Golden orb-web spiders (Nephila clavipes) are sit-and-wait hunters that accumulate prey larders on their webs. These larders are sometimes stolen by other spiders. If prey items are removed from their webs, golden orb-web spiders search for them. They search longer when they have lost more prey items as well as when they have lost larger prey items.

Rafael Rodríguez of the University of Wisconsin-Milwaukee and colleagues wanted to know if these spiders have a sense of numerosity: the ability to track the number of prey items in their larders.

The researchers conducted a field experiment in Costa Rica to determine whether golden orb-web spiders represent the number of prey items or the total mass of their larders in their memories. Rodríguez says these spiders are extremely pleasant to study: "They are large, active during the day, and move relatively slowly, so they are a dream come true for students of behavior."

Ianaré Sévi, via Wikimedia Commons. Distributed under a CC BY 3.0 license.

The key to testing these two hypotheses was looking for a difference in the increase of searching effort with either mass or counts. "It could be that the spiders keep track of counts but one wouldn't know if the increase is the same as for equivalent increases in mass," Rodríguez says. Finding a difference would indicate that they can keep track of both variables.

Rodríguez and his colleagues found golden orb-web spiders on their webs and gave them some amount of prey items (mealworm larvae), either in the form of a given prey size or a given count of prey items of the same size. They let the spiders perform their normal behavior of capturing the prey and beginning to feed on it, and then removed the prey. This caused the spiders to begin searching.

The researchers found that spiders increased their searching efforts both with prey count and prey size, but they did so more steeply with prey counts. This suggests they track both prey numerosity and size, and that they give more weight to prey numerosity.

"We can make a tentative conclusion that these spiders possess a sense of numerosity," says Rodríguez.

This ability puts golden orb-web spiders among many vertebrates and at least a few other invertebrates. "This is interesting because it suggests that brains of very different sizes and organizations can give animals this ability," says Rodríguez.

__Reference: __

Rodríguez, R. L., Briceño, R. D., Briceño-Aguilar, E., and Höbel, G. (2015). *Nephila clavipes *spiders (Aranae: Nephilidae) keep track of captured prey counts: testing for a sense of numerosity in an orb-weaver. Animal Cognition 18: 307-314. doi: 10.1007/s10071-014-0801-9.