safety motion cues had a bigger weight on the flies' decisions when these were exposed to the higher threat, thus overriding differences in perceived threat levels. These findings shed light on the 'safety in numbers' effect, revealing the modulation of the saliency of social safety cues across threat intensities, a possible mechanism to regulate costly defensive responses.

Animals in groups integrate social with directly gathered information about the environment to guide decisions regarding reproduction, foraging, and defence against predatory threats. In the context of predation, usage of social information has acute fitness benefits, aiding the detection of predators, the mounting of concerted defensive responses, or allowing the inference of safety, permitting other beneficial behaviors, such as foraging for food. We previously showed that Drosophila melanogaster exposed to an inescapable visual threat use freezing by surrounding flies as a cue of danger and movement resumption as a cue of safety. Moreover, group responses were primarily guided by the safety cues, resulting in a net social buffering effect, i.e., a graded decrease in freezing behavior with increasing group sizes, similar to other animals. Whether and how different threat levels affect the use of social cues to guide defense responses remains elusive. Here, we investigated this issue by exposing flies individually and in groups to two threat imminences using looms of different speeds. We showed that freezing responses are stronger to the faster looms regardless of social condition. However, social buffering was stronger for groups exposed to the fast looms, such that the increase in freezing caused by the higher threat was less prominent in flies tested in groups than those tested individually. Through artificial control of movement, we created groups composed of moving and freezing flies and by varying group composition, we titrated the motion cues that surrounding flies produce, which were held constant across threat levels. We found that the same level of safety motion cues had a bigger weight on the flies’ decisions when these were exposed to the higher threat, thus overriding differences in perceived threat levels. These findings shed light on the “safety in numbers” effect, revealing the modulation of the saliency of social safety cues across threat intensities, a possible mechanism to regulate costly defensive responses.