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Some birds steal food from other birds to save themselves work, but kleptopredators take it one step further!

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Sources:
https://academic.oup.com/biolinnean/article-lookup/doi/10.1111/j.1095-8312.2008.00954.x
http://rsbl.royalsocietypublishing.org/content/13/11/20170447
http://onlinelibrary.wiley.com/doi/10.1111/ivb.12154/full
http://www.sciencedirect.com/science/article/pii/0040816684900053
https://www.nature.com/articles/s41598-017-08002-0
http://www.sciencedirect.com/science/article/pii/S0960982210007864
http://www.pnas.org/content/102/42/15155.short
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3368621/
https://ir.canterbury.ac.nz/bitstream/handle/10092/9753/12639965_mve_986_proof.pdf?sequence=1 http://www.pnas.org/content/106/46/19416.short
https://link.springer.com/article/10.1007/s10905-012-9338-4
https://www.researchgate.net/profile/Pablo_Alarcon5/publication/261502044_Diet_of_the_Neotropical_Cormorant_Phalacrocorax_brasilianus_in_a_Patagonian_Freshwater_Environment_Invaded_by_Exotic_Fish/links/02e7e53468e2283e87000000.pdf
http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0161841

Images:
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https://commons.wikimedia.org/wiki/File:Eudendrium_ramosum_(Linnaeus,_1758).jpg
https://commons.wikimedia.org/wiki/File:Chromodoris_lochi_(AA3).jpg
https://commons.wikimedia.org/wiki/File:Tritoniopsis_elegans.jpg
https://commons.wikimedia.org/wiki/File:Glossodoris_atromarginata.jpg
https://commons.wikimedia.org/wiki/File:Elysia_viridis_(Montagu,_1804)_1.jpg
https://commons.wikimedia.org/wiki/File:Flickr_-_Lukjonis_-_Jumping_spider_-_Evarcha_arcuata_(Set_of_pictures)_(1).jpg
https://commons.wikimedia.org/wiki/File:Anopheles_mosquito_engorged_with_human_blood.jpg
Every year as fall slides into winter, we’re reminded of the masterpiece that is the turducken.

The meal that asks “how would it taste if I ate a turkey that had just eaten a duck that had just eaten a chicken?” The idea sounds absurd. But there are animals that eat other animals to also eat their undigested meals.

It’s a feeding strategy called kleptopredation, which is really a specialized version of another weird feeding strategy, called kleptoparasitism. That’s when an animal steals a meal caught or prepared in some way by another animal—like how frigatebirds and gulls dive bomb other birds to steal fish. This way, they don’t have to put as many resources into finding or catching top-quality food.

They let other species do the hard work, then reap the rewards. Kleptopredators take this one step further, by eating both the animal and whatever it just ate. The term was coined in a November 2017 paper in Biology Letters to describe a colorful sea slug that eats little coral-like animals called hydroids, mostly for their stomachs full of plankton.

They don’t think this is the only type of sea slug that does this, either—it’s just the only one where they’ve looked for this behavior. At first, this might seem like something predators do all the time. Sometimes prey has food in its stomach, and a predator happens to eat it.

But when you eat a chicken, you aren’t specifically aiming to eat its belly full of scratch grains as well. You want the nutrients stored in its tissues. Whereas kleptopredators want both.

Figuring out if a species is a kleptopredator is tricky. You have to show that it only eats, or at least has a strong preference for, prey that has undigested food in its stomach. But that’s what this sea slug does.

Even though it eats hydroids, more than half of its diet is actually plankton. The slugs don’t have any way of catching these plankton, so they just let the hydroids do it. Then they eat the hydroids — and therefore, the plankton.

Problem solved! The researchers showed that the sea slugs specifically target hydroids after they eat. In experiments, they consumed twice as many full hydroids as ones that hadn’t just eaten, though the team isn’t sure how the slugs know whether a hydroid has just fed.

In some ways, it’s not that surprising that some sea slugs feed like this. This species is in a category of sea slugs called nudibranchs, and as a group, they’re somewhat notorious for stealing from their meals. Some steal their preys’ stinging cells to defend themselves.

And there’s at least one species that researchers think steals chloroplasts from the algae it eats. That’s the part of cells which allows them to photosynthesize to make their own food. The slug then uses these chloroplasts to make its own sugars, basically becoming part plant.

But there is another, more surprising animal that targets its prey after they’ve just eaten: a cute little jumping spider. The East African jumping spider is a mosquito specialist. But it doesn’t go for just any mosquitoes: it prefers female mosquitoes that have just consumed blood.

Back in 2005, biologists detailed the spider’s finicky palate in a paper published in PNAS. Both sexes of the spiders went after blood-fed female mosquitoes over several other options. The researchers ruled out any kind of learning of this preference, since the spiders went for the bloody insects even when they hadn’t seen or tasted one before.

And further studies have found that they can tell male mosquitoes from females by their antennae alone. Part of their strong preference may be because drinking blood makes them dead sexy. Both sexes of spiders can smell if potential mates have eaten blood-filled mosquitoes, and find those that have more attractive.

Since both males and females engage in courtship and mate choice—a remarkably egalitarian system for a jumping spider—it makes sense that both seek blood meals to maintain their swagger. But why this spider species became so enamored with blood is less clear. Several close relatives are also mosquito specialists, but they don’t have the same vampiric tastes.

It might just be their way of showcasing their spidery awesomeness. In the spider’s natural habitat, blood-fed mosquitoes are vastly outnumbered by non-biting flies and other mosquitoes. Being able to find, capture, and consume such rare prey often enough to maintain that sexy blood scent may be an honest signal of quality.

It’s kind of like they’re buying an expensive perfume or cologne. By reeking of blood, these spiders are advertising that they’ve got the goods to make super spider babies. It’s also possible that the spiders benefit nutritionally from minerals like iron or something else unique to vertebrate blood.

But so far, we don’t know much about their dietary needs or how much blood they consume. Regardless of how they gained it, their thirst for blood makes these spiders one of two known species of kleptopredator, even though the term for it wasn’t invented until 12 years after researchers discovered what they were doing. Odds are, though, they’re not alone.

Diet studies find weird things in animal stomachs all the time that are attributed to so-called “indirect” or “secondary” predation. Which makes sense, because at any given moment, your prey /might/ have just eaten. The trick is showing whether any of those animals prefer full prey.

Until then, it’s an elite club of two. Or three, if you count our love of artificial turduckens. Thanks for watching this episode of SciShow, and if those sexy vampire spiders have you wondering about other weird spiders, you can check out our video about 9 of the coolest new arachnid species.