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We collected milk samples from two rodents, Kerodon rupestris and Acomys cahirinus, that live in arid habitats and produce precocial young, in order to determine milk composition throughout the lactation period. Milk from species producing precocial young has been predicted to be low in energy and protein content, while milk from arid-dwelling species has been predicted to be high in total solids. Milk samples were analyzed for total fat, protein, and sugar content. Milk from Acomys was much higher in energy content than Kerodon milk, exhibiting higher concentrations of total solids (41% vs. 20%), fat (21% vs. 6.9%), and protein (12% vs. 66%); however, sugar concentration was lower in Acomys milk (2. 9% vs. 5.3%). Milk composition varied throughout the lactation period in both species, with protein levels increasing and fat levels decreasing in Kerodon and sugar levels decreasing and total solids and fat levels increasing in Acomys. The milk of Kerodon females exhibited a threefold variation in fat content, while milk from Acomys females showed a twofold variation in protein content. The results of this study lend some support to the idea that mammals inhabiting arid environments conserve water by producing milk with a high proportion of solids. Furthermore, these results do not support the hypothesis that milk from rodents with precocial young is low in energy content but instead indicate that precocial species produce milk that is high in total solids and fat.

Changes in diet quality may be as important as changes in energy availability in the reproductive success of small mammals. The effect of dietary protein on fitness of females is most likely mediated by changes in yield or composition of milk. Mus musculus was used as a model to test whether dietary protein restriction leads to changes in milk composition. Primiparous mice were fed a 10% or 20% casein-based diet through 2 consecutive reproductive attempts. There was no effect of diet on food consumption or litter size. However, a reduction in dietary protein led to the production of milk with a 12% decrease in protein content and decreased both prenatal and postnatal growth of young. The impact of low levels of dietary protein on milk protein was greater with the increased reproductive demands of concurrent pregnancy and lactation. Significant variation in the protein content of milk was seen among females on the 20% casein-based diet, but not among females on the 10% casein-based diet. Further work is needed to determine the physiological mechanisms that lactating mice might possess to compensate for changes in dietary protein levels, and to determine if the effect of decreased diet quality on the reproductive success of small mammals is similar to the effect of energy availability.