Mega Mammal Milk Analysis!

Fifty years ago Devorah Ben Shaul published the seminal paper “The Composition of the Milk of Wild Animals” (1963). She had spent ten years aggregating published papers of milk composition as well as directly analyzing dozens of species’ milks. 

Haruo Takino

Eyeballing the data from 101 species, Ben Shaul posited that the composition of milks--the percent fat, protein, and sugar--did not necessarily cluster by the evolutionary history of taxonomic groups (aka phylogeny). She noted that “grizzly bear milk and kangaroo milk had virtually the same basic milk composition(pg 333).” Ben Shaul hypothesized that milk composition may instead reflect environmental pressures or nursing behavior.

Fun Fact: Manatee nipples are in their arm… er… flipper pits.

Ben Shaul posited that species’ milks clustered in relation to the degree of maturity at birth, maternal attentiveness, and nursing frequency, and the exposure to water and ambient temperature. Mammals that parked their infants and foraged for extended periods of time, such as the echidna, rabbit, and lion, produced milk with high fat content to sustain infants until the next nursing bout. High fat milk was also typical in many seals and sea lions in cold, aquatic environments because the pup’s thermoregulatory needs are high, as is the need to develop a fat blubber layer. Species produced more dilute milk when young were “precocial,” born at a relative mature condition, like ungulates, with eyes open, able to follow their mother on foraging trips from an early age, and with frequent suckling access- nursing “on demand.”

Instead of Walk & Talk, It's Strut & Suck

Liz Miller & The Baboon Films Project 

In subsequent decades, as more data both across and within species became available; two more very large reviews of milk composition were helmed by Olav Oftedal. He further considered milk composition through the lens of behavior and environment, as well as phylogeny (Oftedal, 1984; Oftedal & Iverson, 1995). Although milk composition was more likely to be similar among closely related species; behavioral care, environment, and other traits like body mass, also seemingly influenced milk synthesis. 

Mauricio Anton, extinct & extant mammals

However, systematic statistical techniques for rigorously detecting “signatures” of phylogeny, environment, and behavior in milk composition across mammals remained absent from the literature.

Until now.

In the newest issue of Journal of Animal Ecology, Amy Skibiel and colleagues combined data on mid-lactation milk composition from 129 mammalian species, representing 51 families and 15 different orders. For the first time, researchers have analyzed milk composition across egg-laying (montoreme), pouch-touting (marsupial), and placenta-having (eutherian) mammals. They looked at milk composition in relation to maternal body mass, adaptations to arid environment, maternal diet, length of lactation, altricial versus precocial young, aquatic versus terrestrial habitats, and total reproductive output (litter size & neonate size). Aw yeah.

If you don't have this poster, you're doing it wrong.

Skibiel et al. (2013) used two different statistical approaches: one that did not account for phylogeny (ordinary least squares regression) and one that did (phylogenetic generalized least squares regression and regression with an Ornstein-Uhlenbeck transformation). Yeah, at first it seemed like gibberish to me, too. Basically, the problem is that traditional regression models operate on the assumption that all data points are independent. 

BUT we know that milk synthesis has a genetic component and that closely related species will have vast similarities in their genomes. For this reason, two closely related species have a high probability of sharing many genes because they share a recent common ancestor. Regression analyses that compare species without accounting for phylogeny are violating assumptions that data points are independent. As a result, comparative analyses controlling for phylogeny are becoming increasingly important for understanding how selective pressures have shaped biological traits in animal and plant species (Nunn 2011). Phylogenetic methods & statistical analysis- FTW!

Martha Helen Barton is not impressed by gender stereotyping clipart.

