Accounting for Spiders

They don’t have hedge fund managers or spreadsheets, but even animals need to keep track of their assets. If you’ve spent all day hunting for your dinner, you wouldn’t want to lose it—or other kills you’ve made—later on. It seems that counting isn’t just an elementary skill, but a necessity of evolution.

How deep into the animal kingdom does this exigency go? Rafael Rodriguez of the University of Wisconsin, Milwaukee set out to show that golden orb weavers (spiders that you can see in the Academy’s rainforest exhibit) can count the insects they trap in their webs. But, as is often true of animal behavior, there was more.

Rodriguez tested the spiders’ “sense of numerosity” by moving prey around on their webs. When the orb weavers returned to check in on their suppers, they would begin to look around, and wouldn’t stop until they felt satisfied that all was accounted for. Rodriguez’s team also changed the size of the future spider snacks, and noted that these arachnids could tell the difference. When a big meal disappeared, the spiders spent more time searching. Wouldn’t you do the same?

It’s clear, then, that the golden orb weavers “form memories that include some measure of the quantity of captured prey,” as Rodriguez and his colleagues wrote in Animal Behavior. This can save the spiders time and energy, continued the authors, as they “use those representations to regulate searching efforts.”

What if it’s not memory? What if the spiders are just using environmental clues (torn silk in the web, for instance) to prompt the search? Just to be sure, the research team eliminated this competing explanation by replacing prey with debris, old meals, or other similar but false leads. “Searching spiders that encounter plant debris or old prey remnants on their webs should cease searching,” wrote Rodriguez, if environmental stimuli prompted the search. But they didn’t: the orb weavers would keep looking, a sure sign that they remembered what it was they were looking for.

It’s interesting to know that the sense of numbers we humans take for granted extends to some of our most distant animal relatives. Still, I won’t be replacing my calculator with spiders quite yet.

Image: Victor Patel/Wikipedia