Pathogen and immune dynamics during maturation are explained by Bateman's Principle

1. Bateman's principle, stating that female fitness is maximised by increasing longevity, suggests females are likely to exhibit a stronger immune response than males. Applying these ideas to maturation, it is predicted that immature beetles will also show enhanced immunity. This study looks at how life-history strategies interplay with infection responses in a well-established immunological model, the burying beetle, Nicrophorus vespilloides (Herbst (Silphidae)), and a pathogenic bacteria, Photorhabdus luminescens (Thomas and Poinar (Enterobacteriaceae)). 2. To test this, the immune enzyme phenoloxidase (PO) and bacterial loads in sexually mature and immature beetles were tested over time after injection with P. luminescens, and survival was monitored. Breeding after infection was also tested. 3. Bacterial loads were lower in females than males, and clearance seems more prolonged in immature than mature beetles, and both these groups showed increased survival, supporting the application of Bateman's principle to the effects of maturation and sex on immunity. 4. Mature beetles were found to undergo a shorter period of PO suppression after injection with bacteria. However, while high PO was beneficial up to 20 h post-infection, it became detrimental after that. This temporal factor has rarely been investigated but is shown here to be influential on the interpretation of PO activity, which is generally perceived as beneficial after infection. 5. A trade-off between reproduction and immunity was also found, revealed only by a highly pathogenic infection. This contrasts with the effects found in studies using non-pathogenic bacteria, suggesting an enforced, resource-based trade-off is the driving force behind the change, rather than an adaptive strategy.