When you spend six years watching kangaroos, you start to see some strange things. From 2008 to 2013, Wendy King, a doctoral student at the University of Queensland, and her colleagues studied wild grey kangaroos in a national park in Victoria, Australia. All told, King and her colleagues studied 615 animals–194 adult females, and 326 juveniles, known as joeys. The first time King and her colleagues captured each kangaroo, they took a number of measurements and then marked it so they could recognize it later. From time to time, they’d find a juvenile kangaroo in the pouch of a different mother. Sometimes it would climb out, but then it would climb back into the new pouch, getting milk and protection from the adult female for