Nightly selection of resting sites and group behavior reveal anti-predator strategies in giraffe

This study presents the first findings on nocturnal behavior patterns of wild Angolan giraffe. We characterized their nocturnal behavior and analyzed the influence of ecological factors such as group size, season and habitat use. Giraffe were observed using night vision systems and thermal imaging cameras on Okapuka Ranch, Namibia. A total of 77 giraffe were observed during 24 nights over two distinct periods – July-August 2016 (dry season) and February-March 2017 (wet season). Photoperiod had a marked influence on their activity and moving behavior. At dusk, giraffe reduced the time spent moving and increasingly lay down and slept at the onset of darkness. Body postures that likely correspond to rapid eye movement (REM) sleep posture (RSP) were observed 15.8 ± 18.3 min after giraffe sat down. Season had a significant effect with longer RSP phases during the dry season (dry: 155.2 ± 191.1 sec, n=79; wet: 85.8 ± 94.9 sec, n=73). Further analyses of the influence of social behavior patterns did not show an effect of group size on RSP lengths. When a group of giraffe spent time at a specific resting site, several individuals were alert (vigilant) while other group members sat down or took up RSP. Simultaneous RSP events within a group were rarely observed. Resting sites were characterized by single trees or sparse bushes on open areas allowing for good visibility in a relatively sheltered location.本研究首次报道了野生安哥拉长颈鹿（Angolan giraffe）的夜间行为模式相关首批研究结果。我们对其夜间行为进行了系统刻画，并分析了群体规模、季节与生境利用等生态因子的影响。研究团队于纳米比亚奥卡普卡牧场（Okapuka Ranch）借助夜视系统与热成像相机开展长颈鹿观测工作。本次研究在两个独立时段的24个夜间共记录到77只长颈鹿个体，观测时段分别为2016年7-8月（旱季）与2017年2-3月（雨季）。光周期对长颈鹿的活动与移动行为具有显著影响：黄昏时分，其移动时长逐渐减少，且在夜幕降临时愈发多地趴卧休憩。在长颈鹿趴卧后的15.8±18.3分钟，我们观测到了疑似快速眼动（rapid eye movement, REM）睡眠姿势（RSP）的躯体姿态。季节因素对RSP时长存在显著影响，旱季的RSP阶段更长（旱季：155.2±191.1秒，n=79；雨季：85.8±94.9秒，n=73）。进一步针对社会行为模式的分析未发现群体规模对RSP时长存在显著影响。当长颈鹿群体在特定休憩场地停留时，部分个体保持警觉（vigilant）状态，其余成员则趴卧或进入RSP状态。群体内同时发生RSP事件的情况极为罕见。休憩场地多为开阔区域内的单棵树木或稀疏灌丛，此类场地既能提供相对遮蔽的环境，又可保障良好的视野范围。