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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.