Skibiel and colleagues revealed that the “phylogenetic signal” in milk is very strong, in other words- descent from a common ancestor is reflected in milk composition. In fact, the genes underlying milk synthesis change across evolutionary time at a slower rate than do other areas of the genome (Lemay et al. 2009).

painting by Charles Knight

However, after controlling for this phylogenetic signal, species that had longer periods of lactation produced more dilute milk- an extended period of lactation is seemingly not physiologically compatible with synthesizing milk with high fat and protein percentages. This makes sense; mothers would too quickly deplete their fat stores if milk composition was energetically dense and they nursed young for years (like many ape species and human populations).

orangutan mother & infant

Diet also had a strong influence on milk composition--carnivores synthesize milk higher in fat and protein (and consequently with higher dry matter content and caloric density (kcal/g)) than vegetarians or omnivores.

$5 bucks says my mom gets teary-eyed looking at this photo. 
Update: I know my mom! Too bad no one took me up on the bet.

Maternal body mass, arid environment, reproductive output (litter size and mass), and precociality were not associated with milk gross composition in this cross-mammal analysis (Skibiel et al. 2013) despite previous studies showing an association with some of these parameters within some clades, like primates (Hinde & Milligan 2011). Mammals in aquatic environments produced richer milk after controlling for phylogeny, but this correlation was no longer important once diet and lactation length were included in statistical models.

Harbor Seal photo by the awesome Alexis Bunten

Milk synthesis exists at the intersection of the other dimensions of the lactation strategy: the duration of lactation, nursing pattern, and litter size. 

Skibiel and colleagues admirably tackled duration of lactation and total reproductive output (a measure related to litter size). But each of these dimensions involves many untested nuances, including the fact that the period of time between birth and weaning can involve substantial changes in milk composition across time as young transition from colostrum to exclusively milk feeding, to a period of mixed milk and solids feeding (Langer 2008; Trott et al. 2002). Patterns of nursing--the frequency and duration of nursing bouts--can complicate milk throughput in the mammary gland, affecting milk composition (Miller et al. 2013). And litter size doesn’t account for the ratio of sons and daughters in the litter despite a growing literature suggesting that infant sex may influence milk synthesis. 

And the milk synthesis dimension of the lactation strategy is the combination of milk composition and milk volume- the quality and quantity of milk aka the how good and how much. Importantly, the Skibiel and colleagues only investigated milk composition, but not milk yield. Total milk synthesis is going to be a very critical component in understanding the effects of phylogeny, environment, and behavior (Riek 2008).

But the complications don’t end there. How mothers “pay” the costs of lactation may also have important interactions with milk synthesis. For most mammals, synthesizing milk reflects differential reliance on “capital” (body stores) versus “income” (dietary intake) tactics. This varies across species, across individuals within species, and across time within an individual (Hinde et al. 2009; Jönsson 1997). Also, among mammals young have to use calories and building blocks ingested via mother's milk for growth, development, and behavioral activity to varying amounts (also varying across species, individuals, & time). This means that not only should nursing behavior be considered, but also how infants are assimilating and utilizing the milk they ingest from their mother (Hinde 2013).  

Natural selection has shaped adaptations for milk synthesis in the mother as well as adaptations to extract and use milk in the infant. And these adaptations can be in conflict.

Rhesus Photo by Kathy West, Dancing Darwins by Me!

Yeah, it’s a complex system. And for most of these parameters, reliable data are not yet available even for the mammals whose milk composition we know, much less the thousands of other mammals whose milk composition we still haven’t analyzed. Oh, and this is just talking about gross composition—the total percent fat, protein, and sugar—and doesn’t even include description of the specific fatty acids and amino acids, or disaccharides and oligosaccharides, or the hundreds, possibly thousands, of other bioactive constituents in mammalian milk: hormones, minerals, immunofactors, etc. etc. etc. ad nauseum.

Long story short, just like theoretical physicists, we lactation biologists still do not have a MILK “Theory of Everything” because the mega-meta-analysis integrating all key data still does not exist.

Vassily Vassilyevich Kandinsky, Composition VII, 1913

Fun Fact: according to Kandinsky, 

it’s the most complex piece he ever painted.

