The immunocompetence handicap hypothesis postulates that secondary sexual traits are honest signals of mate quality because the hormones (e.g. testosterone) needed to develop secondary sexual traits have immunosuppressive effects. The best support for predictions arising from the immunocompetence handicap hypothesis so far comes from studies of insects, although they lack male-specific hormones such as testosterone. In our previous studies, we found that female mealworm beetles prefer pheromones of immunocompetent males. Here, we tested how juvenile hormone (JH) affects male investment in secondary sexual characteristics and immune functions in the mealworm beetle, Tenebrio molitor. We injected male mealworm beetles with JH (type III) and found that injection increased the attractiveness of male pheromones but simultaneously suppressed immune functions (phenoloxidase activity and encapsulation). Our results suggest that JH, which is involved in the control of reproduction and morphogenesis, also plays a central role in the regulation of a trade-off between the immune system and sexual advertisement in insects. Thus, the results reflect a general mechanism by which the immunocompetence handicap hypothesis may work in insects.

Pheromones function not only as mate attractors, but they may also relay important information to prospective mates. It has been shown that vertebrates can distinguish, via olfactory mechanisms, major histocompatibility complex types in their prospective mates. However, whether pheromones can transmit information about immunocompetence is unknown. Here, we show that female mealworm beetles (Tenebrio molitor) prefer pheromones from males with better immunocompetence, indicated by a faster encapsulation rate against a novel antigen, and higher levels of phenoloxidase in haemolymph. Thus, the present study indicates that pheromones could transmit information about males' parasite resistance ability and may work as a reliable sexual ornament for female choice.

It has been assumed that sexual ornaments have evolved to reveal males' health and vigour for females. Choosy females may indirectly use ornaments as

Summary: 1. According to a recent hypothesis, personality traits should form integrative pace‐of‐life syndromes with life‐history traits. Potential life‐history traits that explain personality variation are immune defence and growth rate.2. We studied whether boldness, measured as hiding behaviour, is repeatable during ontogeny in the field cricket,Gryllus integer, and if it relates to the efficiency of immune function (i.e. the capacity to encapsulate a nylon implant), growth rate, developmental time and size as an adult.3. Hiding behaviour was rank‐order repeatable, and in general, juveniles were bolder than adults. Individuals that were cautious at early juvenile stages had higher encapsulation responses late in life compared with bold individuals. Most clearly, fast‐growing individuals matured early and invested little in immune defence compared with their slower‐growing conspecifics, i.e. showed patterns of a ‘grow fast, die young’ life‐history strategy.4. Our results may arise from a trade‐off between immunity‐dependent survival and bold behaviour. Trade‐offs between investment in survival and behaviour could account for the maintenance of variation in personality traits by favouring certain combinations of behavioural and life‐history strategies (i.e. pace‐of‐life‐syndromes).