Get the Popular Science daily newsletter💡 Breakthroughs, discoveries, and DIY tips sent six days a week. At a wildlife park in Germany, a young ring-tailed lemur did something bold: It launched itself at an adult ruffed lemur and started gently slapping and grabbing her. Rather than putting an end to these antics, the bigger, stronger ruffed lemur just rolled onto her back, flashed a relaxed open-mouth expression, and let the little one tackle her.  The playful wrestling match that ensued between these two different species caught researchers’ attention. In a report published in February in Frontiers in Ecology and Evolution , they document four instances of this “rare” and “potentially risky behavior,” which scientists refer to as “interspecies play.” Why animals play Animals that live in groups love to play.…