Prey animals living under threat of predation must maximise their fitness by minimising their risk of predation through altering behaviour and selecting the most beneficial social environment. I used the Trinidadian guppy (Poecilia reticulata) as a model study species to investigate the potential effects of predation and conspecific phenotype on male social and sexual behaviours. Based on field collections of wild guppies from river populations in Trinidad that vary in their overall level of predation risk, I found significant differences in average male phenotype (i.e. body length and body coloration) among the populations, with males from the lowest predation-risk population being larger whereas males from an intermediate-risk population (where water is turbid due to mining activity) were the least colourful. I found no difference in guppy shoaling behaviour among populations but did note differences in sexual behaviour with males from the high-predation Quaré River more frequently performing sneak copulation attempts compared to males from less risky environments.

In the laboratory, I evaluated whether and how the phenotype of conspecific males affected the social association preferences of focal males. I found that focal males exhibited no shoaling preference for a specific male phenotype when presented with mixed-sex stimulus shoals. However, when females were absent, focal males preferred to associate with large, colourful males. As there was no significant difference among populations, I also tested male association preferences in the presence and absence of chemical alarm cues to ascertain whether a perceived immediate risk of predation would affect social choice. I found an overall preference for more attractive males in both the presence and absence of the alarm cue (with some exceptions), though males from the high-predation river moved between shoals less in the presence of an alarm cue, suggesting minimisation of visual conspicuousness. I also tested whether an increased male shoal size affected female social preference for more attractive males but results did not support this. Overall, my work suggests that wild-caught Trinidadian male guppies prefer to socially associate with certain males that differ in phenotype from themselves, though the evolutionary function(s) of such preferences remain uncertain and require further investigation.