How Lactation Duration and Maternal Fasting Affect Mammalian Milk Composition
All mammals share milk-making, demonstrating an evolutionary need to adapt to different environments and survival pressures. We’ve shared a bit about human milk by reviewing macronutrients in milk: protein, carbohydrates, and fat. Now, we’re looking into how and why milk varies so much among mammals.
There are substantial variations in protein, fat, carbohydrates, and total energy among mammalian species. Human milk comprises of about 87% water, 3.8% fat, 7% carbohydrates, and 1% protein . But, fat can make up as little as 0.2% in black rhinoceros and as much as 60% in some species of seals . Carbohydrates are virtually nonexistent in some pinnipeds (e.g. seals, walruses) but can account for up to 11% in some Diprotodont marsupials (e.g. kangaroos, wallabies) . Protein can be less than 1% in some primate species and almost 16% in eastern cottontail rabbits . This variation is due to many factors such as duration of lactation, environment, nutritive needs of young, maternal diet, immunologic needs of baby, body mass and metabolic needs of mom and baby, and life expectancy of each species. Researchers are only just starting to discover the complexity of the diversity of mammalian milk.
Duration of Lactation
Lactation duration affects milk composition more than you might think. Shorter duration of lactation increases the calorie density in the milk in order to get babies ready to be independent — milk from mammals who feed their young briefly or infrequently must have higher fat and protein content than other mammals. Hooded seals only lactate for 4 days and in that time, their pups double in size . This is because fat makes up a whopping 61% of their milk. Mama hooded seals mobilize 25% of their body fat to produce milk that helps their young quickly develop a thick layer of subcutaneous fat that allows them to survive in the arctic environment [5, 3]. Similarly, milk from echidnas, fur seals, and sea lions contain very high levels of fat because they only nurse their young every 3-12 days . High-fat milk is necessary to maintain proper growth when these infants are only fed sporadically. Plus, shorter lengths of lactation reduce the burden on the mother’s body, allowing her to focus on her own survival instead of also that of her baby’s .
Baby bats must be able to fly to find food, shelter, and safety on their own. So, baby bats keep nursing until they’ve gained enough weight and strength to be fully independent. Bats must continue to nurse until their pups have deposited enough protein and minerals into their muscles and bones to be strong enough to withstand the pressure and torque associated with flying. When bats wean at 3-4 weeks of age, they are already 70% of their adult body mass compared to other terrestrial mammals who wean when they reach 37% of their adult body mass . That means, bats need to gain nearly twice as much body mass while they breastfeed compared to other terrestrial mammals!
On the other hand, mammals who nurse their young for long periods of time often have milk that is lower in fat and protein and their young tend to grow more slowly. Orangutan milk has less fat and slightly less protein than human milk and milk of other apes . Orangutans nurse their young for a whopping 5.5-8 years, which can be up to 20% of their lives ! Researchers are able to tell how long orangutans have nursed by analyzing the barium in their teeth, which can give information about their primary sources of nutrition at various life stages . It is hypothesized that orangutans nurse for so long because it helps them learn social behavior and how to live and survive in an environment with unreliable food resources . This length of lactation has given orangutans an evolutionary advantage as their young are able to continue surviving on milk in times of food scarcity.
The lifestyles and habitats of various species also affect milk composition. Both blue whales and bears depend on energy stored in their bodies to produce milk for their young. When a blue whale is pregnant, she feeds in polar and cold waters where food is abundant, allowing her to store energy in her blubber. When she gives birth in warmer areas, she fasts or eats little but is able to survive and produce an impressive 220kg of milk per day (965,000 kcal!) by using the energy stores from her blubber . By the end of their 6-month lactation journey, mama blue whales provided 167,300,000 kcal to their pup, which is equivalent to feeding 200 humans for an entire year!
Bears also rely on mobilizing their extensive body reserves during lactation, which they accumulate in preparation for hibernation . Bear milk is low in carbohydrates which allows them to minimize the glucose demand of the mammary glands and conserves the bear’s limited energy. Their milk also has lower water content, less protein, and higher fat than other omnivores. It is hypothesized that this is due to the need of the lactating bear to conserve water and protein while fasting during hibernation . They lactate for 2 months in their winter dens before emerging to feed or drink, meaning they maintain their survival and that of their young without leaving the house.
The variability of macronutrient composition of milk across mammals is attributable to the unique evolutionary and life histories of these different species. Understanding lactation from an evolutionary perspective is of great interest to scientists and work is ongoing to help us better understand how species relatedness (phylogeny) influences lactation. We hope you will join us as we ruminate on how maternal diet shapes milk composition in the future.
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