Sometimes spiders live in huge social communities,
and sometimes they share food so equitably
that they all starve to death.
Spiders don’t usually hang out with each
other because they’re highly territorial
– and sometimes cannibals.
But of the 40-thousand-plus identified species
of spiders, we know 25 to be social.
One such species, the South American Anelosimus
eximius, has been observed in colonies of
50 thousand individual spiders living in communal,
3D webs that span 25 feet by 5 feet, arching
through several trees.
These critters cooperate on web upkeep, prey
capture, and childcare. Broodcare?
(It involves female spiders – which comprise
between 78 and 95 percent of any given colony
– regurgitating food for the youngsters.
Including youngsters that aren’t their own.)
They self-select tasks based on what each
unique spider is personally suited for.
Beyond that, the entire neighborhood shares
their food supply when the hunting’s good,
ensuring that no spider goes hungry.
Or that’s how it usually works.
In every generation, about 21 percent of healthy,
established A. eximius colonies
suddenly collapse and die off.
As in, no survivors.
It’s like the end of The Thing, except infinitely
worse because there’s no Kurt Russell and
the entire cast is spiders.
So a team of entomologists out of the University
of British Columbia
tried to figure out why this happens.
They experimented with spider populations
in the lab – but they also used MATH.
They were looking at prey size and individuals’
behavior in growing colonies.
Their mathematical model and experiments showed
that the spiders tend to hog smaller prey
and share larger ones around.
Meaning there’s probably a limit to what
a spider considers worth defending.
Smaller colonies have smaller webs that catch
smaller prey.
In those cases, the spiders are mostly competing
for food.
Some individuals may starve, but the overall
colony persists.
In studies of animal behavior, that’s called
contest competition.
However, when a web gets big enough to catch
all large prey all the time, it seems that
the spiders start sharing.
They even give preferential food access to
the hungrier spiders.
That’s called scramble competition.
But they’re bringing in less overall food
per spider capita.
They wind up sharing it so fairly that no
one has enough.
And boom: Colony extinction, not with a bang
but a whimper.
But don’t worry -- A. eximius populations
remain pretty healthy overall.
Which is great, because studying them has
led to lots of hypotheses about the evolution
of arachnid social behavior.
Further research could help scientists get
a better handle on how competitive animal
behavior is influenced by environmental factors
– and how populations are influenced by
competitive animal behavior.
So what do you think about all this?
Would you want to see an A. eximius web in
person?
Or is this entire topic making you itch for
a flamethrower?
Get in touch and let us know.
And for more about what’s going on in science
now – later – head over to now.howstuffworks.com.
