Some light reading for those with plenty of time:
Loners help society survive
Evolution selects for 'loners' that hang back from collective behavior -- at least in slime molds
It isn't easy being a loner -- someone who resists the pull of the crowd, who marches to their own drummer.
But loners exist across the natural world, and they might just serve a purpose, said Corina Tarnita, an associate professor of ecology and evolutionary biology. She ticked off examples of loners who sit out their species' collective actions: the small herd that skips the great wildebeest migration, the locusts that peel off from the swarm and revert to calm grasshopper behaviors, the handful of bamboo that flower a few days before or after the rest of the species, and the slime molds that hang back from forming the swaying towers studied by Princeton luminary John Bonner.
"Now that we're starting to look for it, we realize that a whole lot of systems are not perfectly synchronized -- and it's tantalizing to think that that there may be something to this imperfect synchronization," Tarnita said. "Individuals that are out-of-sync with the majority of a population exist in humans, too. We call them misfits or geniuses, contrarians or visionaries, very much depending on how the rest of the society feels about their behavior, but they certainly exist."
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Consider the humble slime mold. As seen in the many videos Bonner made over his seven-decade career, when they are threatened by starvation, the tiny amoebae coalesce into slug-like creatures that then aggregate into a large, swaying tower that grows upward with a burgeoning slimy top -- until that top sticks to an unwitting passing insect, the starvation-resistant spores hitchhiking out into the world, while all the individuals making up the base and stalk die. In other words, the collective phase is necessary for survival and dispersal.
"Whenever a system has a collective behavior, it's so eye-catching, and so awesome -- and as humans, we tend to look at what's eye-catching," said Fernando Rossine, a graduate student in Tarnita's lab and one of two co-first authors on the paper.
But what caught Tarnita's eye were the slime mold loners, the amoebae that resist the biochemical call to form the tower. She first noticed them the week before she started her faculty job at Princeton in 2013.
"I was at a conference, and a speaker was showing videos of slime molds doing this very complex collective behavior, all determined to reach the center of aggregation," Tarnita said. "All but some, I noticed: Here and there, some scattered cells on the plate just didn't seem to react at all to this aggregation process."
She inquired about these lonely cells, and the speaker dismissed them as "mistakes." "In other words, how could we even expect millions of cells to aggregate without a few chance stragglers being left behind?" explained Tarnita.
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First, he was surprised to find that loners are more numerous than anyone had imagined. When he began trying to replicate the slime mold experiments of other researchers, Rossine discovered that those scientists had carefully optimized conditions to encourage the maximum number of slime molds to join the tower, but even then, a few loners held back. "Even in these very, very idealized conditions, you couldn't exclude loners, because you just can't -- they're part of the process," he said. When Rossine did experiments with slime molds collected from the wild, he was startled to see that up to 30% chose the loner life over collective action.
Then came the second surprise: Tarnita's initial proposal for the nature of these loners turned out to be only half right. When Rossine accurately counted the loners, he confirmed Tarnita's hypothesis that they are decidedly not random mistakes, but a heritable trait. However, they were not a constant fraction of the initial population of starving cells, as she had theorized. Instead, their number depended on the density of the population. In other words, loners were not flipping a coin and deciding, by themselves, to stay back, as Tarnita had first assumed.
In the smallest populations, they found, all the cells remain loners. Above a certain threshold, there is indeed a steady fraction of amoebae that avoid tower-building -- but with a large enough starting population, the number of loners plateaued.
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"It's a social bet-hedging," said Rossine. "And a fascinating conclusion that follows from our findings is that, at least for slime molds, the decision not to become part of the collective is, in fact, taken collectively. All the cells kind of talk to each other chemically: 'Oh, you're going? I guess I'm staying.' There's communication involved in becoming a loner."
https://www.sciencedaily.com/releases/2020/03/200318143635.htm
