The energy cost of reproduction in small rodents

Reproduction is the mechanism by which animals perpetuate their genetic contribution to future generations. Reproduction therefore has clear advantages, but it also has costs. Principal among these are elevated energy demands. Studies of energy demands in small domesticated mice have revealed that food intake increases only slightly during pregnancy, but much more dramatically during lactation. Although the increase during pregnancy are small, this may reflect competition for space in the abdomen between the alimentary tract and the developing foetal mass and intake may be limited in this situation and impose constraints on the reproductive event. During lactation energy intake increase enormously, reaching an asymptote in late lactation. Studies in wild rodents generally show the same pattern of intake between pregnancy and lactation lending hope that our work in domestic mice may provide more generally applicable insights. Studies aiming to discover the nature of the limit on intake in late lactation have been performed for at least the past 15 years. The suggestion that the limits are imposed by capacities of the alimentary tract to digest food, or the of the mammary gland to secrete milk, do not adequately explain the available data. A novel hypothesis is that the limits may be imposed by the capacity of small rodents to dissipate heat. Heat loss capacity has long been known as a constraint on lactation in large mammals. Its significance in small rodents remains uncertain, but adjustments in the level of thermogenesis from brown adipose tissue that have been classically interpreted as releasing energy to support lactation may actually reflect a reduction in obligatory heat production to avoid hyperthermia. In spite of our advances in understanding in these areas we are still far from using this knowledge to understand even simple life history trade-offs.