Brain size and behavior

A Whole New Meaning for Thinking on Your Feet 

Nephila clavipes, a big tropical spider, has plenty of
room in its body for its brain. Credit: Pamela Belding.

Smithsonian researchers report that the brains of tiny spiders are so large that they fill their 
body cavities and overflow into their legs. As part of ongoing research to understand how 
miniaturization affects brain size and behavior, researchers measured the central nervous systems of nine species of spiders, from rainforest giants to spiders smaller than the head of a pin. As the spiders get smaller, their brains get proportionally bigger, filling up more and more of their body cavities. 
“The smaller the animal, the more it has to invest in its brain, which means even very tiny 
spiders are able to weave a web and perform other fairly complex behaviors,” said William Wcislo, 
staff scientist at the Smithsonian Tropical Research Institute in Panama. “We discovered that the 
central nervous systems of the smallest spiders fill up almost 80 percent of their total body cavity, including about 25 percent of their legs.” 
Some of the tiniest, immature spiderlings even have deformed, bulging bodies. The bulge 
contains excess brain. Adults of the same species do not bulge. Brain cells can only be so small 
because most cells have a nucleus that contains all of the spider’s genes, and that takes up space. The diameter of the nerve fibers or axons also cannot be made smaller because if they are too thin, the flow of ions that carry nerve signals is disrupted, and the signals are not transferred properly. One option is to devote more space to the nervous system.  
“We suspected that the spiderlings might be mostly brain because there is a general rule for all 
animals, called Haller’s rule, that says that as body size goes down, the proportion of the body taken up by the brain increases,” said Wcislo. “Human brains only represent about 2-3 percent of our body mass. Some of the tiniest ant brains that we’ve measured represent about 15 percent of their biomass, and some of these spiders are much smaller.” 
Brain cells use a lot of energy, so these small spiders also probably convert much of the food 
they consume into brain power. 
The enormous biodiversity of spiders in Panama and Costa Rica made it possible for 
researchers to measure brain extension in spiders with a huge range of body sizes. Nephila clavipes, a Smithsonian Institution   News Dec. 12, 2011 SI-528-2011 2 rainforest giant weighs 400,000 times more than the smallest spiders in the study, nymphs of spiders in the genus Mysmena.  
The Smithsonian Tropical Research Institute, headquartered in Panama City, Panama, is a unit 
of the Smithsonian Institution. The Institute furthers the understanding of tropical nature and its 
importance to human welfare, trains students to conduct research in the tropics and promotes 
conservation by increasing public awareness of the beauty and importance of tropical ecosystems. 

Website: www.stri.si.edu 

Spiders carefully move their legs
The web-weaving behavior 
A research on brains and spiders