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

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.