As an organism, you basically have two things to worry about:
Both these things cost quite some energy. For the reproduction part, one often sees that males signal their quality to the female, which costs energy. The female might then decide to mate with the male or not. The fact that signalling costs energy, normally means that females can reliably find strong males, because weak males will not have enough energy to signal very well.
But what happens when an individual doesn’t have to put energy in survival anymore? Imagine this scenario: An individual gets terminally ill. Putting a lot of energy in survival would be useless, as it will die in any case. This energy would be put to better use in trying to make offspring. However, for obvious reasons, sick individuals are generally not very attractive to females.
Mattias Nielsen and Luke Holman built upon other studies regarding this problem using the mealworm beetle. In this beetle, females choose between males based on smell. The researchers fooled male beetles into thinking they were very very sick (a nice trick indeed!). By extracting the smell of ‘sick’ and healthy males, and presenting these smells to females, they show that ‘sick’ beetles smell much more attractive to females.
This might be because the ‘sick’ males don’t have to put energy into survival anymore, and instead try to be as sexy as possible, fooling the female in order to quickly produce some offspring before they die of illness. Surprising but true!
Nielsen, M.L., Holman, L. (2011) Terminal investment in multiple sexual signals: immune-challenged males produce more attractive pheromones. Functional Ecology 26, 20-28
Sadd, B., Holman, L., Armitage, H., Lock, F., Marland, R., Siva-Jothy, M.T. (2006) Modulation of sexual signalling by immune challenged male mealworm beetles (Tenebrio molitor, L.): evidence for terminal investment and dishonesty. Journal of Evolutionary Biology, 19, 321-325