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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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