Stick with fashion

one barn spider Araneus cavaticus eating anotherSo, right outside the same porch mentioned in the previous post has been a pair of large orb webs occupied by barn spiders (Araneus cavaticus), both females – it was only one for a few days, then another moved in. And curiously, their webs were almost stacked; while the centers of the orbs were not aligned, the webs themselves overlapped only about 10cm or so apart, while the spiders sat in their alert positions about 30cm offset from each other. Barn spiders tend to rebuild their webs frequently, usually using the same anchor strands, so their positions changed over a period of about a week. I kept thinking I should do a shot that included them both, but couldn’t work out an angle that would work.

Sometime in the last 12 hours, that opportunity passed, along with one of the spiders. I have no idea what transpired, but one of them was proudly displaying its meal of the other. And you thought beauty pageants could get cutthroat…

Competition among individuals of other species can at times be pretty distasteful to us – sure, we might kill our rivals, but not eat them! Almost never, anyway. But behavior is a widely variable thing, dictated by the winning lottery of natural selection. Think about it: each spider is competing for both food and mating privileges, and in one move (well, okay, it might have been quite an altercation) this was eradicated. Spiders have no reasons whatsoever to cooperate, so the only decision about what constitutes “food” is whether it’s too dangerous to tackle.

It can even illustrate a simplified facet of evolution. If one of these spiders was aggressive and ‘cannibalistic’ while the other wasn’t, one of them is far more likely to win that encounter. And now, with the competition gone, the winner stands a much greater chance of passing on her genes. it is entirely possible that this scenario played itself out numerous times in the past history of this species, and so now all of them will view other females only as a tricky-to-obtain collection of calories. Humans are a cooperative species, because it worked better for us in hunting and farming and such, and to foster this we have strong feelings about interactions among our own species; thus we might look upon this behavior within the same species of spider and find it abhorrent. Yet it’s only about what worked best among the available options, and from a survival standpoint, this works well for the spiders. Sometimes we have to ditch the emotional reactions to fathom the functional aspects.

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