Lactation in the tammar wallaby (Macropus eugenii). II. Intake of milk components and maternal allocation of energy

The intake of milk components (total solids, carbohydrate, protein, lipid, energy) by suckling tammar wallabies (Macropus eugenii), from peak lactation to independence, was measured using a double-isotope dilution technique and chemical analysis of milk samples. The time of peak intake of milk solids (day 256 of lactation or 1126 g of offspring weight) was similar to that for whole milk. Peak intake of carbohydrate occurred earlier than this (235 days) and peak intakes of protein and lipid occurred later (262 days and 266 days, respectively). Intake of gross energy peaked at 262 days and represented a maternal yield of about 207 KJ.Kg-0.75.d-1. This is much lower than peak lactational energy yields in most other mammals, but the duration of lactation is longer in tammars than in other mammals. Total output of energy in milk by tammar mothers was 63 MJ, and this would require an intake of about 98 MJ of metabolizable energy in food. This requirement, which is equivalent to 21 MJ/kg of maternal weight, is similar to those calculated for sheep and cattle, suggesting that there are not large differences between marsupial and placental herbivores in terms of weight-related allocation of energy to reproduction. We suggest that a strategy of minimizing the peak energetic demand of lactation may be an important adaptation for a small, primarily grazing mammal due to size-related physiological constraints on elevating herbage intake and the increasing risk from predators when grazing time is increased.