One focus of my research involves testing assumptions about how animals acquire food efficiently (like what this crab spider is doing to the left). Most studies assume that animals maximize their energy gain over time, but to understand if this is true, requires an understanding of the mechanism by which animals make decisions about where to forage and for how long. To understand decision making in animals, one needs to know what information is important to that animal. For example, an animal may feed in an area as long as food is available and then leave after a period of time elapses where no food is found. The animal may decide to leave when its stomach is empty or when it does not perceive any more food in the area. It may leave based on a memory of how long it remained the last time it was in the area or even a memory of a recent predator encounter. The information used to determine the duration of time spent in a foraging area will influence the predictions of how long the animal will stay, and, ultimately, how efficient it is in finding food.
I have been examining how different types of information influence the duration of time wolf spiders spend in a foraging patch before leaving (residence time). I have tested the influence of different types of sensory information, prey capture and feeding, previous patch experience, prey movement, predator hunger level, age, and sex on foraging patch residence time. I have been particularly interested in the influence of sensory information on residence time decisions in wolf spiders. Wolf spiders do not build webs but rather use a sit-and-wait foraging strategy that employs both visual and substratum-borne vibrations to detect prey. Is the decision to leave a foraging patch based on what a spider perceives or what it eats? If it is based on what it eats, a spider that is allowed access to perceive prey, but not consume them, should leave all foraging patches at the same time regardless of the sensory stimulus. I tested this idea using a series of simple artificial foraging patches.
The foraging apparatus is a series of four patches that are equal in size to the sensory range of the spider. The spiders were allowed to perceive prey (one-week old crickets) behind acetate screens but were not allowed to feed. The sensory cues presented to the spider (shown as a black dot in the center of the picture) include from top left clockwise, 1) a control, no crickets present; 2) vibratory information only, white paper serves as a visual barrier to the spider; 3) visual cues only; the substratum of the crickets and spider are mounted separately; and 4) visual and vibratory cues together. The spiders move between chambers for a one hour period. The results showed that spiders modify their residence time with the presence of different sensory cues even in the absence of feeding. Visual cues were more important than vibratory cues however. See the abstract for more information.
I have also examined and found ontogenetic shifts in the use of different types of sensory information as well as sex-based differences . The types of sensory information important to male and female wolf spiders is different. Further, juvenile and adult spiders use visual and vibratory information differently to make foraging decisions as well. These studies present the first evidence of sex-based differences among juvenile spiders in their foraging habits. This is just another example of how poor our current understanding of spiders is relative to other taxa.
Using a different apparatus, the effects of hunger and differences in prey density were examined. Unlike predictions from many other empirical studies of foraging behavior in animals, hunger had no effect on foraging patch residence time decisions in wolf spiders. However, other studies demonstrate that wolf spiders can clearly assess the quality of a foraging patch via various sources of sensory information and that actual feeding on prey in a patch is not necessary to accomplish this.
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Previous studies by Will McClintock, Eileen Hebets, and George Uetz found that wolf spiders respond to video images as though real. I took advantage of this fact to investigate how prey movement influences a spider's decision to stay or leave a particular foraging patch. The apparatus on the left is what I used to examine the influence of prey movement on patch residence time decisions. By recording images of live crickets, digitizing them on a computer, and then playing them back to a spider on a small micro-television screen, I could carefully control cricket movement. Once cricket movement was standardized, I examined how variation in movement influenced spider locomotor patterns. As it turns out, prey movement has a large effect on residence time decisions
Our current understanding of the mechanisms and functional significance of spider foraging decisions is poor at present, yet this understanding is critical if we are to explore how or if spiders regulate insect population size in a variety of ecosystems.
