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African Spider Craves Human Blood, Scientists Find

James Owen
for National Geographic News
October 11, 2005
 
Scientists have discovered a spider in East Africa with a craving for
human blood.

They say the spider, which hunts blood-sucking female mosquitoes, is the only animal known to select its prey based on what the prey has eaten.

The spider is the also first known predator that deliberately feeds on vertebrate blood by eating mosquitoes.

The finding raises the possibility that other spiders also have a taste for human and mammal blood, the researchers add.

The blood-hungry spider, Evarcha culicivora, is found only around Lake Victoria in Kenya and Uganda. A species of jumping spider, or salticid, it usually hunts insects on tree trunks and buildings. It stalks its prey rather than trapping it in a web.

The study team says the jumping spider uses both its acute eyesight and its sense of smell to seek out the mosquito Anopheles gambiae, a notorious blood-sucker that is the main carrier of malaria in Africa.

The team's findings are published this week in the online edition of Proceedings of the National Academy of Sciences. The research was partly funded by the National Geographic Society.

Smell of Blood

Lab experiments conducted near Lake Victoria showed the spider preferred female mosquitoes fed with human blood over all other prey, including male mosquitoes, which don't feed on animal blood.

Tests of the spider's prey preferences showed it went for blood-engorged female mosquitoes in 83 percent of cases when offered a choice of two similar-size insects.

When it came to making a choice based on smell alone, with the two meal options hidden from view, around 90 percent of jumping spiders selected the blood-filled mosquito.

Although no other predators are known to choose prey based on the prey's last meal, other spiders may also select their victims this way, says study co-author Ximena Nelson, who conducted the research while at the University of Canterbury in New Zealand.

"Spiders have good chemosensory abilities, and if blood were on the menu no doubt many would be able to detect it by smell, even if not all spiders possess vision good enough to be able to detect blood-carrying prey visually," Nelson said.

Although many spiders have relatively poor eyesight—those that use webs to trap prey have no need for acute vision, Nelson says—jumping spiders are an exception.

"Salticids are predators that actively search for prey and mates and typically do not build webs," she said. "They have evolved eyes that support high-acuity vision suited to their active lifestyle."

Spiders don't have the skin-piercing mouth parts needed to feed directly on human blood, but the mosquito-munching jumping spider appears to have got around this. The strategy has other advantages as well, Nelson points out.

"Blood-feeding is a dangerous activity," she said. "Animals that are bitten have a swatting response, and often the insect is killed."

By eating mosquitoes, the spider avoids the risk of being squashed by an unwilling blood donor.

The study team suspects a blood meal is also biologically important to E. culicivora.

They say spiders expend a lot of energy breaking solid food down into liquid by injecting their prey with digestive enzymes.

"Perhaps blood is a ready-made, nutrient-rich liquid meal," Nelson said.

Picky Eaters

Another recent study suggests spiders are surprisingly picky eaters, purposely seeking a balanced diet to maintain their health.

The study, published in January, found that spiders are selective about the nutrients they take from their prey depending on their need for proteins or fats.

Even the web-building desert spider (Stegodyphus lineatus), which has little control over what it catches, was able to balance its diet, according to David Mayntz, a biology professor at Aarhus University in Denmark.

He says the spider likely does this by altering the cocktail of digestive enzymes it pumps into its prey, allowing it to extract extra protein if the last prey it ate was low in protein.

Mayntz describes the African blood-eating spider find as "a very fascinating story indeed."

While he says blood-fed mosquitoes might be easier to catch than other mosquitoes because they are heavier, their higher nutrient value may well make them more appetizing to spiders.

Mayntz adds that the East African spider must have evolved a special ability to handle large amounts of vertebrate blood.

"[The spiders] would need some special enzyme to deal with this more complex protein," he said. "It's likely they would benefit from this protein, which is more or less dissolved already and easy to extract."

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