Arachnid Science: Article Reviews

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Mayntz, D., D. Raubenheimer, M. Salomon, S. Toft, & S.J. Simpson. 2005. Nutrient-specific foraging in invertebrate predators. Science 307: 111-113.

There are multiple ways of restoring balance to a diet that is deficient in certain nutrients. Herbivores and omnivores are known to become more selective about what foods they consume, choosing what will yield the highest concentration of the lacking nutrient. In contrast, carnivores typically do not change what they eat; they simply capture and consume more prey. The goal of this paper was to investigate consumption changes among predatory invertebrates that differ in methods of prey capture. Controlled laboratory experiments were conducted with three invertebrates: (1) Agonum dorsale, a very mobile ground beetle that hunts for prey; (2) Pardosa prativaga, an ambushing wolf spider of intermediate mobility that periodically changes its primary foraging turf; and (3) Stegodyphus lineatus, a desert web-building spider that remains in a single location awaiting prey. With such differences in how prey are encountered, how can these invertebrates adjust their food intake to counter very specific deficiencies and assure that they are receiving adequate nutrition?

The invertebrates' diets were controlled for 24 or 48 hours prior to testing. Researchers provided food that had been augmented to be very high in either lipids or protein. A. dorsale subjects were given a choice of two food sources during testing. P. prativaga subjects were tested with a single food source, either the same as the pretreatment diet or that food source augmented to be higher in the nutrient they were lacking. Total prey mass consumed was recorded for A. dorsale and P. prativaga. S. lineatus was given only one food source during testing to simulate the natural environment of the species and a more detailed analysis of the matter extracted from this single prey item was conducted.

Results for A. dorsale and P. prativaga were similar. Individuals that were pre-fed a lipid-rich diet ate more of the food source that delivered the most protein. Conversely, the protein-rich subjects preferentially ate more lipids during testing. Thus, the strategy of both species was to vary consumption of only a single prey type. Another interesting result was a significant increase in total amount of food consumed by those beetles that had been pre-fed a protein-rich diet. An increase in overall food intake was not observed in the lipid-rich beetles or any wolf spiders.

The desert spider S. lineatus builds a web and does not leave to hunt. Therefore, these spiders have a take-what-comes-along menu in the wild, based on what prey happens to find its unlucky self caught in the web. Given that the spiders must make do with what is available, can they effectively regulate for specific nutritional needs? The researchers tested the idea that perhaps these spiders regulate by selectively extracting what they need from the prey carcass. The researchers allowed spiders to feed for 30 minutes, then removed prey remnants to study how much carbon and nitrogen had been consumed. As predicted, protein-deficient S. lineatus preferentially extracted more nitrogen from the prey.

In summary, this study demonstrated various ways that organisms can compensate for experimenter-induced imbalances in their recent feeding histories. Three invertebrate species, all with different prey capture strategies, showed nutrient-specific regulatory consumption. Given that diets were manipulated for only 24-48 hours to create the imbalance, these compensatory mechanisms kick in quickly to correct an imbalance arising from recent feeding history. Dietary compensation could be achieved by selecting among foods that differ in nutrient concentration, by increasing or decreasing consumption of a single prey species, or by highly targeted extraction of the deficient nutrients from any prey item. That these carnivorous invertebrates were discriminating about what they ate challenges current models of optimal foraging, which assume that carnivores adjust their diets by changing quantity, not quality, of food consumed. This study reveals that this assumption does not hold for all carnivore species.

For more information: www.sciencemag.org

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