Time of year influences social play in juvenile ground squirrels

Animals must allocate available energy according to priorities that ensure survival and maximize long-term reproductive success. Social play behavior is ubiquitous among juvenile mammals and can have important influences on their social, motor, and cognitive development. We evaluated how the amounts of energy young animals devote to play behavior changes over the course of their annual cycle, as other energetic considerations and constraints change. Young animals may change priorities from play to preparation for hibernation when the annual cycle occurs closer to the usual onset of winter. We used Belding’s ground squirrels (Urocitellus beldingi) as a model system; U. beldingi live in habitats with a short annual growing season, and therefore have an annual cycle of a brief active period in the summer and then an extended period of hibernation. We studied a population of U. beldingi at Tioga Pass in the Sierra Nevada over the course of two summers (May to July, 2016 and 2017). We recorded various measurements of juvenile social play during the two weeks following juveniles’ first emergence from their natal burrows; social play in U. beldingi primarily occurs during this two-week developmental period. We also captured and weighed juveniles at the beginning and near the end of the development period. We compared measurements of social play and changes in body mass over the development period between juveniles born earlier and later in the summer. We found that juveniles born later in the summer engaged in lower rates of social play and had shorter-duration play interactions on average than juveniles born earlier in the summer. Moreover, juveniles born earlier in the summer showed greater increases in body mass over the course of the development period than juveniles born later in the summer. These results are consistent with the idea that changes in growth patterns and behavior may be associated with shifts in the annual cycle of U. beldingi