Most spiders produce silk draglines and excreta as they move through the environment. It is believed that draglines provide a "safety thread" to the spider if it should ever fall off a particular object. This function makes sense for spiders that live off the ground, but is of dubious value for those that are ground-dwelling. Our lab is currently in the process of determining the function of dragline and other types of silk deposited by ground-dwelling spiders. Through a number of previous studies, arachnologists know that male spiders can detect pheromones bound up in the silk of females. In this way, draglines are often a communication medium by which females advertise their willingness to mate and males will then follow this silk trail to the female for mating purposes. To the left is a male wolf spider of the species Rabidosa rabida following a silk dragline produced from a conspecific (as an aside, airborne pheromones have been discovered in wolf spiders as well). Although inter-sexual communication is a well known function of draglines, we have discovered that juvenile spiders and adult males produce dragline silk as well , but its function is still unknown.
If silk can be used by conspecifics to communicate, couldn't it also be used by both predators and prey of spiders to locate potential prey or avoid a potential predator? The answer is a resounding YES! Although silk and excreta from spider predators are ubiquitous in the environment, there are still relatively few studies that have examined chemically-mediated anti-predator behavior in terrestrial arthropods.
Spiders are capable of detecting chemical cues produced by insect prey as are various species of insects. Further, wolf spiders, like Hogna helluo shown at the left, appear to be preferentially attracted to the chemical cues from whatever prey they were feeding on last. Thus spiders can not only detect and respond to chemical cues from prey, but also can remember odor cues associated with their most recent prey (even if the spider's last meal was nine days ago). This means that the predator's recent diet may be a direct indicator of the level of predation risk. Therefore, prey that can discriminate between predators that have or have not fed on them may benefit by acquiring more accurate information about predation risk.
As it turns out, there is much more information that may be extracted from a predator than simply the presence or absence of that predator. The wolf spider Pardosa milvina(smaller spider at left) can detect differences in a larger wolf spider's diet based on silk and excreta cues alone. Pardosa shows a stronger antipredator response when the silk and excreta is from a predator that has fed on them versus one that has fed on other prey types. Graded levels of antipredator behavior suggest that there are fitness costs to such behavior. This turns out to be true. Pardosa milvina placed in the presence of silk and excreta from another larger spider will lose weight quickly, reduce feeding rates on prey made available, delay reproduction, and produce less offspring! Most surprising is that chronic exposure to silk cues can actually result in the death of the spider even when the predator isn't present! We have been conducting a number of studies that examine chemically-mediated predator prey interactions between the syntopic species Hogna helluo (larger spider above) and Pardosa milvina (smaller spider above). Pardosa is a much smaller species than Hogna helluo . They are intraguild predators, meaning that they frequently will feed on smaller individuals of the other. However, because of the large differences in size, there is a well-defined asymmetry in their predator-prey interactions.
Given these fitness costs to reduced activity, Pardosa appears to be able to assess predation risk through chemical cues in a variety of ways and show different levels of activity based on these assessments. For example,Pardosa shows strong reductions in activity when the silk and excreta from Hogna is fresh (up to 1 day old), but shows little response when these cues are 1-week old. Given that fresh cues would mean that Hogna may still be in the area, it makes sense that Pardosa would reduce activity in this context and show little response to older cues. Perhaps even more interesting, Pardosa appears able to discriminate between male and female Hogna predators based on chemical information only. They show greater reductions in activity in the presence of silk and excreta from female Hogna and more activity in the presence of male Hogna. This also makes adaptive sense since females are larger in size than males (sexually dimorphic), have higher energetic requirements and generally present a greater predation risk. Pardosa also shows antipredator behavior in the presence of chemical cues from Hogna that are equal in body size to themselves or larger, but shows little response to cues from small Hogna.
The responses of Pardosa to Hogna silk is incredibly fine-tuned. Not only can Pardosa determine the diet, size, and how recently Hogna was in the are based on silk alone, they can also tell if the predator is hungry or satiated by these cues alone.
Chemically-mediated reductions in activity may only be considered antipredatory in nature if it increases survival in the presence of a live predator. This also turns out to be true. Pardosa placed in containers with access to silk and excreta from a Hogna survive much longer in the presence of the predator than when they lack this information. The graded levels of activity in the presence of Hogna chemical cues of different ages, or from Hogna fed different diet directly translates into differential survivorship when actual predators are present.
So is this antipredator behavior learned or innate? We're not sure, but we have collected data that suggest the answer is both. In other words, wolf spiders with no prior experience with chemical cues from another predatory wolf spider show an innate reduction in activity. However, if they have additional experience with the chemical cues or other sensory information about a predator, they will increase their antipredator response which translates into increased survival in the presence of a live predator.
The fact that spiders show behavioral responses to prey and predator chemical cues (even in the absence of a predator) and that such behavior has measurable fitness costs, has far-reaching implications in our understanding of community ecology. Food webs and trophic structure within ecosystems may be largely governed by behaviorally-mediated indirect effects on prey and predators (i.e. reduced feeding, reduced reproductive success) rather than direct effects (death due to predation events) as previously believed. This means that we can only understand community and trophic structure dynamics of ecosystems by understanding the behavioral ecology of animals interacting within these systems.
Recently we have begun to examine interactions between Hogna helluo, Pardosa milvina, and a third, syntopic species, Rabidosa rabida (shown at left). All three of these species may be either a predator or prey of the others. Each species also shows context-dependent behavioral responses to the other species silk and/or feces.
Stay tuned for future developments on this page since this is currently the most active area of research. For a sneak peek of recent and current projects-see the web pages of senior research students