Docile spiders tend to stick closer to home when starting new colonies, and they rarely fight predators or hunt for prey. Aggressive-type spiders, on the other hand, are always looking for a fight. And when it comes time to leave the web, they venture out much further than the docile spiders. Each spider has one personality type or the other, and they pass that personality type on to their offspring. With mixed parents, baby A. studiosus spiders will still end up with one disposition or the other, not somewhere in between.

The question for biologists, then, is why would two such distinct personalities evolve and persist? To answer that, behavioral ecologist Jonathan Pruitt collected some wild female spiders in Tennessee and brought them into his lab. Here, Pruitt did some spider matchmaking. He divvied up the spiders into pairs—either two aggressive spiders, two docile spiders, or one of each.

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What he found was striking: the best spider personality depended on the prevalence of predators. When predators were around, lineages founded by docile spiders, though they produced many more babies, were eight times more likely to go extinct. These spiders spent their time reproducing instead of defending their webs, and so they were eaten by invaders or their prey was stolen from their webs.

In those colonies where invaders were removed, the docile spiders did just fine. Thus the presence of competitors is essentially driving the docile spiders into a lose-lose proposition (in biology lingo, an evolutionary dead end): make babies or protect the nest, but not both. Aggressive spiders, on the other hand, seem able to manage both demands at once. This ability of one species to drive another into dead-end strategies has long been hypothesized, but never demonstrated in the wild before now. The results were published in Ecology Letters last week.

So which is more of an advantage in the end: the docile spider’s ability to reproduce or the aggressive spider’s ability to survive?

Pruitt says that either extreme can lead to extinction, so a mix is probably the best place to be. All things in moderation, spiders.

That was so logical, I wonder if it's true or humans too.