Nephila clavipes, a big tropical spider, has plenty of room in its body for its brain. |
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%
of their total body cavity, including about 25% 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 to 3% of
our body mass. Some of the tiniest ant brains that we’ve measured
represent about 15% of their biomass, and some of these spiders are much
smaller.”
The brains of smaller spiders, like nymphs in the genus Mysmena, extend out of their body cavity into their legs. Photo: Wcislo lab. |
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 rainforest giant weighs 400,000 times more than the smallest spiders in the study, nymphs of spiders in the genus Mysmena.
