Got Milk? This phrase, while ubiquitous in magazine and television advertisements cheekily paired with milk-mustachioed celebrities, is also an easy way to distinguish mammals from other species. While the first mammal you may think of in response to this question is likely a cow, human, or maybe even a goat, one lesser-known (and arguably the coolest) milk-producing animal is the tammar wallaby (Macropus eugenii), an Australian marsupial known for its unique reproductive strategy.
Most mammals, including humans and cattle, have long gestation periods where nourishment is provided to developing embryos in-utero via the placenta. This allows for a relatively short and simple lactation period after birth (approximately as long as gestation). The tammar wallaby has the exact opposite strategy: with only a 26.5-day pregnancy that produces a single, underdeveloped offspring, these wallaby mothers must devote considerably more time and energy to lactation than other mammals. Immediately after birth, the impossibly tiny, 400 mg, half jellybean-sized neonate crawls into the mom’s pouch and semi-permanently affixes itself to one of her teats [1,2]. For the first 100 days, the baby will stick to this teat, in desperate need of nutrients that other species’ offspring would have received prior to birth. From 100–200 days, the baby periodically releases the teat but steadfastly remains in the pouch (and you thought you were bored while stuck at home during the pandemic?). Starting around day 200, the baby (now called a joey) will leave the pouch to munch on grass, but continues suckling from the mom until it’s 300–350 days old.
I know what you’re thinking: “That’s it? The coolest reproductive strategy is giving birth to a jellybean-sized clingy baby?”
Okay, well, I did say half a jellybean, but I digress. We’re building to it.
When the joey leaves the pouch and begins to suckle less frequently, tammar wallabies perform a biological magic trick and resume development on a second, previously dormant embryo. Only 26 days after baby #1 begins to leave the pouch, surprise baby #2 is born! This process is known as embryonic diapause, or delayed birth, and can occur because female tammar wallabies enter estrus almost immediately after giving birth, often resulting in a second conception [1,2]. The sucking stimulus of the first offspring inhibits growth of the second embryo, stalling its development. As the first offspring starts to leave the pouch more and suckle less frequently, development of the second embryo automatically resumes. While you might think that having offspring close in age would be beneficial (life’s already hectic! They have a built-in friend! They can share clothes and toys! Wait…some of these may be human-specific…), remember that tammar wallabies give birth to underdeveloped offspring that rely on an energetically-complex lactation period. How can they make sure the 26-day-old neonate receives appropriate nutrition while also supporting the older joey that’s still reliant on milk?
Magic trick #2: tammar wallabies secrete two different types of milk, from two separate teats, at the exact same time! This process, known as asynchronous concurrent lactation, allows them to secrete dilute carbohydrate-heavy milk to support the rapid development of the neonate’s respiratory system, gut, and nervous system, while providing a concentrated protein- and fat-heavy milk to the older joey [1,2]. The lactation period is split into three phases based on these milk composition changes, all of which are perfectly timed to each stage of the wallaby’s development. For example, asparagine, an amino acid known to be critical for brain development in other species, is present at high levels in phase “2A” milk (day 0–100), and tammar neonate brains are essentially fully developed by the end of this phase . Phase “2B” milk (day 100–250) is high in sulfur-containing amino acids and cysteine, necessary for keratin production, and corresponds with the development of hair and nails . As someone who can never remember if I’ve taken my daily vitamins, let alone vitamins specifically for healthy hair and nail maintenance, this sounds like a sweet deal. Phase “3” (day 250–weaning) is associated with a physical change in the mammary gland and teat sizes to accommodate increased milk production . This, along with the increase in protein and fat, allows for rapid growth of the joey, helping it transition away from the pouch to a grass-based diet .
As baby #1 asserts its bourgeoning independence, baby #2 continues to grow and move through lactation phases, making space for, you guessed it, baby #3. Due to their ability to delay birth and nurse differently-aged offspring at the same time, tammar wallaby females are in a near-constant state of pregnancy and lactation. This unique control of lactation has been of keen interest to researchers for decades as it has generated novel insights into mammalian evolution, lactation control, and an enviable ability to multi-task (the interesting trivia fodder is just a bonus—I’ll take “Categories I would excel at but will never be asked in a competitive setting” for $500, Alex!). So next time you hear the phrase “Got Milk” or eat some cheese, send up a silent toast to the true dairy queens: tammar wallabies.
Blair Downey is a 5th year PhD Candidate studying the development and maintenance of abnormal behaviors in dairy cattle. She is passionate about animals who lactate and translating research into metrics that can be used to assess and improve their welfare. She spends most of her time staring at calf mouths, listening to podcasts, and not sleeping.
- Sharp, J. A., Wanyonyi, S., Modepalli, V., Watt, A., Kuruppath, S., Hinds, L. A., Kumar, A., Abud, H.E., Lefevre, C., & Nicholas, K. R. (2016). The tammar wallaby: A marsupial model to examine the timed delivery and role of bioactives in milk. General and Comparative Endocrinology, 244, 164–177.
- Nicholas, K., Simpson, K., Wilson, M., Trott, J., & Shaw, D. (1997). The tammar wallaby: A model to study putative autocrine-induced changes in milk composition. Journal of Mammary Gland Biology and Neoplasia, 2(3), 299–310.
- Renfree, M.B., Meier, P, Teng, C., & Battaglia, F. C. (1981). Relationship between amino acid intake and accretion in a marsupial, Macropus eugenii. I. Total amino acid composition of the milk throughout pouch life. Biology of the Neonate, 40(1–2), 29–37.
- Bird, P., Hendry, K. A., Shaw, D., Wilde, C., & Nicholas, K. (1994). Progressive changes in milk protein gene expression and prolactin binding during lactation in the tammar wallaby (Macropus eugenii). Journal of Molecular Endocrinology, 13(2), 117–125.
Main image [source]
Edited by Jessica Schaefer