Dietary content of culms and leaves of four Phyllostachys bamboos (P. aurea, P. aureosulcata, P. glauca, and P. nuda) over 2008-2010

Seasonal changes in the foodscape force herbivores to select different plant species or plant parts to meet nutritional requirements. We examined whether the search for calorie-rich carbohydrates explained giant panda's selection for bamboo culm over leaves during spring. Leaves and culms were collected from four Phyllostachys bamboos (P. aurea, P. aureosulcata, P. glauca, and P. nuda) once per month over 18-27 months. Monthly changes in annual plant part nutrients were examined, and compared to seasonal foraging behaviors of captive giant pandas. Although total fiber was greater (p<0.0001) in culm (85.6 ± 0.5%) than leaves (55.3 ± 0.4%) throughout the year, culm fiber was at its lowest in spring (79-85%) when culm selection by giant pandas exceeded 70% of their overall diet. Culm starch also was greatest (p=0.044) during spring (5.5 ± 1.1%) and 2.5-fold the percentage of starch in leaves (2.2 ± 0.6%). The free sugars in spring culm consisted of a high proportion of glucose (35%) and fructose (47%), whereas sucrose made up 42% of the total free sugar content of spring leaves. Bound sugars in culm consisted of 60% glucose and 38% xylose likely representative of hemicellulose. The concentrations of bound sugars (hemicelluloses) in spring culms (543.7 ± 13.0 mg/g) was greater (p<0.001) than in leaves (373.0 ± 14.8 mg/g). These data help explain a long-standing question in giant panda foraging ecology: why consume the plant part with the lowest protein and fat during the energetically intensive spring breeding season? Giant pandas likely prefer spring culm that contains abundant mono- and polysaccharides made more bioavailable as a result of reduced fiber content. These data suggest that phenological changes in bamboo plant part nutrition drive foraging decisions by giant pandas.食物景观的季节性变化迫使植食性动物选择不同的植物物种或植物部位，以满足自身的营养需求。本研究旨在探明，大熊猫在春季偏好取食竹秆而非叶片的行为，是否源于其对高卡路里碳水化合物的需求。研究团队在18至27个月的周期内，每月采集一次4种刚竹属（Phyllostachys）竹子的叶片与竹秆，涉及物种包括P. aurea、P. aureosulcata、P. glauca及P. nuda。本研究分析了植物各部位营养成分的月度变化规律，并将其与圈养大熊猫的季节性觅食行为进行比对。尽管全年竹秆的总纤维含量（85.6±0.5%）显著高于叶片（55.3±0.4%，p<0.0001），但竹秆的纤维含量在春季会降至79%~85%的最低水平，而此时大熊猫的竹秆取食占比已超过其总日粮的70%。竹秆的淀粉含量同样在春季达到峰值（5.5±1.1%，p=0.044），为叶片淀粉含量（2.2±0.6%）的2.5倍。春季竹秆中的游离糖以葡萄糖（35%）和果糖（47%）为主，而春季叶片的总游离糖中蔗糖占比达42%。竹秆中的结合糖主要由60%的葡萄糖与38%的木糖组成，这大概率对应半纤维素的组成特征。春季竹秆中的结合糖（半纤维素类）浓度为543.7±13.0 mg/g，显著高于叶片的373.0±14.8 mg/g（p<0.001）。上述数据有助于解答大熊猫觅食生态学领域一个长期悬而未决的问题：在能量需求极高的春季繁殖期，大熊猫为何会取食蛋白质与脂肪含量最低的植物部位？研究结果显示，大熊猫或更偏好春季竹秆——其单糖与多糖含量丰富，且因纤维含量降低而具备更高的生物可利用性。本研究表明，竹秆各部位营养成分的物候变化，是驱动大熊猫觅食决策的核心因素。