Science

Phidippus audax

The little black jumping spider tickles its way up my forearm. I turn my head to get a better look as it climbs over the knob of my elbow. It stops, sticks its two front legs up in the air, and stares at me. At least I think it does. With all those glittery eyes it’s hard to tell where it’s looking.

Is the spider reacting to me? Legs up could merely be its resting position. It could be stretching out tight muscles after a long crawl or preparing for an acrobatic jump from my arm to a nearby tree. Or it could be trying to steady its balance.

I cock my head to the side. It rocks back on its abdomen but maintains the posture. I slowly extend my finger toward its face. It scuttles backwards a couple steps, then raises its legs again. Three times in a row, my movements triggered a response. It seems to know I’m here.

The spider’s pose is comical, like a gymnast who’s just executed a flawless back flip or a robber who’s been caught red-handed in a bank vault.

If I let the spider go and come back in two days to repeat the same sequence, will it respond the same way again? Maybe it was irritable the first time because it was hungry. If I subject it to the same treatment every other day for a month, maybe it will eventually learn that I’m not a real threat and stop responding to me. Or maybe it will flee instantly, recalling our harrowing first encounter.

Say I conduct this experiment, and every time, the spider does the same thing. Now I’ve learned something: this is a repeatable pattern of behavior. The spider does this when I do that. But what exactly is it doing?

I know two very different cases of jumping spiders performing leg up behaviors. The Myrmarachne mimic ants by tapping their front two legs along the ground as they weave in erratic patterns like they are following a scent trail, and when they pause, they hold these legs above their heads like antennae. Male Maratus use elaborate leg waving and posturing in their flamboyant courtship displays for females. I doubt this spider thinks I’m a potential mate. I don’t know what it thinks I am.

Perhaps it’s posturing to intimidate me. Lifting its front legs reveals an impressive pair of green fangs. Is that a warning? I’ll bite you if you try anything funny.

How can I know whether it sees me as a threat? I would certainly be frightened by a giant hairless, eyeless, segment-less creature invading my forest. I’d probably spring for cover at the slightest movement, or more likely I wouldn’t crawl up the giant’s arm in the first place. I’m intrepid, but I’m not an idiot. Then again, I’m also not a spider.

Do all jumping spiders do this? This could be the only spider in the forest that raises its legs when threatened, if that is indeed what it’s communicating.

I could collect a hundred jumping spiders from this woodlot and expose them to the same sequence of movements. I’m almost certain to get different responses from different individuals. Some spiders may perform the same leg posturing as the first one. Some may freeze but not stick their legs up. Some may instantly leap for the trees. Some may bite me.

Let’s say I repeat the experiment with this group of spiders, and each time everyone repeats their response. Some always freeze, some always leap, some always bite, and some always stick their legs up. Now I’ve identified not one, but four different conserved reactions to the same encounter.

Are they afraid? Are they angered? Are they merely startled? And if so, which behavior is which?

If I interpret from this experiment alone, I might tentatively put their responses in two categories: fearful and bold. Leapers and freezers are fearful. Biters and posturers are bold. Or are leapers and biters fearful, while freezers and posturers are bold? Either way, the legs up posture seems like an intimidation. By raising its front legs and rocking backwards, the spider appears larger and more imposing. Sunlight glints off its emerald fangs. It doesn’t flinch. It doesn’t flee. It faces me head on, bold and defiant.

I’m describing this behavior as bold. What’s more, I’m describing this individual as bold. I don’t know that it is. I’m basing this characteristic purely on my human intuition of how I think a bold spider should behave.

To test my hunch, I need to watch how spiders react to a more ecologically relevant situation. I could expose them to another animal that should provoke a response: a predatory wolf spider. If the posturers raise their legs for the caged predator, that further supports intimidation as an interpretation for their behavior. If leapers keep as much distance as possible, that provides more evidence of fearfulness. Alternatively, all the spiders could cower. Maybe I’m just not as scary as a recognizable predator.

Another point of interest: is this behavior effective? If I let the wolf spider loose, is it less likely to attack spiders that posture than those that don’t? Or is it better to run away? Maybe the behavior works on other spiders but not on birds. I’m not particularly intimidated by a little black spider. I doubt a blue jay would be either. Do individuals respond differently to different predators? Or will this spider always raise its legs to things in the category of ‘threat,’ and it’s the luck of the draw whether it survives? Dozens of questions and years of experiments with thousands of spiders separate me from a true understanding of this one curious act.

Behavioral ecologists are tasked with the nigh impossible: to read the inscrutable expressions of creatures whose lives are so wonderfully unlike ours. I can’t pump hydraulic pressure into my legs and spring nimbly from one blade of grass to another. I can’t see approaching grasshoppers through three different kinds of eyes. I don’t have to look out for hungry birds or larger spiders. But my subjects do. They can tell me their story if I give them the means, then step back and watch without leaping to conclusions. As faithful interpreter, my job is to quell my human sensibilities and describe their experience honestly without translating it into my own.

The spider on my arm raises its legs higher and quivers its fuzzy black abdomen. Taxonomists call this species audax, from the Latin for ‘bold.’ Not all P. audax are bold. This spider may be.

I just need to know how to ask.

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