Locomotor play is vigorous and seemingly purposeless behavior, commonly observed in young mammals. It can be costly in terms of energy expenditure, increased injury risk, and predator exposure. The main hypothesized benefit of locomotor play is enhancement of neuromuscular development, with effects persisting into adulthood. We hypothesized that levels of locomotor play would have evolved as a correlated response to artificial selection for increased voluntary exercise behavior. We studied mice from 4 replicate lines bred for voluntary wheel running (High Runner or HR) at 6–8 weeks of age and four non-selected Control (C) lines. Mice were weaned at 21 days of age and play behavior was observed for generations 20 (22–24 days old), 68 (22–23 days old), and 93 (15 days old). We quantified locomotor play as (1) rapid, horizontally directed jerk-run sequences and (2) vertical "bouncing." We used focal sampling to continuously record behavior in cages containing 4–6 individuals during the first 2–3 h of the dark cycle. Observations were significantly repeatable between observers and days. A two-way, mixed-model simultaneously tested effects of linetype (HR vs. C), sex, and their interaction. Contrary to our hypothesis, HR and C lines did not differ in any generation, nor did we find sex differences. However, differences among the replicate HR lines and among the replicate C lines were detected, and may be attributed to the effects of random genetic drift (and possibly founder effects). Thus, play behavior did evolve in this selection experiment, but not as a correlated response to selection for voluntary exercise.

运动游戏是一种充满活力且看似漫无目的的行为，常见于年轻的哺乳动物中。就能量消耗、受伤风险增加和捕食者暴露而言，它的成本可能很高。运动游戏的主要假设好处是增强神经肌肉发育，其影响持续到成年。我们假设，运动水平的演变是对增加自愿运动行为的人工选择的相关反应。我们研究了 4 个 6-8 周龄自愿轮跑（High Runner 或 HR）复制品系和 4 个未选择的对照 (C) 品系的小鼠。小鼠在 21 天时断奶，观察第 20 代（22-24 天龄）、68 代（22-23 天龄）和 93 代（15 天龄）的玩耍行为。我们将运动游戏量化为（1）快速、水平方向的急跑序列和（2）垂直“弹跳”。我们使用焦点采样来连续记录包含 4-6 个个体的笼子在黑暗周期的前 2-3 小时内的行为。观察者之间和天数之间的观察结果具有显着的可重复性。双向混合模型同时测试了线型（HR 与 C）、性别及其相互作用的影响。与我们的假设相反，HR 和 C 系在任何一代中都没有差异，我们也没有发现性别差异。然而，检测到重复 HR 系和重复 C 系之间的差异，并且可能归因于随机遗传漂变的影响（以及可能的创始人效应）。因此，游戏行为确实在这个选择实验中进化了，但并不是对自愿锻炼选择的相关反应。