Adaptation, phylogeny, and covariance in milk macronutrient composition

BACKGROUND: Milk is a complicated chemical mixture often studied through macronutrient concentrations of fat, protein, and sugar. There is a long-standing natural history tradition describing interspecific diversity in these concentrations. However, recent work has shown little influence of ecological or life history variables on them, aside from maternal diet effects, along with a strong phylogenetic signal. METHODS: I used multivariate phylogenetic comparative methods to revisit the ecological and life history correlates of milk macronutrient composition and elaborate on the nature of the phylogenetic signal using the phylogenetic mixed model. I also identified clades with distinctive milks through nonparametric tests (KSI) and PhylogeneticEM evolutionary modeling. RESULTS: In addition to the previously reported diet effects, I found increasingly aquatic mammals have milk that this is lower in sugar and higher in fat. Phylogenteic heritabilities for each concentration were high and phylogenetic correlations were moderate to strong indicating coevolution among the concentrations. Primates and pinnipeds had the most outstanding milks according to KSI and PhylogeneticEM, with perissodactyls and marsupials as other noteworthy clades with distinct selection regimes. DISCUSSION: Mammalian milks are diverse but often characteristic of certain higher taxa. This complicates identifying the ecological and life history correlates of milk composition using common phylogenetic comparative methods because those traits are also conservative and clade-specific. Novel methods, careful assessment of data quality and hypotheses, and a “phylogenetic natural history” perspective provide alternatives to these traditional tools.