Why Did the Rooster Lose Its Penis? | Summary and Q&A

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.

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.

Transcript

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Questions & Answers

Q: Why do over 95% of bird species lack penises?

Scientists have several hypotheses, including sexual transmitted diseases, female choice in sexual selection, and selectional pleiotropy, but there is still no definitive answer.

Q: How did scientists determine that all amniotes had penises before the lineage split?

By studying chicken and tuatara embryos, scientists observed that both species start out developing penises during embryonic development, which supports the theory of a single early evolution of penises in amniotes.

Q: Why did penis reduction occur separately in different amniote groups?

Different lineages had different selective pressures and evolutionary changes that led to the reduction or loss of penises. It is still unclear why these changes occurred independently.

Q: Are there any exceptions to the lack of penises in bird species?

Yes, some bird species, such as ducks and kiwis, still have penises. However, there are various factors, including egg size and mating and parenting styles, that influence the presence or absence of penises.

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.