International Journal of Comparative Psychology

Play and tool use are controversial in part because both have been challenging to define. Play behavior continues to elude specific definition but is currently recognized as a legitimate behavioral classification, especially when an activity involves handling objects (toys), although play does not require object handling. In contrast, animal tool use behavior requires object handling that also meets criteria of purposeful and conditional handling in a specific context to achieve a goal. This report describes a form of object handling, grass-wearing behavior, exhibited by free-ranging bottlenose dolphins in St. Petersburg, FL, USA, to see if play or tool-use-like behavior explains it. During 9,551 sightings of 311 dolphins across 8 yrs of study (Jan 2006 – Dec 2013), N = 79 dolphins were observed with one or more blades of grass splayed across the dorsal fin 190 times. Grass-wearing was unrelated to activities conducted in seagrass meadows, age-sex class, or adult female reproductive phase. Grass-wearing was primarily related to changes in group composition (fusion events). It occurred in larger groups that were significantly more likely to be socializing in affiliative, explicitly sexual and playful contexts with only one observation during conflict, although grass-wearing occurred during travel, forage/feeding, and resting. The behavior was partly explained by play and tool-use-like behavior but is more consistent with dolphins self-decorating with grass as a stimulus enhancement in greeting or bids for attention.