for National Geographic News
Spiders and insects that eat other creepy crawlies purposely seek a balanced diet to maintain their health, according to a new study.
Scientists found that three predatory invertebratesall of which use different hunting methodsadjust their feeding to correct nutritional deficiencies.
Researchers behind the studyto be published tomorrow in the journal Science say other, much larger predators, like leopards and sharks, may also monitor what they eat to maintain a balanced diet.
While it's known that plant eaters and omnivores often eat a wide selection of foods to ensure the intake of various nutrients, carnivores aren't thought to be that fussy. Yet the study showed that predators also "read the label" when selecting their prey.
Scientists based in England, Denmark, New Zealand, and Israel tested a quick ground beetle, an ambushing wolf spider, and a web-building desert spider to see if they selectively forage for fat (lipids) and protein.
The animals were first given an unbalanced diet, skewed in favor of either lipids or protein. Their subsequent feeding, after they were given a choice of foods, was then monitored.
Previously fed a lipid-rich diet, ground beetles (Agonum dorsale) subsequently ate protein-enriched food to compensate for the imbalance. The reverse happened when they were initially fed protein-laden food.
It was a similar story for the wolf spider (Pardosa prativaga), according to co-author David Mayntz, a zoologist at Oxford University, England.
He said, "Wolf spiders don't build webs but sit and wait for prey to appear and then ambush them, so we didn't think they would be able to go out and select their diet. They have to deal with whatever they catch. But we found what they eat from the prey they do catch will depend on how much protein is in the prey and what [the predator's] last meal was. If they had a lipid-rich meal the day before, then the next day they would eat more prey containing lots of protein."
The web-building desert spider (Stegodyphus lineatus) has even less control over the type of prey it eats, Mayntz says.
"It cannot do anything to attract specific animals with specific nutrients," Mayntz said. "It has to deal with whatever ends up in the web."
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