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."
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 ineven 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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