Use of maternal reserves as a lactation strategy in large mammals

Abstract

The substrate demands of lactation must be met by increased dietary intake or by mobilization of nutrients from tissues. The capacity of animals to rely on stored nutrients depends to a large extent on body size; large animals have greater stores, relative to the demands of lactation, than do small animals. The substrate demands of lactation depend on the composition and amount of milk produced. Animals that fast or feed little during lactation are expected to produce milks low in sugar but high in fat, in order to minimize needs for gluconeogenesis while sustaining energy transfers to the young. The patterns of nutrient transfer are reviewed for four taxonomic groups that fast during part of or throughout lactation: sea lions and fur seals (Carnivora: Otariidae), bears (Carnivora: Ursidae), true seals (Carnivora: Phocidae) and baleen whales (Cetacea: Mysticeti). All these groups produce low-sugar high-fat milks, although the length of lactation, rate of milk production and growth of the young are variable. Milk protein concentrations also tend to be low, if considered in relation to milk energy content. Maternal reserves are heavily exploited for milk production in these taxa. The amounts of lipid transferred to the young represent about one-fifth to one-third of maternal lipid stores; the relative amount of the gross energy of the body transferred in the milk is similar. Some seals and bears also transfer up to 16-18 % of the maternal body protein via milk. Reliance on maternal reserves has allowed some large mammals to give birth and lactate at sites and times far removed from food resources. Lactation involves the mobilization and export of substantial quantities of nutrients in milk, placing large nutrient demands on the mother. If she is unable to meet these added demands by dietary intake, she must reduce milk output, mobilize nutrients from maternal tissues or employ some combination thereof. However, reduction in milk output may compromise survival of the young. In nature, lactation is often timed to periods of food abundance so that the mother can match dietary intake to lactational needs, and does not have to choose between her own future and that of her offspring.An alternative approach is to disengage lactation from maternal food intake by storing nutrients in tissues during periods of abundance and then mobilizing these nutrients into milk at a later date. Such an approach is only possible if the types and amounts of nutrients stored in tissues are sufficient to cover the demands of lactation. The sufficiency of nutrient storage is largely governed by the allometry of energy expenditure (Oftedal, 1993): since small animals expend energy at a much higher rate (relative to body mass) than large animals, small animals are physically unable to store sufficient energy to cover expenditures for any length of time.A simple calculation for 4 g shrews illustrates this point. If a shrew deposits body fat equivalent to 250 mg/g body mass (M), the energy content of the fat stores will be about 39 kJ, while lea...

The evolution of milk secretion and its ancient origins

“…The ability to lactate allows mothers to collect nutrients and deposit them in their tissues at one time and location, but then provide these nutrients to their young as milk at a later time and/or another location, such as in a nest or burrow (rodents), cave (bats, bears) or snow den (polar bear) or on an isolated island (sea lion, fur seal) or on floating rafts of pack ice (various seals, walrus;Oftedal, 2000). Some bears lactate in the winter den for 2 months before emerging to feed and drink, a feat made possible by mobilization of extensive body reserves, production of milk low in sugar (minimizing the glucose demand of the mammary glands), and recycling of water and perhaps nitrogen from cubs back to the mother when she ingests their excreta (Oftedal et al, 1993b).…”

A teleonomic model describing performance (body, milk and intake) during growth and over repeated reproductive cycles throughout the lifespan of dairy cattle. 1. Trajectories of life function priorities and genetic scaling

“…The balancing of body reserves is also inhibited at the onset of lactation, consistently with the fact that anabolism of body tissues is known to be hindered in early lactation (Vernon and Flint, 1984;Chilliard and Robelin, 1985;McNamara, 1997). Mobilization of X at parturition contributes to the supply of energy for milk secretion dedicated to the survival of the neonate, which is the main feature in mammals (Pond, 1984;Oftedal, 2000). Moreover, mobilization is assumed to be proportional to the level of body reserves (Broster and Broster, 1998;Martin and Sauvant, 2002).…”

“…Since U 1 N 1 S 5 0 during the non-reproductive stages of life, priority R 5 1 2 (G 1 A), quantifies the level of potential investment in reproduction, that is, a 'quantity of priority' potentially conveyable from R to U, which increases during development and decreases in older animals. The key role of R as a source for priority to reproduction in the model reflects the crucial importance of energy storage in body tissues in reproduction of mammals (Pond, 1984;Oftedal, 2000). As seen in Figure 2, priority R oscillates during the reproductive stages of life defining 'opportunity peaks' for reconstitution of body reserves between reproductive cycles.…”

Understanding mammary activity in red-rumped agouti and implications for management and conservation of this Neotropical game species

“…The length of lactation in mammals is influenced by the quality and quantity of milk produced, body size, and age of the mother (Oftedal, 2000). The mammal's ability to produce milk is determined by the level of activity and number of secreting cells (Hughes, 1950;Kim et al, 1999).…”