Sperm's lifespan. It takes around 10 weeks for the sperm to reach maturity. Once the sperm is inside the woman's body it can live up to 5 days or even longer under favorable conditions. However, when it is outside the body, it can only survive from few minutes to several hours.
The UX developer role is a discipline under the overall user experience umbrella. This means that to become a UX developer you need to have at least a general understanding of the core UX principles, philosophies, and methodologies.
Stakeholders write user stories. An important concept is that your project stakeholders write the user stories, not the developers. User stories are simple enough that people can learn to write them in a few minutes, so it makes sense that the domain experts (the stakeholders) write them. Use the simplest tool.