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When you think of milk, you might think of mammals like humans and cows, but there are other species that give food to their young, in their own weird ways.

Hosted by: Stefan Chin

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References
https://www.jstor.org/stable/2166840?seq=1#metadata_info_tab_contents
https://bmcgenomics.biomedcentral.com/articles/10.1186/1471-2164-12-452
https://nagonline.net/wp-content/uploads/2014/02/Ward-Nutrient-Composition-Of-American-Flamingo-Crop-Milk.pdf
https://onlinelibrary.wiley.com/doi/abs/10.1111/j.0908-8857.2008.04053.x
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https://www.sciencedirect.com/science/article/abs/pii/0040816674900093
https://askentomologists.com/2016/07/31/cockroach-milk-is-not-the-next-superfood/
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https://www.sciencedirect.com/science/article/abs/pii/0020179077900233
https://www.forbes.com/sites/brucelee/2018/05/28/cockroach-milk-what-must-happen-for-it-to-become-a-superfood-trend/?sh=4610abcf5fcd
https://science.sciencemag.org/content/362/6418/1052
https://www.jstage.jst.go.jp/article/asjaa1936/28/2/28_2_71/_pdf
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‘caecillian

https://www.nature.com/articles/nature04403.epdf?sharing_token=c7vgh2Bipvc1BV8p1CZSyNRgN0jAjWel9jnR3ZoTv0OpSbm-8t_8IMIbOoRm4AfpZ8R8-XVr--bq2kwUZHzgCU_yFz4AIPo2NfNFJ0EqhyzemJOqyGTnI2iVpu7iIDr4kIq_P1V6tE9etOkj_ITrxVegrT8y9eGGDNKgGkLBpIRdlc1GGRd7yywrzwZeMoO9kwq_PzNK4EdbiC_9kYRoaA%3D%3D&tracking_referrer=www.bbc.com

Fish


https://www.nature.com/articles/nature04403
https://journals.biologists.com/jeb/article/213/22/3787/33456/Biparental-mucus-feeding-a-unique-example-of
https://www.jstor.org/stable/2458829?mag=how-non-mammals-nurse-their-young&seq=1#metadata_info_tab_contents
https://academic.oup.com/biolinnean/article/128/4/926/5602696
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https://onlinelibrary.wiley.com/doi/abs/10.1111/jen.12096


IMAGES

https://www.istockphoto.com/photo/red-cow-with-dreamy-eyes-and-pink-snout-on-a-blue-background-gm1282514033-380222171
https://commons.wikimedia.org/wiki/File:Rosaflamingo-K%C3%BCken_-_Fuetterung_mit_Kropfmilch.jpg
https://commons.wikimedia.org/wiki/File:PigeonAnatomy.png
https://www.shutterstock.com/image-photo/feeding-young-cuban-flamingo-phoenicopterus-ruber-103128599
https://www.flickr.com/photos/162338887@N06/25357104657/in/photolist-ECHMMB-6rGvBa-E8EScM-57ZJdn-GqTBs-bGefkt-aiQxtJ-8wa1TS-9u5pb5-9u2nqV-imGeJ3-aYn2cV-5XRJZ-bDXzUa-br3Ewb-85cKhW-6v3tjt-GPP4d-9u5oLu-67pxqV-xPuteS-cUCgzy-aaSY45-T8GWjM-eXUb8j-fqA7va-YJ3ck-7zev8A-imGkL1-aCvq19-cV7Kof-6vN7pM-d8exL-fAF5pk-5roQor-cHJeG-6jjjaS-3BkXoF-ahEJnU-vqg4KY-af7u5f-d8eyW-4ZDxx1-aRnC3x-cV7Kus-Co51nh-4HcD2s-br3EyY-4CGfoB-fAETZD
https://commons.wikimedia.org/wiki/File:Myrmarachne_magna_male_lateral.jpg
https://commons.wikimedia.org/wiki/File:Myrmarachne_magna_male_frontal_01.jpg
https://www.istockphoto.com/photo/house-pseudoscorpion-gm1302846007-394469717
https://www.istockphoto.com/photo/extreme-closeup-of-a-false-scorpion-pseudoscorpiones-on-wood-gm1247561879-363332748
https://www.istockphoto.com/photo/caecilian-gm1278468811-377401526
https://www.storyblocks.com/video/stock/symphysodon-discus-in-an-aquarium-hkly4tangix6aliqr
https://commons.wikimedia.org/wiki/File:Symphysodon_aequifasciatus_(1)_03May2010.JPG
https://www.storyblocks.com/video/stock/stingray-swimming-in-an-aquarium-underwater-world-r3bt41edsk16v2ztj
https://commons.wikimedia.org/wiki/File:Stingray_giving_birth.jpg
https://www.istockphoto.com/photo/baby-ray-fish-face-close-up-view-gm1270565273-373463369
https://www.istockphoto.com/photo/cows-on-the-beach-gm802937940-130189243
[♪ INTRO].

