There’s a hidden war going on for your fruit. The apple snatchers and orange thieves aren’t what you might think, though. Humans and other fruit-loving species are locked in an ongoing evolutionary battle against the microbes that also want to feast.
Now, researchers believe they have found out why competition between mammals and microbes is so fierce, and how it explains why fruit goes bad.
The Point of Fruit
Fruit evolved to be eaten. Over millions of years, plants discovered that if they wrapped their seeds in a delicious, sugary package, they could convince animals to disperse their seeds for them. Animals eat the fruit in one location and expel the seeds in their waste elsewhere, with feces providing a nutritious start for the seedlings.
Still, ecologist Daniel Janzen was perplexed. If fruit was meant to be eaten, then why did it contain secondary chemicals such as anthraquinones, tannins, and flavonoids that were less appealing to would-be fruit eaters and seed dispersers? Wouldn’t it be in the plant’s best interests to make the fruit as appetizing as possible? His 1977 breakthrough occurred when he realized that microbes were also competing for the fruit. In a paper published in American Naturalist, Janzen argued that plants produced these secondary compounds as antimicrobial agents to deter microbes from gobbling up the fruit before animals could eat it and move the seeds.
The microbes, however, weren’t simply watching the action from the stands. While plants tried to deter them, they tried to deter animals at the same time.
Since animals don’t like eating food contaminated by microbes, even just a hint of spoilage would cause them to leave the food alone. Janzen theorized that spoiling microbes gained a benefit over non-spoiling microbes because they could better hog the food for themselves. Both the plant and the hungry vertebrates lose when this happens. Fruit spoils, the theory said, because microbes are good—very good—at what they do.
But like lots of signals in nature, the “keep away” chemicals produced by microbes are costly to make. Some mathematical models have found that the cost of producing noxious chemicals to drive away animals overshadows any benefits gained by the spoiling microbes. In fact, it was the perfect opportunity for freeloaders: non-spoiling microbes would get to enjoy the fruit without synthesizing the costly chemicals.
More recently, however, scientists revisited that 2006 study and changed one key assumption. Instead of assuming that any piece of fruit could by colonized by any number of microbial species, the researchers hypothesized that the microbes were much less mobile. Microbes, they proposed, exist as a “metapopulation” – spatially separated but still occasionally interacting. This slight shift in initial conditions tilted the competitive playing field, making it less likely that the two types would go head-to-head on one piece of fruit.
That change made all the difference. In a study published recently in Proceedings of the Royal Society B, behavioral ecologist Graeme D. Ruxton and colleagues presented a new model of microbial spoilage. The scientists determined that even when there was a high cost to synthesizing stinky compounds, spoilers could still have an advantage. Fruit carrying spoiling microbes lasts longer in the environment, ensuring a longer life for the microbes. This advantage only occurs when spoiling microbes can get to the fruit before their non-spoiling brethren arrive.
So the next time you inspect your fruit drawer and suspect there might be something almost sentient hiding out, you’re probably right. As you wrinkle your nose to dispose of your furry fruit, remember it’s one more war the microbes are winning.
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