Why Did the Rooster Lose Its Penis?

TL;DR

Over 95% of bird species do not have penises, and scientists are still unsure why, but hypotheses include sexually transmitted diseases, female choice in sexual selection, and selectional pleiotropy.

Transcript

over 95 percent of bird species don't have penises and to us that might sound bizarre but it works they bring their nether regions together much like mammals do but there's no penetration involved and to be up front we don't have one definitive answer as to why but we have some pretty good ideas and by looking at those ideas we can learn a lot abou... Read More

Key Insights

• 💁 Fertilization has to happen internally before the formation of tough eggshells, and penises are effective at delivering sperm precisely where needed during internal fertilization.
• 😀 Penis loss occurred separately at least four times in amniotes, indicating that different lineages faced different evolutionary pressures.
• 👨‍🔬 Hypotheses for penis loss include sexually transmitted diseases, female choice in sexual selection, and selectional pleiotropy, but more research is needed to confirm these theories.

Summary & Key Takeaways

• Scientists have discovered that all amniotes, including birds, reptiles, and mammals, had penises before the lineage split, but penis loss occurred separately in different groups.

• One hypothesis suggests that penis reduction was naturally selected for because it reduced the contact males had with the potentially bacteria-infested cloaca.

• Another hypothesis proposes that penis loss is driven by female choice in sexual selection, as female birds may prefer males with smaller penises to ensure the best quality genes for their offspring.

• The possibility of selectional pleiotropy exists, where the loss of penises could be a side effect of genes involved in limb development or teeth.