Imagine that you’re being attacked by a lion. Or if you happen to be a lion-wrestler, imagine that it’s a shark. How hard are you going to fight back? Probably with everything you’ve got. This is about as dangerous as a situation as you can be in, and you’re going to kick, punch, bite and use everything at your disposal to stay alive. Now imagine that a Chihuahua puppy runs up to you and bites your ankle (the lion/ shark has left by this point). It’s pretty annoying, and perhaps even painful, but you’re unlikely to respond with as much aggression.
Deciding how you're going to respond to different threatening situations is based on risk assessment. By assessing what the risk is of you being seriously harmed or killed, you make the decision of how much effort you’re going to put into defending yourself. Of course, everyone’s ability to risk assess is slightly different.
A recent study used the western widow spider to see how this animal might alter its behaviour based on its perceived risk of the situation at hand. Black widow spiders will use their venom to attack prey, including their mates, but also in defence. However, venom is expensive stuff to make, and the spider doesn’t want to waste it unnecessarily. People watching these spiders in the wild have observed that when they’re being attacked by rodents (the black widow equivalent of a lion or shark) they actually throw silk on to the attacking animal instead of biting it, in an action known as ‘silk flicking’.
To see whether black widow spiders would alter their behaviour, both in terms of whether they used venom and whether they flicked silk, scientists squeezed them to see how they would respond. As they didn’t fancy using their own fingers to do this, they made fingers out of gelatin to use instead.
To simulate a low-threat condition (the equivalent of a Chihuahua barking at you), they poked the spider in its web with a gelatin finger. The medium-level threat, the equivalent of perhaps a Chihuahua biting your foot, was 60 prods by the finger to the poor spider. For the high-level threat the spider was coaxed on to the gelatin finger, where it was pinched between two gelatin fingers three times, simulating the grasp of a predator like a rodent that might eat it in the wild. Each spider was exposed to all three types of threat, although each spider encountered them in a different order.
The spiders did indeed behave differently depending on how threatening the situation was. With just a few pokes, the spiders generally moved away from the obtrusive jelly fingers, and none of them bit. When being prodded repeatedly, the spiders would move away, and often play dead and flick silk, but actually only 2% of the spiders attempted to bite. In fact, the spiders only responded with biting when they were pinched between the gelatin fingers, saving this tactic as a last resort.
Some people are extremely wary of black widows given the strength of their venom, but this experiment goes to show that you really have to be squeezing them to make them want to bite you – they’d much rather back away or flick silk in your direction. This is illustrated in this video here, where the photographer Thomas Shahan handles a black widow
In a second part of this experiment, the scientists also got the spiders to bite a tube in order to collect the spider’s venom. The spiders did this either after having been attacked incessantly or at longer intervals. This time the attacks consisted of either being pinched on the leg with forceps, or by being squeezed on the belly by a gloved hand.
In most single bites by spiders, the spider didn’t actually inject any venom at all (so-called ‘dry bites). In addition to this, when the spiders did eject venom, they controlled how much depending on the situation. When their body was pinched (more threatening than just having a leg pinched) spiders ejected larger quantities of venom, around 1.8 fold more per bite.
Interestingly, when being grabbed by the leg, spiders ejected most venom in their first bite when they bit multiple times. However, when being grabbed by the body, they ejected increasing amounts of venom with every bite. This is again likely to be because body-pinches are more life-threatening, so it makes sense to keep upping the counter-attack with every attack.
When there were longer intervals between attacks, spiders also used more venom. This is likely to be because they saw each new attack as a new predator, so it makes sense to keep using lots of venom.
How exactly the spiders ‘decide’ how forcefully to attack is not known. It may all be reflexive responses to the type of attack, or there may be more information that they use in making a decision of how to behave. It is also not known how their attacks might differ with different types of predator, and it would be interesting to know if they attack differently, for example, when being attacked by a wasp, centipede, mantis or rodent. Hopefully the answers to some of these questions will become clear with further experiments.
Chihuahua - David Shankbone
Black widow - Ryan Somma
Black widow being pinched - taken from Nelsen et al. (2014)
Black widow video - Thomas Shahan
Nelsen, D. R., Kelln, W., & Hayes, W. K. (2014). Poke but don't pinch: risk assessment and venom metering in the western black widow spider, Latrodectus hesperus. Animal Behaviour, 89, 107-114.