When you think of milk, you probably  think of mammals like humans or cows. I mean, “Mammalia” literally  translates to “of the breast,” so milk is kind of our thing.

Mammalian milk is a nutritious blend of  protein, fat, and sugars that’s produced in the parent’s mammary gland to ensure babies are getting the nutrients that they need.   But mammals aren’t the only ones that  can produce a nutritious bodily fluid. There are other species  that have evolved completely different ways to provide their offspring  with their own milk alternatives. So here are six animals that take  an unconventional approach to giving “milk” to their young, and the weird  ways that their bodies make it.

Some birds not only regurgitate food  they swallow to feed their chicks, but actually produce extra bodily  fluids to help out with their growth. The bird version of milk is formed from  skin cells in the lining of their crop, a food storage organ that  birds use to moisten meals before they make their way through  the rest of the digestive tract. And a number of birds do this,  including emperor penguins, flamingos, and all species in the pigeon and dove family!

But a flamingo’s milk alternative is  actually kind of disturbing looking. And that’s because, at least in  the first few weeks of feeding, flamingo crop milk is bright red. And I mean blood red.

So much so that people witnessing this  feeding habit confuse it with actual blood. And so did scientists at one  point, but we now know that it doesn’t really contain red blood cells. Flamingos’ milk is likely this color  because of their parent’s high intake of carotenoids, the pigments found  in the microorganisms they eat, which gives them their lovely pink hue.

But while the milk doesn’t have any blood  in it, it does have a lot of nutrients. And it has a lot of the same macronutrients  you’d find in mammalian milk, even though it’s produced by a  completely different process. And that process is a whole family affair.

Both parents make crop milk to  support the rearing of their chicks. And flamingo parents are in it for the  long haul, producing this supplement for their chicks for sometimes  as long as six months. Next up, one species of jumping spider,  Toxeus magnus, is also a bit of an oddball.

First of all, it’s clearly  pretending to be an ant. Just look at it! Nice try, spider, but we’ve got you figured out.

In addition to being an ant mimic,  it has a few other surprising habits. For one, these jumping spiders  have been known to form small crews of multiple adults that stay  together in a breeding nest. Which is a pretty strange behavior for a species that’s usually considered to fly solo.

But scientists realized these  clusters of spiders could be a sign of an unexpected maternal care adaptation. It turns out these female spiders feed  their spiderlings with their own special “milk” droplets, which have about four  times more protein content than cow’s milk! At first, the spider places those droplets  around its nest for spiderlings to eat.

But eventually, spiderlings  suck the droplets right from a specialized area of the parent’s  abdomen called the epigastric furrow. And surprisingly, these spiders  continue to take care of their offspring long after they’re able to feed themselves. After the first 20 days, the young  start leaving the nest and finding other things to eat, but they can still  come back for milk for the next 20 days!.

Once they reach sexual maturity, the  female spiderlings are allowed to keep coming back to the nest to drink milk, while males are driven off to fend for themselves. This extra attention skews  the sex ratio of this species, with females making up almost  85% of all individual adults, which might actually help combat  the ill effects of inbreeding. So for all the fear and judgment  we tend to have towards spiders, they can really make some excellent parents.

A cousin of the spider known  as the house pseudoscorpion is frequently found around human structures. At some point, you may have  found these scorpion-esque looking critters around your home. They’re often thought of  as uninvited house guests, but pseudoscorpions are helpful arachnids  with unique maternal adaptations.

Since their eggs only contain  a small amount of yolk, they need to be nourished another way. So females create a nutritious  fluid to feed their embryos. But unlike their distant arachnid relatives  the jumping spiders, in pseudoscorpions, the fluid is formed in their ovaries and oviducts, where eggs are produced and transported.

