Scientists from the University of Exeter, University of York and University of the West Indies, St Augustine, observed Trinidadian guppies and found the fish developed stronger and more stable social bonds when they thought predators were in the area. Intriguingly, this also coincided with social groups being smaller - suggesting a possible conflict between being able to form strong social relationships and being able to live in larger social groups."

Predator threat boosts friendships among guppies
https://www.eurekalert.org/pub_releases/2017-02/uoe-ptb013017.php

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Abstract
"Social relationships can have important consequences for fitness in animals. Whilst numerous studies have shown that individuals often join larger groups in response to perceived predation risk (i.e. fear of predation), the importance of predation risk in driving the formation and stability of social relationships within groups has been relatively ignored. We experimentally tested how predation threat influenced fine-scale social network structure using Trinidadian guppies (Poecilia reticulata). When perceived predation risk was high, individuals developed stable and more differentiated social ties compared to when perceived risk was low. Intriguingly, social differentiation coincided with shoals being somewhat smaller under high-perceived risk, suggesting a possible conflict between forming stable social relationships and larger social groups. Individuals most at risk of predation (large and bold individuals) showed the most exaggerated responses in several social measures. Taken together, we provide the first experimental evidence that proximate risk of predation can increase the intensity of social relationships and fine-scale social structure in animal populations."

From:
Fear of predation drives stable and differentiated social relationships in guppies
https://ore.exeter.ac.uk/repository/handle/10871/25280