Spiders Watch Their Diets Too, Study Says

James Owen in London
for National Geographic News
January 6, 2005
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 invertebrates—all of which use different hunting methods—adjust their feeding to correct nutritional deficiencies.

Researchers behind the study—to 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."

Desert Spider

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."

Yet the study team discovered that the desert spider is able to extract nutrients selectively from a single fruit fly, depending on the spider's previous meals. It's unclear how desert spiders manage this, though the researchers suspect it's to do with enzymes the spider pumps into its prey to digest its insides.

"The spiders let the enzymes work for a while, then suck up the half-digested juice," Mayntz said. "Apparently they are able to take more protein out via the enzymes if the last prey they ate was poor in protein."

Possible mechanisms that allow insects and other invertebrates to alter their feeding according to nutritional need have been identified by two of Mayntz's colleagues at Oxford University's zoology department, Stephen J. Simpson and David Raubenheimer.

Working with plant-eating locusts, they identified a novel "taste-feedback" mechanism, whereby levels of nutrients in the blood indicate a locust's nutritional state, giving it a taste for the type of food it needs most.

Simpson said: "This provides direct, nutrient-specific control over food selection and consumption and allows insects to make sophisticated nutritional decisions without requiring complex neural integration."

Food Odors

The locusts were also shown to associate certain smells with beneficial foods. Simpson says they were specifically attracted by odors previously associated with foods containing nutrients the insect was deficient in—even after only a few hours of deficiency.

For predatory bugs, Mayntz says different food will also have different nutritional values.

"Some prey can have as much fat as a sausage, while another will be more like a lean steak," he said.

Mealworms, for instance, contain high amounts of lipids.

"If protein is needed, a predator might go for something like a mosquito, which has a huge amount of muscle compared with body lipids," Mayntz added. "Mosquitoes also suck up blood, which is full of protein."

Some larger predators have also been shown to be choosy about what they eat. For instance, experiments with carnivorous fish reveal they are able to compose diets based on nutritional value.

Biologists at the University of Murcia in Spain found that rainbow trout, when offered a range of foods, went for the high-protein option, while cutting out fats and carbohydrates. And it's possible that much bigger carnivores, such as leopards and sharks, show similar feeding behavior.

Mayntz says leopards might get particular nutrients by concentrating on certain parts of a carcass. "If they can't eat a whole antelope at once, perhaps they will start eating the bits they need most and then try to hide the rest," he said.

Indeed, Mayntz and his colleagues intend to move up the food chain in their investigation of predators' eating habits, starting with mink and cats.

Who knows? Maybe even great white sharks consult their diet sheets when deciding what next to sink their teeth into.

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