How this milk is formed and  consumed actually changes over time. Their supplemental feeding begins  as a secretion of uterine fluid that nourishes the growing eggs,  referred to as histotrophy. But as they develop, this nutrition  instead comes from secretions from the surrounding organ tissues,  which start to fragment.

That becomes the new source of baby  formula, now called histophagy. As the thick surface cells  within the ovary get secreted, this provides extra nutrients to  the eggs and developing babies, which are stored in a brood  sac on the parent’s abdomen. So this special formula actually starts  to feed them long before they even hatch, and keeps on nourishing them as babies!

So, maybe the ‘skin’ off your back  isn’t the first thing that comes to mind when you think of milk, but for the Taita  African caecilian, skin is the new milk. These snake-like, legless amphibians  supplement their offspring with their own special nutritious baby formula. But this time, this supplement  is coming from the skin.

The brooding females' bodies  transform their skin into a strange, nutrient-rich mass to  support their growing babies. During this time, their skin  looks paler, is twice as thick, and much like mammalian milk,  is full of fatty goodness. The babies actually develop  special teeth used to peel off and munch down on their mother’s skin!

Called dermatotrophy, this form of  parental care is very rare in tetrapods. In fact, caecilians are the  only ones known to do it. And yes, they’re still technically tetrapods  even though they’re actually legless!

And while it’s not uncommon for  species to eat their shed skin to take advantage of the extra nutrients,  they usually wait until it’s off their bodies! This bizarre grainy skin secretion  is vital to their growing babies, and boosts their growth  significantly, increasing their length by 11 percent in just one week of feeding! And since these babies don’t  appear to be feeding on anything else during this time, it seems  like it’s pretty vital for their survival.

Now while it might be  incredibly rare for tetrapods, caecilians aren’t the only  vertebrates producing skin-based milk. Cichlids, a diverse family of fish  that we often keep in aquariums, produce a mucus secretion from their  skin to feed their developing babies. And similar to what you might expect  from mammalian milk production, this mucus production peaks when  their babies first begin to swim, and once they begin feeding on other  sources, mucus production slows down.

In many cases, both parents provide  for their babies after hatching, and it’s pretty widespread for  them, with at least 30 species pulling off this type of parental care. And in at least one particular  species of Amazonian cichlid, called the discus fish, it  might be vital for survival. In a well-kept aquarium, cichlids  wouldn’t be encountering too many dangers and health risks, but even  in these protected settings, their offspring don’t survive  without parental help.   That’s because the discus fish are  likely passing along much more than just nutrition to their  young through the mucus milk, but also hormones, beneficial  microbes, and antibodies.

And the babies take that necessity seriously. As humans know, patience is a  virtue when it comes to parenting, and it seems like discus  fish need it in abundance. The discus fish offspring basically  become enthusiastic biters, nipping at the bodies of their parents  continuously to glean this mucus from them.

So it seems like these fish have  to put up with a lot of attitude from their offspring in their efforts  to ensure reproductive success. Next up, stingrays. They aren’t just strange in  that they’re milk producers.

How they have babies to begin with  is also a bit out of the ordinary. Instead of laying eggs like most fish do, red stingrays give birth to  live wee little stingrays. They gather together in estuaries while  they carry their young, eventually giving birth to around 25 newborn rays  that look like mini versions of the adults.

Stranger yet, stingrays secrete milk from  inside their uterus to give their offspring a strong start to life before they’re born, and help them develop faster inside the uterus. This milk, which is rich in  proteins and fatty acids, gives their embryos a serious boost. This means that in addition to  the nutrients of the egg yolk, their developing babies are getting  added nutrients from this milk, likely first absorbing it through their  gills, and then later through the mouth.

The composition of the milk also changes over time to suit the developing babies’ needs. The milk isn't as nutritious in the  early stages of their development, since the developing eggs are also  still utilizing their egg’s yolk sac. That said, its protein content is very  similar to human milk during early lactation!

As time passes, the quantities of protein,  fat, and total solids in the milk bump up, eventually doubling the calories  you’d find in human or cow milk, preparing these rays for life in the water! So while you might think of milk  production as a strictly mammalian trait, it turns out that providing  additional nutrients in the form of milk-like substances is more  widespread than you’d think. And it makes sense, given the huge  advantage it can provide offspring at a period when consistent, quality  nutrition is of utmost importance!

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