But even though we don’t yet have the data available for the ONE ANALYSIS TO RULE THEM ALL! we can be more confident about the things that we think we know. Milk reflects the immediate conditions of the mother and the environment as well as the conditions experienced by ancestors in the past. Species can have milk of the same composition as a result of different selective pressures. Conversely, species living in the same environment, with the same diet and same length of lactation, may still have very different milks because they evolved from different recent ancestors. Similarly, populations of a species may have recently expanded into a new environment, possibly leading to consumption of new types of food or tangling with new predators that require different vigilance and interactions with their young, all of which are likely to have important consequences for milk.

In this way, milk synthesis reflects both the “here and now” AND the “there and then.”

Modern Morgan horse & earliest known horse Sifrhippus sandrae from the beginning of the Eocene, ~55.8 million years ago. 

Image: Danielle Byerley, Florida Museum of Natural History

An earlier version of this post is featured in this month's SPLASH! Milk Science Update. Check out the many awesome articles about milk you'll find there!

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Literature Cited

Ben Shaul DM. 1963. The composition of the milk of wild animals. Int. Zoo. Yearb. 4, 333-42.

Hinde, K. (2013). Lactational programming of infant behavioral phenotype. In Building Babies (pp. 187-207). Springer New York.

Hinde, K., & Milligan, L. A. (2011). Primate milk: proximate mechanisms and ultimate perspectives. Evolutionary Anthropology: Issues, News, and Reviews, 20(1), 9-23.

Hinde, K., Power, M. L., & Oftedal, O. T. (2009). Rhesus macaque milk: magnitude, sources, and consequences of individual variation over lactation. American journal of physical anthropology, 138(2), 148-157.

Jönsson, K. I. (1997). Capital and income breeding as alternative tactics of resource use in reproduction. Oikos, 57-66.

Koskela, E., Mappes, T., Niskanen, T., & Rutkowska, J. (2009). Maternal investment in relation to sex ratio and offspring number in a small mammal–a case for Trivers and Willard theory?. Journal of Animal Ecology, 78(5), 1007-1014.

Langer, P. (2008). The phases of maternal investment in eutherian mammals. Zoology, 111(2), 148-162.

Lemay, D. G., Lynn, D. J., Martin, W. F., Neville, M. C., Casey, T. M., Rincon, G et al. (2009). The bovine lactation genome: insights into the evolution of mammalian milk. Genome biology, 10(4), R43.

Miller, E. M., Aiello, M. O., Fujita, M., Hinde, K., Milligan, L., & Quinn, E. A. (2013). Field and laboratory methods in human milk research. American Journal of Human Biology, 25(1), 1-11.

Nunn, CL. ( 2011). The Comparative Approach in Evolutionary Anthropology and Biology.  University of Chicago Press.

Oftedal, O. T. (1984). Milk composition, milk yield and energy output at peak lactation: a comparative review. In Symp. Zool. Soc. Lond (Vol. 51, pp. 33-85).

Oftedal, O. T., & Iverson, S. J. (1995). Comparative analysis of nonhuman milks. A. Phylogenetic variation in the gross composition of milks. Handbook of milk composition, 749-788.

Riek, A. (2008). Relationship between milk energy intake and growth rate in suckling mammalian young at peak lactation: an updated meta‐analysis. Journal of Zoology, 274(2), 160-170.

Skibiel, A. L., Downing, L. M., Orr, T. J., & Hood, W. R. (2013). The evolution of the nutrient composition of mammalian milks. Journal of Animal Ecology. 82: 1254–1264. doi: 10.1111/1365-2656.12095

Trott, J. F., Simpson, K. J., Moyle, R. L., Hearn, C. M., Shaw, G., Nicholas, K. R., & Renfree, M. B. (2003). Maternal regulation of milk composition, milk production, and pouch young development during lactation in the tammar wallaby (Macropus eugenii). Biology of reproduction, 68(3), 929-936.