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 densit...