Orca milk smells dizzyingly fishy. Seal milk has a rich orange hue. Reindeer milk, perhaps fittingly, is as thick as eggnog. Not that I am tempted to try it, or any of the other unusual milks I can see stacked on shelves from floor to ceiling. I have donned a puffy winter jacket and stepped inside the freezer that houses the largest collection of animal milk in the world, containing that of everything from tree shrews to two-toed sloths and giant anteaters.
The collection, housed at Smithsonian’s National Zoo in Washington DC, is more than a cabinet of curiosities – it is a critical resource for staff at this zoo and others around the world tasked with feeding orphaned infants. By studying all this white – and not-so-white – stuff, Smithsonian scientists can create custom infant formulas that will give the animals in their care the best possible start in life.
As their understanding of milk has improved, however, they have realised that their formulas are missing an important component: microbes. Now, as they explore the microbial diversity contained in different milks and the benefits these organisms bring, they are striving to replicate this in lab-made milk – not only to better assist young animals in the zoo, but also to aid the survival of some of the rarest species in the wild.
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“The objective isn’t necessarily to freeze milk, archive it and not touch it,” says , who leads the Smithsonian’s cryopreservation initiative. “It’s to use it to understand and sustain biodiversity.”
Milk is the essential first food for mammals. From mice and marmosets to humans, mammals are born incapable of feeding themselves. By producing milk, mothers can give their offspring the key nutrients they need to develop and grow.
Evolutionary adaptations
Here at the milk bank, researchers have collected over 16,000 samples from more than 200 species – around 3 per cent of the roughly 6000 mammal species in the world. New samples are constantly being added, many from animals kept at the Smithsonian zoo, but also from other zoos and conservation organisations. Some animals can be milked by hand – including a , trained to be milked using a handheld pump – while other samples are collected when an animal is under anaesthesia for a medical procedure. “As long as the sample is good quality and taken ethically, we’re not picky – we accept everything,” says Comizzoli.
The result is a collection that illustrates the diversity of mammalian milk. Looking at the colours and consistencies of the samples in the freezer, it is striking just how different they are – the milk of each species is the result of millions of years of evolutionary adaptations.
Take howler monkeys, which have milk that is around 1 per cent fat. “If you nurse your baby frequently, which primates do, you can afford to have a fairly dilute milk,” says , curator of the milk repository. Tree shrews, by contrast, produce milk that is around one-third fat and “oozes like butter”, says Power, because they only visit their offspring for 10 minutes every two days to nurse them. This strategy avoids drawing predators’ attention to the nest of infants, but it means mothers need to pack lots of fat – and hence energy – into a small amount of milk.
Understanding these differences is vital when it comes to looking after orphaned infants or those that aren’t thriving. This is where the milk bank comes in. When screaming hairy armadillo pup Sherman wasn’t growing after his mother stopped producing milk, for example, zookeepers knew the few commercially available infant formulas, for common species such as dogs, cats, sheep and cows, wouldn’t be right for him. Instead, they turned to the milk bank for a custom formula.
The nutritionists and vets at the bank usually begin by thawing one of the frozen milk samples, some of which date back decades. They then separate each sample – which can be as small as 5 millilitres – into its components like fat, protein and sugar. They also look at minerals in the milk, mainly calcium and phosphorus.
Their analyses of armadillo milk revealed it has an “astronomical level of calcium”, needed to build the animals’ bony carapaces, says Smithsonian clinical nutritionist . It also has a higher-than-usual level of protein, to increase calcium absorption.
Once the researchers know what they are aiming for, they turn to the commercial formulas, tinkering with them until they create the closest match. The whole process can take a matter of days and, in the case of Sherman, it did the trick. When I stopped by the zoo’s small mammal house to see the now adult armadillo, he was blissfully scavenging for mealworms.
Milk microbiome
There are occasional mishaps, however, says lab technician . When mixing seal milk, an intern once stirred it for too long and “accidentally made seal butter” and had to start again. On the day I was visiting, she was in the process of separating casein proteins from tapir milk. The result might best be described as tapir cheese, with a ricotta-like consistency.
But milk is more than just the sum of its proteins and minerals – it also contains bacteria. “Every animal has a microbiome, so every animal that produces the milk is going to have microbes in their milk,” says , a microbial ecologist at the zoo.
To find out more about this microbiome, Bornbusch, Power and their colleagues looked at the milks of 47 mammal species, from giraffes to wolves, and compared the microbes they harboured. They found that , but diet and habitat also played a role. “What that implies to me is that the microbes that get into that milk are not just random,” says Power. Indeed, they play many important roles, including building an infant’s immune defences. “Those incoming microbes from mom’s milk start to train the infant’s immune system by teaching it what to tolerate in terms of bacteria, and [what] not to tolerate,” says Bornbusch.
But she says it is too soon to start adding specific microbes to batches of formula, as we don’t want to accidentally introduce harmful bacteria. In cases where an infant’s gut needs to be seeded with microbes, the researchers whip up what they call a “poop milkshake” – blending its infant formula with its mother’s faeces. “We try and get all of those microbes that mom has into the baby to help spark their microbiome,” says Bornbusch.
With orphans, however, the researchers lack the mother’s faeces and so can’t make a poop milkshake. This is a particular problem in species where poaching has left many in need of formula, such as the critically endangered black rhino. The team is now working on how to seed infant formula for black rhinos with microbes, says Bornbusch, which may help them digest their fibrous diet. She expects microbe-rich rhino formula will be available within the next few years, and the recipes will be shared with partners abroad in order to help as many youngsters as possible. Hopefully they, too, will soon get a better start in life.
This article is part of our festive holiday special in which we explore:
Article amended on 20 December 2023
We have corrected who made seal butter, the diet of rhinos and removed a reference to Vlad the two-toed sloth
