Data from: Trade-offs in lactation and milk intake by competing siblings in a fluctuating environment

Income breeders which forage without their offspring in attendance should adjust nursing frequency and the quantity and composition of milk to prevailing ecological conditions, and increase the quantity or quality of milk transferred if the frequency of nursing visits declines. When milk delivery to a litter is insufficient, sibling competition should skew milk consumption in favor of dominant litter members. We quantified milk nutritional composition and gross energy density, nursing bout durations, per capita milk transfer (kg), and milk transfer rate (g/min) per nursing bout in singletons and twins in a social carnivore, the spotted hyena. In our study population, the energetic cost of lactation is rank dependent and particularly high because large fluctuations in local prey abundance necessitate long-distance foraging, reducing nursing frequency and intensifying sibling competition, more frequently for low-ranking than high-ranking females. Females increased milk gross energy density, driven by fat content, and increased nursing bout durations as nursing frequency declined. Milk transfer increased as social status declined. Females below median rank also increased milk transfer as nursing frequency declined. Milk transfer was independent of nursing frequency in females above median rank and higher for singletons than cubs in twin litters. Dominant siblings achieved significantly higher amounts and rates of milk transfer than their paired subordinate littermates; this bias increased as sibling competition intensified. Our results show how the interplay between ecological and social constraints determines the quality and quantity of milk transferred, and how socially dominant siblings use aggression to skew milk intake in their favor.那些在不携带幼崽随行的情况下外出觅食的收益型繁殖者（income breeder），应根据当前的生态条件调整哺乳频率、乳汁分泌量及乳汁成分；若哺乳探访频率下降，则需提升乳汁的分泌量或品质。当同窝幼崽获得的乳汁供应不足时，同胞间的竞争会使乳汁消费偏向于处于优势地位的同窝幼崽。我们针对群居食肉动物斑鬣狗，对单胎和双胎幼崽的乳汁营养成分、总能量密度、哺乳回合时长、每头幼崽的乳汁转移量（kg）以及每次哺乳回合的乳汁转移速率（g/min）进行了定量测定。在本研究种群中，哺乳的能量消耗存在等级依赖性且尤为高昂：当地猎物丰度的大幅波动迫使雌性需要长距离觅食，这会降低哺乳频率并加剧同胞间的竞争，且该情况在低等级雌性身上比高等级雌性更为常见。随着哺乳频率下降，雌性会提升乳汁的总能量密度（该变化由脂肪含量升高所驱动）并延长哺乳回合时长。乳汁转移量随社会等级降低而升高。等级低于种群中位数的雌性，其乳汁转移量也会随哺乳频率下降而提升。等级高于种群中位数的雌性，其乳汁转移量与哺乳频率无关；且单胎幼崽获得的乳汁转移量高于双胎同窝幼崽。优势同胞获得的乳汁转移量及转移速率均显著高于其配对的从属同窝幼崽，且这一偏向性会随同胞竞争加剧而增强。我们的研究结果揭示了生态与社会限制因素的交互作用如何决定乳汁转移的品质与数量，以及社会优势同胞如何通过攻击行为使乳汁摄入向自身倾斜。