Trade-offs in Energy Allocation During Lactation

SYNOPSIS. During lactation, mothers require energy to meet both maternal and offspring requirements. If a mother exports too much energy to dependent offspring (in milk), her weight loss may be excessive and maternal risk may increase. Conversely, too little energy allocation to offspring may reduce the growth rate or induce mortality of dependent offspring. This paradigm was evaluated in cotton rats (Sigmodon hispidus) supporting small (3 pup) and large (6 pup) litters from early to late lactation. Several types of evidence indicate that physiological constraints limit the ability of mothers with large litters to provide resources to offspring. 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 intake or weight loss (catabolism of maternal tissue) could have provided additional energy to offset the low growth rate of pups in large litters. However, mothers with large litters did not substantially increase their food intake or weight loss compared with mothers supporting small litters. These results indicate that the maternal support of offspring in large litters is limited. The pattern of energy allocation shown by cotton rats with large litters likely reflects a compromise between meeting maternal and offspring energy requirements (cf., Parker and Macnair, 1979). The energy flow is greater than optimal for the parent but less than optimal for the offspring. Less maternal-offspring conflict occurs in small than large litters because offspring in small litters maintain a high growth rate at a relatively low maternal cost. Yet, under favorable environmental conditions, the reduction in maternal-offspring conflict has no apparent fitness benefit.