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.