Evolution of individual variation in behaviour and behavioural plasticity under scramble competition

• Males tend to mount others indiscriminately under scramble competition.
• We measured male sensitivity to another male or a mating pair.
• Sensitivity traits showed consistent between-individual variation.
• Males also showed between-individual variation in the plasticity to context.
• Sensitivity in both contexts influenced mating success of males.

In scramble competition mating systems, individual behaviours related to male–male competition are expected to be plastic across sociosexual environments. However, the evolutionary forces that shape individual differences in behavioural plasticity remain poorly understood. We measured behaviour associated with scramble competition in male water striders, Tenagogerris euphrosyne, in two social contexts. Male T. euphrosyne tend to mount other individuals or pairs indiscriminately and then dismount if their target is either another male or a mating pair. We measured the speed with which a focal male dismounted (i.e. his sensitivity to having made an inappropriate mounting) in each of these contexts. We quantified temporal consistency of dismount speed within each context, as well as individual variation in behavioural plasticity across contexts. We then estimated the effect of the behaviour and of plasticity on individual fitness in different sex ratio conditions. We found that individuals differed in their recognition sensitivity traits, and also in how plastic their sensitivity (i.e. dismount speed) was to context (mating status or sex). When we measured the mating success of each male within experimental groups kept at four different sex ratios, recognition sensitivity traits in both contexts interacted to influence mating success, and there were significant differences in linear selection on sensitivity traits between sex ratio treatments. We discuss how insensitive males evolve, and how individual variation in behavioural plasticity could be maintained by selection or environmental variability.