The impressive abilities of ant colonies come in part because of a division of labor. Large soldiers protect more vulnerable workers, while an even larger queen ensures that there's a large population of replacements. In these cases, the specialization of labor is inseparable from a physical specialization. But it doesn't have to be that way. In leaf-cutter ants, as the cutting parts wear down with age, workers shift to carrying material cut by others. In many other species, there's no obvious difference between animals that do different jobs.

Now, researchers have described an example where task specialization is the product of a behavioral trait rather than a physical one. And it comes in a group we don't normally associate with social behavior: spiders. In a species of spiders that builds a shared nest and collectively rears its young, an aggressive personality dictates the jobs that the spiders normally take on.

Since the term "personality" is normally associated with complex human behaviors, it's worth stepping back and seeing how the authors define animal personality: "consistent variation in individual behavior across contexts." In other words, the spider must respond in similar ways throughout its life rather than switching between aggression and more passive behavior. And it has to show that aggression consistently; it can't rush to attack prey while backing off from intruders.

The spiders in question, Anelosimus studiosus, have been extensively studied, and females show two distinct personalities: aggressive and docile. The animals' behavior is exactly what it sounds like, and it isn't associated with any obvious physical differences. These females also behave socially, collectively creating a single web, sharing food and the rearing of offspring. Past studies have shown that colonies that are a mix of these two personalities outperform those composed of animals with a single personality type.

In the new paper, three researchers (Colin Wrighta, Tate Holbrook, and Jonathan Pruitt) look into why this might be the case by tracking individual spiders in small colonies composed of two aggressive and two docile individuals.

The aggressive individuals were twice as effective at capturing prey and were three times as likely to attack intruders. In cases where the spiders attacked, the aggressive individuals were far more likely to actually drive an intruder off compared to their more docile compatriots. Webs built by aggressive individuals also held prey for 65 percent longer than the ones built by docile animals. Consistent with all of this, aggressive animals were far more likely to engage in all of these activities.

This would seemingly be in keeping with one idea to explain the existence of docile spiders: they're freeloaders, saving their energy for reproduction while their compatriots do all the work. But the authors looked at what the docile spiders were doing and found they were raising the next generation of young. And they were rather good at it.

When the nests were given 10 offspring, both docile and aggressive parents handled their care equally well. When that number was bumped to 25, however, things started to go badly for the aggressive moms, and only about a third of the offspring in their care survived—about half the rate of survival of offspring cared for by docile females. What happened to these missing offspring? At meal time, the aggressive spiders treated them like an appetizer. The authors observed a "significantly higher incidence of infanticide during cofeeding events with aggressive mothers, with 29 instances of infanticide with aggressive mothers, compared with only 4 instances with docile mothers."

Although there are no obvious physical differences between aggressive and docile spiders, it's entirely possible that there are some non-obvious ones. Differences in the nervous system may underly the distinct behavioral types seen in this species.

But even without the mechanism underlying the behavior sorted out, the authors argue that we should think more carefully about animal personalities when examining things like the division of labor seen in these spiders. Stable personality traits, they note, have been seen in a large range of animals.

PNAS, 2014. DOI: 10.1073/pnas.1400850111 (About DOIs).