Webs of Per­cep­tion

Un­der­stand­ing the tan­gled sci­ence of how spi­ders “out­source” processing power for their tiny brains

Canadian Wildlife - - WILD THINGS - By Jay In­gram Illustration by Spencer Flock

It makes in­tu­itive sense to hu­mans that the big­ger the brain, the more in­tel­li­gent the an­i­mal. Hu­mans, chimps, go­ril­las, dol­phins and whales all are good ex­am­ples of in­tel­li­gence pow­ered by large, com­plex bun­dles of grey mat­ter. But this big­ger-is-bet­ter as­sump­tion might be a sig­nif­i­cant over­sim­pli­fi­ca­tion. Some re­cent stud­ies sug­gest it would be worth­while fo­cus­ing on in­trigu­ing ex­am­ples at the other end of the scale.

Most peo­ple are fa­mil­iar with the way bees per­form near-mirac­u­lous feats of ex­plo­ration and com­mu­ni­ca­tion us­ing fewer than a mil­lion neu­rons (we hu­mans have 86 bil­lion). Spi­ders present an­other fas­ci­nat­ing ex­am­ple. For in­stance, jump­ing spi­ders, those that hunt down their prey, have im­pres­sive men­tal skills. One species in par­tic­u­lar, Por­tia africana, at­tacks other spi­ders and in­sects and is known to have the abil­ity to spot a tar­get, then ad­vance to a po­si­tion from which it can at­tack, even if that ap­proach takes the prey out of sight mo­men­tar­ily. Ap­par­ently, it holds that in­for­ma­tion in its tiny brain (which, at about 600,000 neu­rons and smaller than the head of a pin, mer­its the word tiny).

Some of the lat­est ex­per­i­ments with Por­tia have shown that it can, in a sense, count. If sci­en­tists place a sin­gle ed­i­ble spi­der in its view, it will un­hesi­tat­ingly choose an at­tack route that briefly ob­scures its view. How­ever, if by the time the prey is back in sight, one or two ex­tra spi­ders have joined the orig­i­nal, Por­tia will pause. The num­bers don’t jibe. Th­ese ex­per­i­ments sug­gest Por­tia can keep track of one, two and many — a feat that de­mands men­tal skills that seem un­likely to emerge from a brain that size.

Com­par­ing spi­ders whose brains vary widely in size also sug­gests smaller brains are no hand­i­cap. All spi­ders make mis­takes when weav­ing webs, but small-brained species are no more er­ror-prone than their large-brained cousins.

In a pa­per pub­lished in early 2017, bi­ol­o­gists Hilton Japyassú and Kevin La­land sug­gested that spi­ders use “ex­tended cognition,” or, in their words, are ca­pa­ble of “out­sourc­ing in­for­ma­tion processing to the body or en­vi­ron­ment.”

What comes to mind im­me­di­ately is stor­ing your gro­cery list on your cell­phone rather than in your frontal lobes, but there are more vivid ex­am­ples. Oc­to­puses, renowned for their smarts, have dis­persed their cen­tral ner­vous sys­tem through­out their bod­ies: their brains con­tain only about two-thirds of their neu­rons, the rest re­sid­ing in the eight arms. Those arms make de­ci­sions on their own, like de­cid­ing where to bend the arm so that the sucker hold­ing a food item is brought di­rectly to the an­i­mal’s mouth. But at least the oc­to­pus “brain,” scat­tered though it may be, still re­sides within the an­i­mal’s body. That may not be the case with spi­ders.

Japyassú and La­land con­tend that one of the is­sues con­fronting very small an­i­mals is how to main­tain their smarts. It’s a com­plex prob­lem. Even though as the body shrinks the brain vol­ume does too, the brain oc­cu­pies a pro­gres­sively larger per­cent­age of to­tal body vol­ume, ac­cord­ing to a bi­o­log­i­cal prin­ci­ple called Haller’s Rule. How to cram this or­gan into the head and how to fuel it as it be­comes pro­por­tion­ally larger be­come se­ri­ous chal­lenges. In some spi­ders, like the tiny Anapisona si­moni, the brain even over­flows into the legs! And re­duc­ing the size of the brain’s neu­rons, while help­ful, can only go so far.

The prob­lem is par­tic­u­larly acute for spi­ders, which, as preda­tors, must ex­e­cute elab­o­rate hunt­ing ma­noeu­vres. Even weav­ing a web that does the hunt­ing for them is a de­mand­ing task. But there’s more to it: a spi­der can ad­just the tension of the web threads to tune the web to de­tect larger or smaller prey. It’s a two-way street, be­cause as the spi­der’s brain ad­justs the web, the web feeds dif­fer­ent in­for­ma­tion to the spi­der. That is ex­tend­ing cognition be­yond the body.

It’s tempt­ing to ar­gue that the web, how­ever beau­ti­fully de­signed and re­spon­sive it is, is a tool just like a chim­panzee’s ter­mite-prob­ing branch, or even a beaver’s dam. But Japyassú and La­land main­tain that the key to an ex­tended cog­ni­tive sys­tem is that it must work in both direc­tions. The ter­mite stick feeds in­for­ma­tion to the chimp but not vice versa. The dam it­self isn’t com­mu­ni­cat­ing with the beaver; the an­i­mal is ad­just­ing the height and com­po­si­tion of the dam de­pend­ing on the ma­te­ri­als avail­able and the wa­ter level. In con­trast, the web it­self is in­form­ing the spi­der just as our fin­ger­tips in­form us about tex­tures they’re touch­ing. We squint or cup our hand be­hind an ear to heighten our senses; the spi­der tweaks its web.

As strange as the idea is that we hu­mans might one day en­hance our own cognition by em­bed­ding a com­puter chip, stranger still is the no­tion that we are way be­hind the curve when com­pared with spi­ders.

AS THE SPI­DER AD­JUSTS ITS WEB, THE WEB FEEDS IN­FOR­MA­TION TO THE SPI­DER. AMAZ­INGLY, IT IS EX­TEND­ING COGNITION BE­YOND ITS BODY

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