Maturity matters for movement and metabolic rate: trait dynamics across the early adult life of red flour beetles

Transitioning between life stages involves significant changes to the physiology, structural morphology, biochemistry and behaviour of an organism. Eclosion, metamorphosis and the onset of sexual maturity have consequences for the life history evolution of an organism by initiating reproductive and dispersal-related behaviours that are both energetically costly and directly related to fitness. Animal movement, particularly dispersal when sexually mature, is critical for mate location, controlling population density and promoting gene flow. Here we examined changes in dispersal-related and physiological traits during a significant transitional period in red flour beetles, Tribolium castaneum. We measured the metabolic rate, spontaneous activity, body size and movement behaviour traits of individuals of known age and sex. Traits were compared between immature and sexually mature adults as well as during early adult life when there is a strong tendency to disperse and reproduce. Spontaneous activity, movement speed and metabolic rate were distinctly reduced in immature adults prior to the onset of sexual maturity, and immature individuals moved more intermittently than mature ones. We found that these traits increased with age following eclosion, up to a relatively stable mean once sexual maturity was attained. The reduced metabolic expenditure found in immature individuals was attributable to a reduced energy demand due to relative inactivity, which we hypothesize to be a protective mechanism from conspecifics while the cuticle is undergoing sclerotization. Understanding the precise developmental trajectories of behavioural and physiological traits allows us to interpret the trait syndromes that underlie dispersal and their evolution.