The phases of maternal investment in eutherian mammals

This investigation presents a comparative data set that characterizes the maternal investment period of eutherian mammals, which starts at conception and ends when the offspring becomes nutritionally independent. I discuss the possible subdivision of the maternal investment period into three phases. Special consideration is given to lactation that can be divided into a first phase when the offspring ingests exclusively milk and a second phase when both milk and solid food are taken. Data on reproductive biology, on pre- and postnatal nutritional characters (composition of milk and character of adult food), on the status of maturity at birth, and other life history traits, were included in the analysis. Effects of phylogeny on life history traits were accounted for by calculating phylogenetically independent contrasts. No relationship between body mass and the content of nutrients and ash in milk and food quality could be found. The absolute and relative length of the milk-only phase and any of the other characters was not related to milk and food quality. Large litters of altricial (i.e., nidicolous) young are born after a short gestation time, while birth of precocial (i.e., nidifugous) young follows a longer gestation time. In the nutritionally strained postnatal situation the mother tends to shorten the milk-only phase. I suggest replacing the traditional subdivision of the maternal investment period into gestation and lactation phase by the above-mentioned three phases. Also, I suggest that description of the weaning period as "maternal-offspring conflict" does not fully grasp the relationship between generations. An alternative interpretation suggests that the nutritional and thus also reproductive, effort of the mother might be "subsidized" by her offspring during the mixed feeding phase, when the young animal supplies solid food itself and relieves her of part of the nutritional load.