Perhaps I am mistaken, but I thought there was a theory that essentially all mammals stop producing lactase as they grow because it promotes weaning. Babies no longer want to nurse when milk starts giving them stomachaches. Once mum is no longer feeding her babies, she can physically recover from childbirth and nursing, and get ready to have more babies, meaning that a hard limit on lactase production promotes the mother having more offspring.
I think essentially all mammals become lactose intolerant as they grow. Humans are the exception, because (some) rely on milk grown by other animals.
I have no idea but since they don’t nurse from their own mothers (nursing being a defining trait of mammals) I tend to wonder whether any non-mammals have evolved to produce lactase at all. I expect the only time they get any lactose is when they kill and eat prey that is nursing and the lactose they receive that way must be a trivial part of most non-mammalian diets. How many mommy mice must a snake eat before lactose intolerance becomes a problem for the snake?
I dunno. Typo and brain glitch or not I read your posts here as a good day!
I’m putting it together as a three hit filter.
People without lactase persistence start adding milk in small amounts to their diets and tolerate small amounts fine.
The mutation develops, or has been there already randomly, helps avoid a bit of gassy stomachaches here and there from too much milk albeit people learn from the mistake, but no huge pressure to spread. Until
disease and famine force using milk as a bigger portion of the diet exactly when doing such is most likely to kill those without lactase persistence off. Then lactase persistence is a huge selection advantage.
The surviving population is overweighted to lactase persistence, and culturally selected for eating more milk.
Lactose may not have been on the ingredient list in the first milks—at least not on its own. Monotremes and marsupials are the oldest mammal lineages and have oligosaccharides (carbohydrates made of 3–10 monosaccharides) as their primary carbohydrates [1-3]. Their milks are not completely lactose free—there is usually a lactose molecule at the end of the sugar chain—but they lack any free lactose molecules [2]. Coupled with this observation is the small amount of alpha-lactalbumin made by monotremes. Urashima [3] have argued that this was the ancestral condition: the first milks contained oligosaccharides and most likely lacked any free lactose. Over time, mammary glands began to make more alpha-lactalbumin, which in turn increased the production of lactose. Finally, lactose became an important energy source for some marsupials and eutherian infants, and was produced separately from oligosaccharide chains [3].
Sounds good as a summary. But not definitive until more studies replicate Evershed.
Agreed but Machine_Elf wasn’t asking whether non-mammal animals evolved milk. He asked whether “[non-mammal animals] are generally lactose-intolerant, or not, or does it just vary from species to species?”
In theory, a non-mammal animal could evolve lactose tolerance/lactase persistence even if those animals did not themselves nurse or generate milk (lactose dominant or not). So far as I know, that has never happened.
You seem to be implying that the only way a non-mammal animal could develop lactase persistence would be if the non-mammal animals also developed milk and that milk was lactose dominant. I understand why lactase persistence would more likely evolve in animals that produce milk but I don’t think it’s a requirement. For an example, if a non-mammal animal was a parasite of nursing mothers, it might evolve lactase persistence/lactose tolerance. Again, I’m not aware of this ever happening but I’m not a zoologist.
I find the scenario so implausible that I’ve never taken time to even look for it. Certainly, though, I’ve never seen such a suggestion in anything I’ve read on the history of milk.
Evolution springs some coping mechanisms that are unbelievable until seen, true. Yeast have evolved to produce the lactase enzyme that breaks down lactose into glucose and galactose: they are used for commercial lactase products.
Animals with stomachs and intestines would need far more availability and dependence to need to produce a counter. Lactose is basically not found in nature outside of milk. A carnivorous advanced parasite (more advanced than yeast) who takes in low-lactose milk as a significant part of its diet - humans have the highest lactose percentage of animals (there may be an exception I can’t think of offhand, but that’s the way it’s usually presented) - would be so noticeable as an outlier that it would as talked about as marsupials.
I also have to note that the presence of lactose in a meal is not even an issue for many non-mammals. A snake which swallows mammals whole uses strong stomach acids to beak down even bones and hoofs. A little lactose wouldn’t be any factor.
I used a ton of qualifiers in everything else I said, but I’ll be absolute on this. Although we can’t prove that no previous non-mammal fit your description, the need for β-d-galactosidase evolution never arose and doesn’t exist today.
My personal coping mechanism… I homebrew Milk Stout, which is named thus because the sugar content of the beer is provided by lactose, rather than glucose. The yeast does the work.
I’m very amateur though, so have not tried any experiments, eg, the obvious: glucose instead.