Digestive and metabolic flexibility allows female degus to cope with lactation costs

Lactation is the most energetically demanding period in the life cycle of female mammals, and its effects on digestive flexibility and the size of internal organs have been extensively studied in laboratory mice and rats since the early 1900s. However, there have been only two studies on this topic for wild rodent species. Here, we analyzed digestive flexibility--that is, changes in gut content, activity of digestive enzymes, and gut morphology--during lactation in the caviomorph rodent Octodon degus. In addition, we evaluated changes in the size of other internal organs and analyzed their relationship with the resting metabolic rate. We found that gut content, the dry masses of digestive chambers, the dry mass of liver, and resting metabolic rate were greater in lactating than in nonbreeding control females. In contrast, fat stores were higher in control subjects. Maltase and aminopeptidase-N specific activity did not change with lactation, and both enzymes had greater activity values in the middle portion of the small intestine. Thus, our data indicate that the previously reported increase in food assimilation that occurs during lactation in O. degus is related to a mass increase in several central organs, leading, in turn, to higher energetic costs. Fat stores may help to mitigate these costs, but, as expected for small animals, to a limited extent. Our study reveals a complex interplay among energy acquisition, storage, and expenditure processes that ultimately determine an organism's fitness.