Trade-offs in Energy Allocation During Lactation1

SYNOPSIS. During lactation, mothers require energy to meet both mater-nal and offspring requirements. If a mother exports too much energy to dependent offspring (in milk), her weight loss may be excessive and ma-ternal risk may increase. Conversely, too little energy allocation to off-spring may reduce the growth rate or induce mortality of dependent off-spring. This paradigm was evaluated in cotton rats (Sigmodon hispidus) supporting small (3 pup) and large (6 pup) litters from early to late lac-tation. Several types of evidence indicate that physiological constraints limit the ability of mothers with large litters to provide resources to off-spring. Mothers with large litters produced a dilute, energy-poor milk and their rates of food intake, weight loss and energy export per litter appeared to approach physiological maxima. Whereas the energy exported to pups in small litters increased from early to late lactation, the energy flow per pup in large litters was consistently low; consequently, offspring in large litters had low growth rates. An increase in either maternal food intak