Imroze and Arun’s paper describing immunosenescence in flour beetles will be out in the January 2016 issue of the Journal of Animal Ecology! You can find the paper here.
Here’s a brief summary of the work described in the paper. In many animals, immune function decreases with age so that older animals are more likely to die from infections. We found that this pattern is also true for flour beetles (Tribolium castaneum): older beetles are more susceptible to infection by Bacillus thuringiensis. Oddly though, individual components of innate immune function (such as phenoloxidase activity) did not decrease with age. This mismatch could arise due to tradeoffs with other fitness components (e.g. fecundity or external antibacterial secretions). If young beetles invested in other aspects of fitness, they may not be able to invest more in immune function, and hence the levels of innate immune components may be lower than optimal. However, we did not find evidence for such a tradeoff. Another possibility is other factors affecting immune function end up muddying the expected relationship between immune components and age. Indeed, we found that a beetle’s sex and mating status also affect its immune function, and complex interactions between these factors determine immune function. The molecular mechanisms mediating these effects remain unclear and it is likely that we are missing other important factors that alter immune function. However, our work shows that a deeper understanding of life history, tradeoffs and fitness is necessary to understand how and why animals become more susceptible to infections as they age.