Armadillo pups rapidly grow a bony carapace, suggesting a large transfer of calcium and phosphorus from mother to pups via milk. Because Ca and P in milk are bound in casein protein micelles, we predicted armadillo milk to be high in protein content. Milk samples (n = 25) from 10 lactating nine-banded armadillos (Dasypus novemcinctus) collected at days 1-6, 14-15, 33-38, and 49-51 after birth were assayed for macronutrient composition (water, fat, protein, sugar, ash, Ca, and P). Gross energy (GE) was calculated from protein, sugar, and fat. Protein concentration (8-11%) was the highest of the milk solids at all time points, and ash (total mineral) values increased from 1.6% to 3.6% and were higher than sugar values (2.4%) after 1 month. Calcium concentration increased from 0.4% to over 1.0% by 1 month of lactation. Calcium and phosphorus were strongly correlated with milk protein (r = 0.796 and 0.815, respectively; P < 0.001). Protein contributed the majority of milk GE over the first 2 weeks of lactation (51%) and was equal to fat after 1 month (both about 47% of GE). The proportion of GE from sugar declined from 14.5% to 6.6%. We suggest that a milk high in protein was an ancestral trait for armadillos that allowed the evolution of a bony carapace by enabling large amounts of Ca and P to be transferred via casein micelles. Based on data from giant anteater (Myrmecophaga tridactyla) milk, a high-protein milk may be ancestral to all Xenarthrans, but this suggestion awaits confirmation from data on the milk of sloths. We suggest that armadillo pups likely catabolize a significant amount of milk protein for metabolic energy.
