Maternal care in house mice (Mus musculus): II. The energy cost of lactation as a function of litter size

Abstract

In mammals, lactation performance may influence future reproduction of a mother and her young. We measured the quantity and quality of milk produced by female house mice (Mus musculus) during a period of 28 days after birth of a litter. We aimed to analyse how females cope with the energy demands of different sized litters as a tradeoff between current and future reproduction. Litter sizes examined were small (six young), intermediate (7.3 0–7), and large (12 young). Females met the energy demand of a growing litter both by increasing the amount of milk given to the young and by improving the quality (through an increase in total solid and fat concentrations) to a peak during days 9–16. At the onset of weaning (day 17), milk production decreased and young shifted to solid food. The main energy source in the milk of house mice was fat, which provided more than 80% of the energy to the suckling young. Average lipid concentrations were 20%, and peak values of 33% were reached during days 13–16. Protein concentrations were 6–8% and carbohydrate (lactose) concentrations 3–4%. With larger litters, females increased both the amount of milk and the absolute amount of major nutrients (lipids, proteins, lactose). However, regulation was imperfect. When litter size was doubled from six to 12, amount of milk produced rose by 44%, and energy content only rose by 30%. The reduction in milk supply for individual young in larger litters was reflected in slower growth and lower weaning weight. The efficiency of conversion of milk energy into biomass of young was highest for intermediate litters (51%; for small litters, 43%; for large ones, 36%). During days 5–16, a single pup in an intermediate litter needed less energy for metabolism and growth than pups in the other litter sizes examined. Although individual young of small litters have a relatively high weaning weight‐which might improve their future reproduction‐females gain higher reproductive success by dividing the energy available per litter between the largest number of young they can raise to a weaning weight of on average 9 g. For the time‐lifen reproductive success of a female house mouse‐at least for the strain used in this study‐an intermediate litter size of seven seems to be best because of a favourable ratio of energy cost of lactation to number and size of young produced.

Does kin selection influence fostering behaviour in Antarctic fur seals (Arctocephalus gazella)?

“…The evolution of mammalian fostering behaviour, where females provide parental care for non-filial young, is particularly challenging to explain because lactation imposes physiological stress on mothers that may reduce their survival and future reproductive success (Konig et al 1988;Clutton-Brock et al 1989;Iverson et al 1993). Furthermore, even if milk production did not entail any fitness penalty, nursing directed towards unrelated offspring should be selected against because it would only serve to increase the fitness of competing individuals in the population.…”

Living together, feeding apart: How to measure individual food consumption in social house mice

“…Milk production is costly not only in terms of energy, but also in terms of decreased future reproduction (Konig, Riester, & Markl, 1988). With increased litter size and correspondingly increased milk production, the birth of the female's next litter is delayed.…”

Lactation, maternal behavior and infant growth in common marmoset monkeys ( Callithrix jacchus ): effects of maternal size and litter size

“…Optimal lactation performance and maximal neonatal growth for a species generally occurs at a modal litter size typical for feral representatives of that species, with poorer growth and survival in both litters that are unusually large (Mattingly and McClure 1982;Thompson 1983;Knight et al 1986;Konig et al 1988;Leamy 1992;Arnbom et al 1997) or unusually small (Knight et al 1986;Hammond et al 1996). Given these findings, we propose that marmosets will be adapted to produce a maximal individual growth rate in twins.…”