So this probably isn't one we're as familiar with as, say, our appendix or the panda's insatiable appetite for foods that aren't really that good for them. But the hyena's sexual organs should get their due for just how cruel an adaptation can be.
First off, the female hyena has an enlarged clitoris that's difficult to distinguish from the penis of the male. That's fine and good; we don't judge organs for superficial reasons here. And, in a twist that might seem downright convenient, the clitoris is used for urination, mating and giving birth [source: Michigan State University]. A pretty useful structure for multitasking.
Unfortunately, it has a nasty side effect. The clitoris -- which is about 6 to 7 inches (15-18 centimeters) long -- is not ideal as a birth canal [source: Michigan State University]. And we should say that the cubs do travel through a vaginal canal before being delivered eventually through the clitoris. The penis-like structure is only about an inch in diameter, and cubs can suffocate during birth [source: Michigan State University]. Females giving birth can be injured, too. Which would cause one to assume that evolution would bring an easier, more lenient system to hyena procreation.
Author's Note: 10 Worst Adaptations in the Animal Kingdom
There's something a little uncomfortable about calling an animal adaptation "bad." As the zoology professor and hyena expert Kay Holekamp points out, the fact that hyenas have evolved -- and kept -- a delivery system that could suffocate young implies that the benefits outweigh the costs [source: Hyena Research Special Report.] It's possible the same could be said for any evolutionary adaptation we've identified as the so-called "worst."
- Barras, Colin. "Appendix evolved more than 30 times." Science Magazine. Feb. 12, 2013. (Oct. 2, 2013) http://news.sciencemag.org/plants-animals/2013/02/appendix-evolved-more-30-times
- Barrett, Paul. "Ask a grown-up." The Guardian. Sept. 6, 2013. (Oct. 2, 2013) http://www.theguardian.com/lifeandstyle/2013/sep/07/ask-grown-up-tyrannosaurus-rex
- Briggs, Helen. "Sloth's 'Lazy' Image a Myth." May 13, 2008. (Oct. 7, 2013) http://news.bbc.co.uk/2/hi/science/nature/7396356.stm
- Buchen, Lizzie. "Could pandas be an evolutionary mistake -- or proof of an intelligent designer?" Discover Magazine. Aug. 5, 2008. (Oct. 2, 2013) http://discovermagazine.com/2008/aug/05-could-pandas-be-an-evolutionary-mistake2014or-proof-of-an-intelligent-designer#.UkmV6mR4ZLo
- Gavin, Mary. "What's an Adam's apple?" KidsHealth.org. September 2013. (Oct. 2, 2013) http://kidshealth.org/kid/talk/qa/adams_apple.html
- Glausiusz, Josie. "And here's why you have an appendix." Discover Magazine. Jan. 15, 2008. (Oct. 2, 2013) http://discovermagazine.com/2008/jan/function-of-appendix-explained#.Ukmi3mR4ZLo
- Goldstein, Miriam. "Perverted cannibalistic hermaphrodites haunt the Pacific Northwest!" The Oyster's Garter. March 24, 2008. (Oct. 2, 2013) http://theoystersgarter.com/2008/03/24/perverted-cannibalistic-hermaphrodites-haunt-the-pacific-northwest/
- Gonzalez, Robert T. "10 vestigial traits you didn't know you had." iO9. Aug. 15, 2011. (Oct. 2, 2013) http://io9.com/5829687/10-vestigial-traits-you-didnt-know-you-had
- Hughes, Janet, Peter Clark and Leslie Klenerman. "The Importance of the Toes in Walking." The Journal of Bone and Joint Surgery. March 1990. Vol. 72-B, no. 2. Page 245-251. (Oct. 7, 2013) http://www.bjj.boneandjoint.org.uk/content/72-B/2/245.full.pdf
- Hyena Research Special Report. "Spotted Hyenas." Michigan State University. (Oct. 2, 2013) http://special.news.msu.edu/hyena/establishingdominance.php?dominance
- Miller, Brooke L. W. "Project Page." University of California Santa Cruz. June 1, 2004. (Oct. 2, 2013) http://bio.research.ucsc.edu/grad/weaver/
- The National Aviary. "Two-toed sloth." The National Aviary. 2013. (Oct. 2, 2013) http://www.aviary.org/animals/two-toed-sloth
- National Geographic. "Giraffe." National Geographic. 2013. (Oct. 2, 2013) http://animals.nationalgeographic.com/animals/mammals/giraffe/
- National Geographic. "Tyrannosaurus Rex." National Geographic. 2013. (Oct. 2, 2013) http://animals.nationalgeographic.com/animals/prehistoric/tyrannosaurus-rex/
- Padian, Kevin. "Tyrannosaur." Encyclopædia Britannica Online Academic Edition. 2013. (Oct. 7, 2013) http://www.britannica.com/EBchecked/topic/611883/tyrannosaur>.
- Pikul, Corrie. "6 women's health myths you can stop believing right now." Oprah.com. June 25, 2012. (Oct. 2, 2013) http://www.oprah.com/health/Debunking-Womens-Health-Myths-and-Misconceptions/2
- Spinney, Laura. "Five things humans no longer need." NewScientist. May 19, 2008. (Oct. 2, 2013) http://www.newscientist.com/article/dn13927-five-things-humans-no-longer-need.html?page=1#.UkxfO2R4ZLp
- Wolman, David. "10 Worst Evolutionary Ideas." Wired. July 20, 2009. (Oct. 2, 2013) http://www.wired.com/science/discoveries/magazine/17-08/st_best
- Zhang, Sally. "Do I really need my pinky toe?" Popular Science. May 15, 2013. (Oct. 2, 2013) http://www.popsci.com/science/article/2013-05/fyi-do-i-really-need-my-pinky-toe
The flightless Aldabra rail only lives on the Aldabra Atoll in Madagascar. But this example of iterative evolution descended from birds that soar.