Developmental temperature, not inbreeding, shapes life history and locomotor behaviours in juvenile guppies ( <i>Poecilia reticulata</i> )

Inbreeding plays a strong role in shaping life-history traits and behaviours. Supporting evidence for this role often comes from observational studies. Experiments that establish causality and formally test how environmental factors moderate any effects of inbreeding remain underutilized. We ran an experiment to test how developmental temperature and inbreeding influence key life-history traits (growth rate, size and age at maturity, survival, and sex ratio) and locomotor behaviours (boldness and habituation) in juvenile guppies (Poecilia reticulata). We used a controlled breeding design to generate inbred and outbred individuals that were then reared under a control (26 °C) or an elevated temperature (30 °C) until maturity. Developmental temperature strongly affected life-history traits; both sexes matured earlier at 30 °C, but only males exhibited slower early growth and reduced size at maturity. Female growth and size at maturity were unaffected. The higher developmental temperature reduced boldness in both sexes; however, only females at 26 °C habituated to the novel test environment. In contrast, inbreeding had no significant effects on any of the measured traits, nor did it significantly interact with temperature. In sum, under our experimental conditions, developmental temperature is the primary driver of phenotypic plasticity in guppies, generating sex-specific responses in both life-history traits and behaviour, while inbreeding between siblings had no detectable effects on any of the measured traits. Our findings highlight the key role of temperature in shaping developmental and behavioural trajectories, and reveal that a single generation of inbreeding may not always affect life-history traits, even under environmental stress.