Could a cow produce bottled milk? The answer is, of course, no; there is no way a cow could do that; obviously. Perhaps I should be more clear on what I'm asking.

Is there any way a mammalian creature could evolve to create a substance within its body that is somewhat similar to one of the compounds we refer to as glass, give that material a defined shape, and somehow expel it from its body?

In the title, when I say "bottled" I am referring to the classic glass bottles that milkmen would deliver milk in, as seen below.

Milk bottles

Of course, the lids on these bottles are made from something other than glass, but just ignore that for now.

So my question is: could a glass making creature exist; and how would it do what it does? (OK, two questions, so sue me.)

P.S. If you're wondering why this question exists or how it came to be, don't; just don't.

Edit: Any scenario that in some way resembles what I described above would be acceptable, the material does not absolutely have to be silicon-based.

  • $\begingroup$ Are you expecting the cow to produce bottles on a daily basis, fill them and have them ready for collection? $\endgroup$
    – nzaman
    Apr 28, 2018 at 17:37
  • $\begingroup$ @nzaman Pretty much $\endgroup$ Apr 28, 2018 at 17:39
  • $\begingroup$ Then the answer is clearly: NO! If you were simply interested in watertight containers, then there are plenty of molluscs to suit your needs. $\endgroup$
    – nzaman
    Apr 28, 2018 at 17:41
  • $\begingroup$ What would the evolutionary advantage be? (After all, the purpose of a teat is to make it easy for a baby to drink.) $\endgroup$
    – RonJohn
    Apr 28, 2018 at 18:04
  • $\begingroup$ Sorry, but it's a big no-no. There is no evolutionary mechanism for which this could ever happen. $\endgroup$ Jul 3, 2018 at 6:20

4 Answers 4


Some living creatures are actually able to create glass in their bodies, typically in the form of silicate shells or exoskeletons. The answers to this question provide some examples, which I will summarize here.

I'm not entirely sure how the existing real-life glass-producing species, such as diatoms, radiolarians, glass sponges, and many grasses, create their shells, spicules, and phytoliths, but as far as I can tell, they use a combination of enzymes and organic acids to dissolve silica from sand, then convert the solution to alkaline sodium silicate (aka "water glass"), then reintroduce it to an acid to harden it into glass. Your cows could use this same process to convert silica obtained from plants (as I mentioned earlier, many grasses create silica phytoliths to wear down herbivores' teeth) into glass bottles.

Alternatively, your cows might create glass by way of "spin-on glass", a compound composed of carbon, hydrogen, oxygen, and silicon that, when cured, transforms into a randomly-arranged network of SiO2- that is, glass. Curing spin-on glass is normally done by exposure to ultraviolet light, but I would not be surprised in the least if your cows found an enzyme that could do it as well.

I imagine your cows could have a sort of "bottle gland" in their udders surrounding the mammary gland that uses either the water glass or spin-on glass pathway to fabricate the glass bottle at the same rate that the mammary gland fills it with milk. The milk-filled bottle is extruded out through the teat as it forms until it becomes too heavy, prompting the bottle gland to narrow down the top of the bottle, seal it, let it fall free, and begin forming a new bottle.

  • $\begingroup$ Note however producing silica biologically is SLOW don't expect to collect a bottle every day or even every week. $\endgroup$
    – John
    Sep 29, 2019 at 1:04

Imagine something much like an egg, filled with milk. Not remotely like a glass bottle (transparent and with a removable cap) but a step in the right direction, maybe.

I could even think of a sort-of-twisted evolutionary part that leads there.

  • Start with an egg-laying creature.
  • Clearly it is a benefit for the individual hatchlings to consume unhatched eggs. This is part of a gradual transition from a low-K to high-K strategy, so it is an evolutionary advantage for the parents to accept that.
  • In another long adjustment process, it is an advantage for the individual hatchling to hatch early. By some twist of fate a mechanism evolves which allows eggs to hatch in the uterus for live births.
  • The most successful breeders are those who produce the best ratio of live-born hatchlings (not to many) and stillborn eggs with a maximum of useable egg-white (enough to feed the hatchlings).
  • Human breeding created animals with enough "milk eggs" to give a surplus for human consumption.

A bit like histotrophic viviparity, if I remember the biology correctly.

  • 3
    $\begingroup$ Slight adjustment: a bird might lay multiple milk-only eggs ahead of the real eggs in order to store up nutrients for the young when the mother won’t be able to leave the nest. $\endgroup$
    – SRM
    Apr 28, 2018 at 20:36
  • $\begingroup$ @SRM when the egg cracks, the milk runs out. $\endgroup$
    – RonJohn
    Apr 28, 2018 at 22:35

For that to be possible the creatures body temperature would have to be that of a furnace around 1700 degrees Celsius. Glass is a compound made from silica, various metal oxides, lime, soda, magnesia, and potash. In order for the proper chemical reactions to occur you need a fairly high temperature. Your creature that excretes glass would probably be some form of silicon based life living within a lava flow or on the surface of some incredibly dense and large planet very close to its sun with a crazy heavy atmosphere. As for if silicon based life is even possible, that's a whole other can of worms.

So its at least within the vague realm of possibility for an animal to exist that excretes glass, but no, it is not even remotely possible for it to just crap out fully manufactured and filled milk bottles.

  • 3
    $\begingroup$ Diatoms serve as a reminder that terrestrial organic life can shape silicon without extreme temperaturs en.wikipedia.org/wiki/Diatom $\endgroup$
    – pojo-guy
    Apr 28, 2018 at 17:52
  • $\begingroup$ Silica is an ingredient in glass, but it is not glass. $\endgroup$
    – TCAT117
    Apr 28, 2018 at 18:35
  • $\begingroup$ OP statement: "somewhat similar to the one of the compounds we refer to as glass," $\endgroup$
    – pojo-guy
    Apr 28, 2018 at 18:48
  • $\begingroup$ @TCAT117 I disagree: en.m.wikipedia.org/wiki/Fused_quartz $\endgroup$
    – Dubukay
    Apr 29, 2018 at 15:35

Not exactly a glass bottle but, We can think of this process something like this:

  1. Imagine a cow with multiple teats (7-10).
  2. Each teat has a capacity of 1 liter.
  3. There is no hole at the end of teat (so no leakage of milk).
  4. outer covering of teats is made up of elastic collagen (let's call it sack and it's replaceable).
  5. The cow produces milk, it starts to fill up the sack (in shape of a gourd).
  6. Once the sack reaches its capacity, some sort of hormonal signal tells the sack to harden up and lose its elasticity, now it resembles the shape of a bottle.
  7. The upper part of the sack shrinks up to seal it from the top.
  8. and now the weight of the sack is enough to drop it.

The entire process would look like a fast-forward development of gourd. or A balloon is filled with milk and tied from the top.


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