“…The majority of work to date on positive emotion and potential sources of PED in nonhuman animals is based on ethological observations of naturally occurring behavior published by well-established field workers (e.g., Bekoff, 2013; Berger, 1979). Specifically, this includes the rigorous behavioral observation and field studies of social play and related behaviors such as laughing across a wide range of species including golden marmots (e.g., Blumstein, 1998), Australian marsupials (e.g., Byers, 1999), bighorn sheep (e.g., Berger, 1979), fur seals (e.g., Harcourt, 1991), rodents (e.g., Panksepp, 2007; Panksepp & Burgdorf, 2003; Pellis & Pellis, 2010), and canids including domestic dogs, coyotes, and wolves (e.g., Bekoff, 1995, 2013). Social play behavior is of particular importance given it is a common form of social interaction in young mammals, and thought to be highly rewarding (e.g., Vanderschuren, Niesink, & van Pee, 1997) and critical for adaptive social and cognitive development (e.g., Bekoff, 2013; Burghardt, 2005; Fagen, 1981; van Kerkhof et al, 2013).…”