Male jumping spiders will literally size each other up before they decide to duke it out. But it’s not just size, it’s the size difference, according to researchers who confronted live spiders using computer-animated, eight-legged opponents. The findings were published in Behavioral Ecology this week.  

Red-headed and with vividly colored forelegs, magnolia green male jumping spiders (Lyssomanes viridis) seem to rely on the giant fangs hanging in front of their colorful mouth parts (or chelicerae) to intimidate rivals and impress females. Back in 2011, Cynthia Tedore and Sönke Johnsen of Duke showed that male jumping spiders with bigger fangs may win more contests. Two dozen males squared off in pairs for 10 minutes, and over the course of 68 cage matches, it appeared that males with the smaller fangs will slink away without a fight. "The males wave their forelegs at each other for a period, and then the smaller male runs off," Tedore said in a Duke release back then. "That's why we think they're using vision to size each other up. Most of the time, the smaller one will run away before it comes to blows." These spiders make decisions and navigate the world using their tiny brains, and they appear to follow rules of some kind. But is it fang size, chelicerae size, or just size? 

For this study, the duo pit male jumping spiders against computer-animated rivals to better understand the visual signals they use when deciding whether to fight or not. With the animated opponents, the researchers were able to control for all potential factors that correlate with size. The computer-animated spiders come in the three different sizes with weapon- and nonweapon-appendages that were elongated differently. They scored the increasing levels of escalation in the live males’ responses to the animated rivals. From the escalation intensity, the team was able to judge the predictive power of three variables: male size, animated opponent size, and the difference in size between the male and the animated opponent. 

Size difference, they found, was the best prediction of escalation intensity: The effect of opponent size disappeared when size difference was included in the same model. Surprisingly, the males were just as willing to fight animated spiders with long legs and fangs than those with less imposing weaponry, Science explains. The male’s own size, on the other hand, didn’t really predict escalation intensity, suggesting that these guys are using a mutual assessment strategy.