Building a better mouse (er, grasshopper) trap

People often don't seem to appreciate the plight of the predator. These hard-working creatures need to kill something else on a semi-regular basis just to keep going. Just think about that: in order to eat, they have to force another organism into the position it least wants to be in (i.e., death). Life for a predator is not as simple as finding a nice plant and nibbling some leaves for a bit, but rather a constant struggle to obtain the energy needed to grow, develop, and reproduce. This is why I often find myself as the odd one out while watching nature documentaries: I always root for the predator!

We should not be surprised when we see predators doing “clever” things to capture prey. Dolphins, chimpanzees, and crows are famous for coming up with novel ways of capturing their prey, but what about luring prey in? Some examples that jump to mind include physical lures in fish and turtles that mimic worms, as well as the amazing chemical lure of the bolas spider. Less well-documented are prey lures based solely on predator body color. Evolution will tend to produce prey that are not drawn to the colors on their predators, but there are limits to the power of natural selection. If the prey are normally drawn to, say, a white stripe that may represent shelter, food, or a mate, then any predator exploiting this by displaying a white stripe on itself may be able to lure prey in. This is known as exploiting a sensory bias, and it can make the situation tough on prey.

Enter the nocturnal, overall drably-colored fishing spider Dolomedes raptor. Adult females (but not immature or male spiders) have little white patches towards the end of their legs. The stripes do not appear to be used for mating, but play into a hunting behavior used only by adult females. They sit motionless at the water's edge, waiting for semi-aquatic grasshoppers, fish, or something else edible to come by. Could the white stripes on their legs act as lures for grasshoppers, their most common prey?

To find out, the authors first developed a model for grasshopper vision, which had never been done before. It took some careful measurements and fancy math

to show that, yes, the grasshoppers would be able to see the white leg stripes but not the spider's body at night. Pretty cool!

Then, dummy spiders were made of cardboard so that half had white leg patches and half didn't. Night vision cameras recorded when grasshoppers approached to within “striking range” of both types of dummies set out along the side of streams. The dummy spiders with stripes attracted about twice as many grasshoppers as those without the stripes, providing some good evidence that the stripes lure in prey.

Back inside, the authors also recorded live spiders preying on grasshoppers. Importantly, they also shaved off the white hairs that compose the stripe to see how the spiders would fare with simple brown legs. By comparing prey capture success with and without the stripes, we can get a good estimate of whether or not the stripes function as a lure. The results of this experiment confirmed the field results with the dummy spiders; white stripes significantly increase prey capture.

While it is intuitive to conclude that the stripes are a lure, one has to pause and ponder some things. First, if the spider is exploiting a sensory bias of the grasshoppers, then what is it the grasshoppers are looking for? The authors could not think of anything small and white that they would normally move towards, so there is some mystery here. The grasshoppers that got within striking range were all captured, so there doesn't seem to be much of a chance for learning to occur.

I wonder how such an odd lure might have evolved. The authors admit that it may function to deter predators or do some other kind of signaling, but I'd be surprised if that were true. This seems to be a case where prey luring is the function, but we need more data to understand the selective pressures that produced the lure. Are there closely related species that have the stripes? What about distantly related species that hunt the same prey? I'd expect lots of nocturnal predators to have white stripes, but I'm not sure that's very common.

Overall, the combination of approaches (visual modeling, field, and laboratory) provides strong evidence for the existence of the visual lure, especially since it only appears on the female's legs. The authors don't list it as a future research direction, but we obviously need to know how the spiders feel about Jack White's solo career.