Spider, Anelosimus studiosus: To be or not to be social

You might think that sociality is a fixed biological trait for some species.  For instance, humans might be considered a social species as we instinctually form social units from clans and tribes to states and nations.  We also prefer social life, but all these factors are related to our biology.  We humans require a lot of care for many years to reach an age where we can take care of ourselves and societies seem to be the best way to incorporate and sustain as huge of a diversity of individuals as compose humanity.  But for some species like the spider Anelosimus studiosus sociality is dependent on the forces of environment and biology.  They are facultatively social, meaning that they will only form colonies under certain circumstances.  Most spider species that are social are web builders.  Amongst spiders social behavior is believed to have independently evolved 12-13 times.  These social spiders can be split up into two groups: colonial and cooperative.  In colonial species, many spiders share the same web, but they compete for territory and hunting rights on the communal web.  In cooperative species, many males, females and juveniles build/maintain a common web and rather than competing for hunting space on the web like colonial species, they cooperatively capture and share prey as well as collectively raise the next generation.  In cooperative species there are often more females than males.  Many spiders that aren't strictly social still have maternal care (that's right, motherhood has a long evolutionary history!).

Anelosimus studiosus

Anelosimus studiosus can be found from Argentina to New England.  The study I read dealt with differences in social behavior between populations of Anelosimus studiosus in Florida and Tennessee.  In Florida a typical colony was described as:

Composed of one adult female, her juvenile offspring and a few unrelated males that don't participate in web maintenance or communal prey capture.  The mother guards her egg case and feeds the newly emerged spiderlings through regurgitation.  As the juveniles grow, they increasingly participate in web maintenance and prey capture.  During this time, the mother accepts the entry of foreign juveniles and males into the nest while driving off intruding adult females.  The colonies are ephemeral because the spiderlings disperse upon reaching maturity and the mother eventually dies.  The young males often disperse first to go searching for mates while the young females are later driven off by their mother.  When the mother dies, it isn't unusual for another adult female to come along and use the web to raise her own offspring. (1)

In Tennessee, many colonies exist that are like the single-female-and-offspring ones described in Florida, but a new type of cooperative colony behavior appears that consists of:

Multiple females (3.7 females on average) cooperatively sharing a web, with cooperative foraging, communal brood care (baby spider daycare), and communal building/maintaining of the shared web. (1)

Why are there only single-female-and-offspring colonies down South in Florida?  And why is there a mixture of single-female-and-offspring colonies as well as multiple-female-and-offspring colonies further North in Tennessee?  Other trends to take note of with Anelosimus studiosus colonies are that the density of colonies goes down, and the spider webs get much bigger as you move from Florida-to-Tennessee.  Can we decipher any environmental, evolutionary and/or biological reasons that might explain these patterns?

A key to this puzzle is that juveniles require a lot of parental care for an extended period of time.  An advantage to communal parenting is that "if a mother dies in a multiple-female colony, the surviving females can foster the deceased female's brood." (1)  If the single mother of a single-female colony dies, so will her brood.  But if this is the case, why aren't all of the spiders communal?  Wouldn't natural selection favor communal parenting because it's a hedged-bet against the sometimes irrational environment?  Yes and no.  As it turns out, communal, adult females tend to have less offspring and have to spend more energy on colony activities on average than females who strike out on their own.  So the trade-off is between security
and reproductive success.  So what is it about Florida vs. Tennessee that makes being a lone mother vs. a communal mother good strategies, respectively?

A clue to the kind of sociality Anelosimus studiosus practices can be found in the environment.  Anelosimus studiosus spiders are most commonly found near water bodies like rivers and lakes.  From Tennessee to Florida there is great variation in air and, correspondingly, water temperature (Average air temperatures: @ latitude 26 degrees North in Southern Flordia is 23.5 degrees Celsius.  @ latitude 31 degrees North in Northern Florida is 19.5 degrees Celsius.  @ 36 degrees North in Tennessee is 14.5 degrees Celsius.)  It tends toward the waters being warmer in Florida and colder in Tennessee.  For Anelosimus studiosus as air and water temperature decreases so does the rate of development of the offspring (Mean time for juveniles to reach independence of mother: @ 22 degrees Celsius, 45.5 days.  @ 27 degrees Celsius, 28.7 days.  Thus about 5 degrees difference in average temperature affects almost a two-fold difference in rate of development!).  This means that where air and water temperatures are colder there is a greater likelihood that the mothers will die while raising their offspring because the cold causes the spiderlings to develop slower.  There is also the issue that there is higher juvenile mortality in colder climes.  Thus, where it is colder it is a good strategy to form colonies because if a mother dies the other mothers will pick up the slack.  Additionally, there is a higher risk that juveniles will die in colder places without proper care.  A community, evidently, is a better parent in these circumstances than an individual mother.

We can explain why the average size of colony webs becomes larger as you move north.  As you move North the proportion of multiple-female colonies goes up.  In Southern Florida there are only single-female webs, in Northern Florida there is only a small percentage of multiple-female colonies, and in Tennessee there is an even higher proportion of multiple-female colonies though it should be noted that for all locations single-female webs were the most common.  The seeming reason why colonies become more widely dispersed as you move North is that the environment becomes harsher by Anelosimus studiosus spider terms (meaning colder and perhaps less rain and vegetation). 

One of the interesting statement/conclusions in the paper I derived this blog from is that:

cooperation can allow populations to expand into and persist under harsher circumstances than individuals could otherwise endure.

It isn't that the spiders become tougher as you move North that allows them survive, but the fact that they work together toward a common destiny that allows them to endure harsher conditions.  Something to keep in mind for our lives I think.

A question that comes to my mind is if this example of facultative-sociality is just a case of instincts being switched-between based upon environmental conditions or if there is a level of recognition and agreement between the female spiders.  Do they recognize the potential for their own mortality in some harsh environments and the affect it would have on their brood?  Do the spiders recognize the utility of cooperating in these circumstances?  Afterall, not all of the spiders choose to form communities, even in colder climes.  Is it the less "fit" individuals that find it too hard to survive on their own in the cold locales who choose to band together as a solution, communally building/maintaining nests, cooperatively foraging, and communally caring for the baby spiders?  If this was true, it might be, as Darwin said in The Descent of Man, that humans became social because we were too weak to survive on our own.  If you're strong and self-sufficient you'd be better off not cooperating.  Yet it does seem to be weakness that brings us together, but united we become so much stronger than the sum of strong individuals competing for their own, selfish interests could ever be.

I hope to instill all my readers with a sense of complexity about all the creatures I speak of.  Assuming simplicity about things has been the source of much confusion and suffering.  It's taken a lot for people to see each other as more than black or white, or, good or evil.  Similarly, we too often assume that organisms other than humans are just instinctually-driven automatons.  But with just a little effort at delving into the world of those we don't understand, we soon find ourselves coming to the realization that nothing and no one can be simply defined and disregarded.

-Seth Commichaux

Sources Cited:
1) Jones, T., Riechert, S., Dalrymple, S., Parker, P., 2006.  Fostering Model Explains Variation in Levels of Sociality in a Spider System.  Animal Behavior vol. 73, pg 195-204.