| Summary: | Social play is a widespread behaviour among mammalian lineages typically occurring among juveniles in periods of plentiful resources. Adult play is rare, as adult mammals typically use their energy for survival and reproduction. Polar bear social play in the Western Hudson Bay region is a double anomaly because play occurs between adult bears during a fasting period. Polar bears should be conserving as much energy as possible during their fasting period, as they are forced onto land away from their primary food sources. I investigated factors that influence the structure of polar bear social play including body condition, time since sea-ice break-up, role (initiator or terminator of play), and energetic cost. I found no significant effect of body condition on the duration or occurrence of social play, nor the effect of body condition on role. I found that as time since sea-ice breakup increased and minimum daily temperature decreased, occurrences of social play per day increased. Through the use of thermography, I found that polar bears use energy during social play, inferred from the increase in their surface temperature after a bout of social play. However, I was not able to quantify the amount of energy expended and conclude that thermography is a useful tool to non-invasively measure surface temperature during activity but needs to be validated with internal temperature probes and in stricter environmental conditions. During the fasting season, polar bears rely on fat stores built up over the hunting season, and any energy expenditure is considered costly to their fitness. I suggest that polar bear social play is an important life event that must be maintained despite the energetic costs associated with social play. October 2020
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