Why don't we like mouldy food? It seems like a no-brainer, but why, in fact, should most foods become unpalatable when colonized by microbes? The existence of Roquefort, after all, shows that mould does not always equal bad food.
1977, Dan Janzen suggested that microbes spoil food so that they can keep it for themselves — it's a trick that helps them compete for resources with animals, and avoid being eaten themselves. It's not just humans that prefer fresh food to rotten — birds do to, given the choice.
The idea has attracted a lot of favourable comment from researchers, but, say Tom Sherratt and his colleagues in Ecological Modelling, it has never been properly analysed. They have done this — and it's not good news for Janzen's idea.
Sherratt's team present a mathematical model that shows that if producing chemicals that spoil food is costly, then the microbes that do it will be displaced by free-loaders, that take the benefit of living on spoiled food, but don't pay the cost.
The 1970s (around the time the Selfish Gene came out) were a bit of a golden age for ideas like Janzen's, that propose for ingenious adaptive explanations for biological phenomena. It's surprising that no one got around to putting the idea through the theoretical wringer before now, but not so surprising that the idea hasn’t stood up to close analysis, because freeloaders are probably a powerful force against group behaviour of this sort.
If all the microbes in a group belong to a genetically identical clone, then spoiling might work, but Sherratt and co suggest that microbes move around too much for clones to maintain their integrity. (Entirely tangentially, for my PhD I worked on a similar problem regarding the evolution of altruism, in the form of soldier behaviour, in aphids.)
The researchers suggest, more prosaically, that spoiling chemicals might be used in competition between microbes sharing a food source, or they might just be a by-product of the process of decomposition.