Laterality, the partitioning of information processing into specific brain hemispheres, is widespread across animal taxa. Substantial unexplained variation in this trait exists, particularly between the sexes, despite multiple identified advantages of lateralisation. Here, we demonstrate a relationship between laterality (measured as directional biases), reproduction and experience of mating and parenting. Using three-spine sticklebacks Gasterosteus aculeatus, a species with uniparental male care, we showed that individuals of the caring sex (males) were more strongly lateralised than the non-caring sex (females) during reproduction, and that laterality was reduced outside the breeding season in males. Additionally, males with experience of mating and parenting were more strongly lateralised than males without this experience. Our findings suggest that fitness related behaviours that vary between the sexes, such as reproductive behaviours including courtship, spawning and parenting, are significant but previously unidentified sources of variation in laterality.

ID: Unique individual ID used to account for repeated measures.

Sex: Sex of the fish (male or female).

Stage: Stage of reproduction. This describes what reproductive behaviours individuals fish had performed when each measurement was taken. 

Right turns: Number of turns to the right side out of a total of 10 in a standard detour test.

Left turns: Number of turns to the left side out of a total of 10 in a standard detour test.

Relative laterality index: Relative laterality index, L_R, is calculated as [(right detours – left detours) / (right detours + left detours) x 100] and assesses directional biases of fish at the population level. L_R ranges from -100 to +100, representing a population that detoured consistently leftward or rightward respectively.

Absolute laterality index: Absolute laterality index, L_A, is calculated as |L_R| and assesses the strength of laterality at the individual level. L_A ranges from 0, individuals that turned left and right an equal number of times, to 100, individuals that turned consistently in one direction. 

Detours: Total number of detours in a single trial (10).

Notes: Any additional notes for individual fish e.g. deaths etc. 

Treatment: Describes whether fish were in the treatment group (reproductive breeding males) or control treatment (non-breeding males). 

Order: The order in which repeated measures occured for each individual.

Body size: Standard length of each individual (cm).