Humans didn't invent the idea of combining sex and commerce. Plenty of animals are willing to mate, but only if the price is right. Here are 10 animals who will offer sex for reasons other than just preserving the species.

Top Image: Christopher Michel

10. Adélie Penguins are not Child-Friendly

This is something you didn't see in Happy Feet. Remember those penguins that ran around collecting rocks to make nests in order to attract a mate? Well, when the happy couple finally gets together, they build their nest up together. Both the female and the male go out to discover or steal rocks. Ah, but the female has a trick she can pull if her mate isn't watching. If she sees another male with a particularly fine rock in his beak, she'll have sex with him in exchange for the rock and then happily go back to the nest. Her official mate is none the wiser.

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9. Macaque Monkeys have Mistresses

Macaques can't pay directly for sex, but they can offer a life of luxury that allows them to have more sex with a more faithful partner. Grooming is critical to the health and well-being of monkeys. Female macaques will mate far more with males who offer a lot of grooming. What's more, when they are properly groomed, they curb their inclination to test drive alternate mates. As long as their partners keep them in style, they stay (relatively) faithful to the macaque that's doing their grooming.

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8. Hummingbirds Are Too Hungry to Be Abstinent

Hummingbirds have to eat regularly to maintain their ultra-quick metabolism, and keep from starving to death. They eat the nectar of certain kinds of flowers, and those flowers are clustered together on bushes. The male hummingbirds dominate the best areas of the bushes, while the female birds are left with mere scraps. Either they can pair up long term with a male, or else they can trade sex for temporary access to the male bird's flower trove.

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Cricket Image: Hugo A. Quintero G.

7. Crickets Need to Bring a Big Gift if They Want to Reproduce

A male decorated cricket will give a female a gift while he tries to inseminate her. The mating process involves attaching a sac of sperm to her body. If she's done eating before he's finished, she'll reach back, grab the sac, and eat it as well. And there goes his chance to spread his genes. So not only does he have to bring her a present, whatever present he gives her has to be big enough that she's full when she feels ready to reproduce.

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6. Chimps Blur the Lines Between Sex Work and Marriage

A male chimp will often share meat with a female one. Originally, scientists thought that this was payment for sex. When researchers observed the behavior happening when neither animal was in estrus, they assumed it was more like maintaining a chimp marriage. But chimps will share with one female for a while, and then another for another period of time. So, we wonder, what is the analogy? Is it a stable relationship, or serial marriage, or mercenary sex. And what does it say about us that we can't tell?

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Chimp Image: Tim Strater

5. Capuchins' Interests Include Both Sex and Porn

When researchers gave capuchin monkeys tokens that could be exchanged for snacks, an economy sprang up. Not only would these monkeys have sex in exchange for tokens, they would also trade tokens for extended looks at pictures of other (and presumably hot) monkeys. It seems capuchins like to fantasize.

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Dolphin Image: NOAA Photo Library

4. Dolphins Will Pimp Their Mates

Most people reading this will know already that dolphins are evil. They engage in infanticide and recreational porpoise murder. When a female pairs off with a male, sometimes another male will join them — with the consent of the existing male. Pairs of male dolphins will separate a female from the herd and guard her from other males. They will both have sex with her, dividing their chances of having any offspring but greatly increasing their chances of success compared to the free-for-all that would happen if the female were with the rest of the herd.

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3. The Gray Shrike Is a Serial Killer in a Seller's Market

The gray shrike, a bird, will give its intended female love a gruesome present. It will actually impale insects and small mammals on thorns of a bush, let them stay there until they die, and then present the kabob to its possible mate. But what if, once she accepts, a prettier shrike flits into view? The first gift establishes a sort of "mating price." The second female shrike knows what the going rate is — and she will only acquiesce if she gets an even bigger Death Kabob.

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2. Female Seed Beetles are Just Sad

Seed beetles live out in the desert. There are some times when life is lush in the desert, and then there are years that are lean even by the standards of arid regions. When it's dry enough, female seed beetles will have sex with males purely for the hydration. This isn't an easy choice for the male. The male seed beetles, since they lose out on water, often die for the slight chance of passing their genes along. So it could be argued that they're even sadder.

Image: Limbatus

1. The Praying Mantis's Payment is More Like a Token

Perhaps the most famous instance of a male paying too much for sex is the praying mantis. During copulation, the female sometimes bites off and eats the male's head. That's a steep payment, to be sure, but not as tough as it sounds. It seems the male mantis is controlled by nerves in his abdomen, not his head. So she's not really extracting the ultimate price. A male can last through the heat removal, although he probably will die without eyes or a mouth to help him catch and eat food.

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[Via Chimpanzees Trade Meat for Sex, Mating in the Material World, Macaque Monkeys Pay for Sex, Weirdest Animals Sexual Behaviors, Monkeys Learn to Use Money, Pay for Sex, The Surprising Sex Life of Beetles.]