By Felicity Muth
Regardless what you might think of spiders, once you find out a bit about their behaviour it’s hard to deny that they are at the very least, interesting animals. I’ve previously written about how male spiders select females to mate with that are the least likely to eat them, how spiders can sniff out human scent and how they can control how much venom to inject (although they prefer not to inject any venom at all).
While many people freak out when finding a spider in their house, we rarely stop to think how our presence might be affecting them. However, when sharing an environment with spiders we often impose on their lives more than they impose on ours. I used to live in an apartment in Scotland where we had a large house spider that lived behind the toilet in the bathroom. After a few months it grew to be part of the family, and I would occasionally throw it bits of food when the fly population was low. Living between the toilet and the wall meant that every time the toilet was flushed, the spider would scuttle from one part of its web (near the flush handle) to darker depths behind the tank, but I can only assume that it became used to such disturbance as it never moved home. I should perhaps add that the poor spider came to a sad end when a new housemate moved in and killed (murdered?) it.
The point of this slight diversion is that animals can habituate to environmental disturbances. What’s more, they can get very good at telling the difference between stimuli that are relevant to them, and those that aren’t. Tree frogs can tell the difference between vibration caused by a predator and vibration caused by rain, even when these cues are extremely similar. Similarly, caterpillars living on leaves can tell the difference between vibrations caused by other caterpillars, predators, wind and rain.
Spiders build webs on human-built structures such as pipelines, fences, road signs and wire rods, all of which are made out of materials not present in their evolutionary history. This means that they will absorb vibrations from the environment differently to a more natural place a spider might build its web, for example a plant. If these human-built objects are anywhere near humans (which they are likely to be) they are also probably affected by human noise. For example, a spider that has built a web near a road will be subject to the vibration caused by cars driving by. This matters particularly to spiders because they use vibration so much in guiding their behaviour. Indeed, you can even imagine the web to be an extension of the spider itself, such that the vibrations on the very outside of the web travel down to the spider situated in the centre and tell it whether it’s being ‘touched’ by prey, a mate, wind or rain.
If a spider and its web are being vibrated by anthropogenic (human-caused) noise, how does this affect the spider’s ability to detect its prey and other cues accurately? This might be the equivalent of us trying to hear the difference between different people’s voices if there is music blaring in the background.
Using the European garden spider, scientists recently carried out a study where they first measured how vibrations affected the different substrates the spiders built on differently. They did this for both ‘natural’ substrates (various types of leaves) and ‘artificial’ ones (plastic, glass, concrete, metal, human-manufactured wood). They found that the artificial substrates absorbed a lot more of the noise than did the natural substrates. The scientists then vibrated spiders’ webs at different intensities to see how this would affect the spiders’ ability to detect prey. Each spider experienced three different noise levels: ‘low noise’ (0 dB); ‘medium noise’ (21.6 dB, the equivalent of wind blowing) and ‘high noise’ (35.5 dB, the equivalent of being next to a construction site). They found that the spiders were most responsive to prey at the intermediate noise levels, not the ‘quiet’ level as one might expect.
The likely reason for this is because the intermediate noise level most similar to what spiders would encounter on a natural substrate like a leaf just from the wind blowing. Having this base level of stimulation may increase a spider’s sensitivity to the vibration of prey getting caught in the web, whereas too much stimulation (at the ‘high’ level) would mask the prey’s vibration. This indicates that the spiders’ predatory performance is best when they build their webs on natural substances if there’s not a lot of anthropogenic noise around.
When thinking about how our noise might affect animals, we often consider the auditory effects on animals, such as the ability for birds to hear each other singing, and the effect of noise on fish finding prey. However, perhaps because the vibratory world is not one that generally affects us too much, it has been somewhat forgotten when considering our impact on other animals. This study goes to show how vibrations caused by us can make it harder for animals to catch their prey.