Spanish fly isn't just a mythical drug. It's a real thing, and these male birds use it to get females to mate with them. Learn how spanish fly works, but only when males take it.
Spanish fly should be called "Spanish beetle." The name of the drug came from beetles that men used to consume as a sort of sexual stimulant and male enhancement rolled into one. It's not recommended today, and the true name of the beetles gives a good indication as to why. A Spanish fly is also known as a melloid beetle, or more casually, a blister beetle.
It's an accurate name. Milky secretions from the beetle's legs contain an irritant that can raise blisters and cause inflammation on any part of the body, inside or outside. Men used to swallow some of the secretions. The chemical in them would make its way through their body and irritate their urethra, producing greater sensation during sex and long-lasting inflammation in the area. Think of that the next time you get an email for an all-natural male enhancement drug.
It's ironic that the poison developed by blister beetles to deter predators made them more likely to be killed and eaten - and that irony exists across species. Great bustards are large, terrestrial birds that live in Europe and central Asia. When the males compete for females, they seek out blister beetles and consume them even when there is other food available. Researchers believe this helps them mate for two different reasons:
In addition to anthelminthic properties, cantharidin was effective against gastrointestinal bacteria that cause sexually-transmitted diseases. . . . The male-biased consumption suggests that males could use cantharidin to reduce their parasite load and increase their sexual attractiveness. This plausibly explains the intense cloaca display males perform to approaching females, and the meticulous inspection females conduct of the male's cloaca, a behaviour only observed in this and another similar species of the bustard family. A white, clean cloaca with no infection symptoms (e.g., diarrhoea) is an honest signal of both, resistance to cantharidin and absence of parasites, and represents a reliable indicator of the male quality to the extremely choosy females.
So male birds use Spanish fly to lower the chance that they'll spread parasites to their mate and to prove they're tough enough to consume a near-lethal dose of poison.
Image: Francesco Veronesi
[Sources: Males of a Strongly Polygynous Species Consume More Poisonous Food Than Females.]