Diversity in animal groups is often assumed to increase group performance. In insect colonies, genetic, behavioural and morphological variation among workers can improve colony functioning and resilience. However, it has been hypothesized that during communication processes, differences between workers, e.g. in body size, could also have negative effects. Tandem running is a common recruitment strategy in ants and allows a leader to guide a nestmate follower to resources. A substantial proportion of tandem runs fail because leader and follower lose contact. Using the ant Temnothorax nylanderi as a model system, we tested the hypothesis that tandem running success is impaired if leader and follower differ in size. Indeed, we found that the success rate of tandem pairs drops considerably as size variation increases: tandem runs were unsuccessful when the leader–follower size difference exceeded 10%, whereas ~ 80% of tandem runs were successful when ants differed less than 5% in body length. Possible explanations are that size differences are linked to differences in walking speed or sensory perception. Ants did not choose partners of similar size, but extranidal workers were larger than intranidal workers, which could reduce recruitment mistakes because it reduced the chance that very large and very small ants perform tandem runs together. Our results suggest that phenotypic differences between interacting workers can have negative effects on the efficiency of communication processes. Whether phenotypic variation has positive or negative effects is likely to depend on the task and the phenotypic trait that shows variation. Significance statement Diversity is often assumed to increase colony performance in social insects. However, phenotypic differences among workers could also have negative effects, e.g. during communication. Tandem running is a common recruitment strategy in ants, but tandem runs often fail when ants lose contact. We used the ant Temnothorax nylanderi to test the hypothesis that body size differences between tandem leader and follower impair tandem communication. We show that the success rate of tandem pairs drops considerably as size variation increases, possibly because ants of varying size also differ in walking speed. Our study supports the hypothesis that phenotypic variation among workers might not always be beneficial and can negatively impact the efficiency of communication processes.

Large body size variation is linked to low communication 1 success in tandem running ants 2 3 4 Abstract 22Diversity in animal groups is often assumed to increase group performance. In insect colonies, 23 genetic, behavioral and morphological variation among workers can improve colony 24 functioning and resilience. However, it has been hypothesized that during communication 25 processes, differences between workers, e.g. in body size, could also have negative effects. 26Tandem running is a common recruitment strategy in ants and allows a leader to guide a 27 nestmate follower to resources. A substantial proportion of tandem runs fail because leader and 28 follower loose contact. Using the ant Temnothorax nylanderi as a model system, we tested the 29 hypothesis that tandem running success is impaired if leader and follower differ in size. Indeed, 30 we found that the success rate of tandem pairs drops considerably as size variation increases: 31 only ~7% of tandem runs were successful when the leader-follower size difference exceeded 32 10%, whereas 80% of tandem runs were successful when ants differed less than 5% in body 33 length. One possible explanation is that ant size is linked to the preferred walking speed. Ants 34 did not choose partners of similar size, but extranidal workers were larger than intranidal 35 workers, which could reduce recruitment mistakes because it reduced the chance that very large 36 and very small ants perform tandem runs together. Our results suggest that phenotypic 37 differences between interacting workers can have negative effects on the efficiency of 38 communication processes. Whether phenotypic variation has positive or negative effects is 39 likely to depend on the task and the phenotypic trait that shows variation. 40 41 42 Social groups consist of individuals that differ from each other in a number of ways. For 43 instance, people working in a company may differ in experience, training, gender, ethnicity or 44 skills and this diversity can affect group performance and success. Scientists interested in 45 organizational theory have found that group diversity often has positive effects on group 46 performance, most likely because diverse groups possess a broader range of knowledge, 47