Milk Consumption and Energetics of Growth in Pouch Young of the Tammar Wallaby, Macropus-Eugenii

Abstract

Milk consumption rates of young tammar wallabies, Macropus eugenii, were measured during the first 32 weeks of lactation by means of 22*Na turnover. Milk consumption rates were low during the first 18 weeks or so of lactation, rising from 1.2 to 9.0 ml d-'. Milk consumption rose to 43 ml d-' at 30 weeks post-partum. Changes in milk consumption were paralleled by increases in the size of mammary glands and weight gain by the sucklings. The mass gain for each millilitre of milk consumed (crude growth efficiency) averaged 0.23 g ml-I during the first 26 weeks of lactation, but then increased to more than 0.35 g ml-'. The increased growth efficiency was associated with massive increases in the lipid and energy content of milk from 26 weeks onwards. The conversion efficiencies of milk protein and energy to body materials were intermediate (55-87%, 23-36%, respectively) to those of other mammals, which suggests that the slow growth of pouch young during the first 17 weeks of lactation is the result of restricted milk production by the mother. Milk production and the growth of young were closely correlated with maternal mass (r=0.96; r=0.80, respectively) at 16 weeks post-partum.

Milk Composition during Lactation Suggests a Mechanism for Male Biased Allocation of Maternal Resources in the Tammar Wallaby (Macropus eugenii)

“…These data are consistent with studies on Red deer [13], [14], Rhesus Macaques [15], [16], humans [36], [37] and the Trivers-Willard Model [5] that provides support for a mechanism by which wallabies with greater investment ability can allocate resources in favour of sons [21]. Lactation quantity and composition in macropods follows an endogenous maternal program independent of the pouch young sucking stimulus [26], [27] or the size of current pouch young when cross-fostered [38]. Therefore investment ability must depend on a pre-partum measure of condition [21].…”

“…Therefore investment ability must depend on a pre-partum measure of condition [21]. Green et al [26] found that the conversion of protein and milk energy to body materials in tammar wallabies is intermediate compared with other mammals, suggesting that slow growth rates in the early stages of development may be due to restricted milk production by mothers. Mothers investing in sons are generally in a condition when the lactation program is set, that is indicative of high investment capacity.…”

“…This ability to re-activate a stored blastocyst before the winter solstice allows for early termination of a females current investment early in the season and reinvestment in a new pouch young with minimal cost to the mother. A great deal is known about the composition of milk and development of pouch young in tammar wallabies [26], [27]. The composition of milk changes dramatically throughout lactation with four phases recognised (phase 1: pre partum ∼26 day gestation, phase 2A: 0–100 days post partum, phase 2B: 100–215 days post partum and phase 3: 215–360 days post partum).…”

Digestive plasticity of the small intestine and the fermentative hindgut in a marsupial herbivore, the tammar wallaby (Macropus eugenii)

“…Notably, two females in the natural forage treatment carried very small pouch-young (< 3 g) by the end of the experiment, but these young were each less than 20 days old and would have had a negligible impact on the mother's food requirements (see Green et al 1988) and were therefore unlikely to have affected digestive tract morphology.…”

Biological and Environmental Drivers of Energy Allocation in a Dependent Mammal, the Antarctic Fur Seal Pup

“…California sea lion (Zalophus californianus Lesson) pups exhibited a similar decrease in energy available for growth between their first and second months of life, likely for similar reasons . In contrast, some species showed an increase in the proportion of energy available for growth with age (Green et al 1988;Merchant et al 1996; table 2). If the current study had extended to the period after pups completed molting, when pups are larger and have an adult coat, pups might have allocated more energy toward growth/storage, as seen in other species (Merchant et al 1996;Donohue et al 2002).…”