It’s time again for our listener mail round-up, and this week, the theme is borders and boundaries. We learn what it means to define the “end of an era,” explore how close is too close to a black hole, and discuss the power of animal urine. Plus, we hear the story about the time the U.S. and Great Britain almost fought a border war… because of a pig.  How do different animals mark their territory? How does surveying work? How close do you have to be to get pulled in by a black hole? How do historians decide how to categorize chunks of time? Featuring Eric Salovich, Naoufal Souitat, Dagomar Degroot, and Christine Wilkinson.   SUPPORT Outside/In is made possible with listener support. Click here to become a sustaining member of Outside/In.  Talk to us! Follow Outside/In on Instagram or Twitter, or discuss the show in our private listener group on Facebook.  Submit a question to our Outside/Inbox. We answer queries about the natural world, climate change, sustainability, and human evolution. You can send a voice memo to outsidein@nhpr.org or leave a message on our hotline, 1-844-GO-OTTER (844-466-8837).   CREDITS Host: Nate Hegyi Reported and produced by Nate Hegyi, Taylor Quimby, Justine Paradis, Felix Poon and Jeongyoon Han.  Edited by Taylor Quimby Executive Producer: Rebecca Lavoie  Music by Blue Dot Sessions. Outside/In is a production of New Hampshire Public Radio Learn more about your ad choices. Visit megaphone.fm/adchoices