Limited Evidence That Developmental Diet and Density Affect Adult Female Earwig Behaviour Despite Repeatable Individual Differences.

Developmental environments can have lasting effects on adult behaviour, shaping mean activity, repeatability and within-individual variability in potentially independent ways. We used a split-brood experiment to test whether developmental diet quality and social density affected adult open-field behaviour in female European earwigs (Forficula auricularia sensu lato). Nymphs were reared on a standard or nutrient-diluted ad libitum diet at one, four or eight individuals per dish. Adult females were assayed six times over 16-21 days. Females showed repeatable individual differences in open-field behaviour, particularly distance travelled. A beta-binomial mixed-effects analysis showed substantial treatment-associated differences in survival, which was lower on the nutrient-diluted diet and lower in group-reared than individually reared nymphs. These differences confirmed biological consequences of the treatments and indicated treatment-dependent selective survival that may have filtered phenotypes before testing. Multivariate Bayesian mixed-effects models provided limited evidence that developmental diet or density consistently affected mean behaviour, repeatability, individual change across trials or residual within-individual variability among surviving adult females. Distance travelled and centre use were strongly negatively correlated among individuals; because both were derived from the same arena assay, we interpret this as a shared axis of arena use rather than an independent behavioural syndrome. Repeatable individual differences in open-field behaviour were thus detectable but not clearly explained by the developmental treatments, with treatment-dependent selective survival an additional interpretive constraint.