I was drinking a glass of milk the other day and that got me thinking that no other animal to my knowledge drinks milk past their infant stages. One could argue that cats might but it isn't good for them to do.
Are humans the only animal that are able to drink milk as adults and not have it cause issues?
Of course, I know some people do have lactose intolerance too.
Good observation!
Gene LCT
Mammals have a gene (called LCT C/T-13910) coding for the lactase enzyme, a protein able to digest lactose. Lactose is a disaccharide sugar found in milk.
Expression of LCT
In mammals, the gene LCT is normally expressed (see gene expression) only early in development, when the baby feeds on his/her mother's milk. Some human lineages have evolved the ability to express LCT all life long, allowing them to drink milk and digest lactose at any age.
Today, the inability to digest lactose at all ages in humans is called lactose intolerance.
Three independent mutations
Tishkoff et al. 2007 found that the ability to express LCT at an old age has evolved at least three times independently. Indeed, they found three different SNPs (stands for Single Nucleotide Polymorphism; it is a common type of mutation), two of them having high prevalence in Africa (and people of African descent) and one having high prevalence in Europe (and people of European descent). The three SNPs are G/C-14010, T/G-13915 and C/G-13907.
Pastoralist populations
Lactose tolerance is much more common in people descending from pastoralist populations than in people descending from non-pastoralist populations, suggesting a strong selection for lactose tolerance Durham 1991.
Selective sweep
On top of that, Tishkoff et al. 2007 focusing on the locus 14010 (one of the three SNP's mentioned above) showed that there is a clear selective sweep (which is a signature of past and present selection) around this locus.
They estimated the age of the allele allowing lactose tolerance at this locus (allele C is derived, the ancestral being G; see nucleotide) at around 3,000 to 7,000 years (with a 95% confidence interval ranging from 1,200 to 23,200 years) and a selection coefficient of 0.04 - 0.097 (with a 95% confidence interval ranging from 0.01 to 0.15).
I recommend reading Tishkoff et al. 2007. It is a classic, is short and is relatively easy to read, even for someone with only basic knowledge in evolutionary biology.
I don't really know... but I would think so, yes!
Drink vs digest thoroughly
As @anongoodnurse rightly said in his/her answer
"Drink" and "digest thoroughly" are two different things
Pets
According to many dog health websites (such this one for example) claim that there is also variance among dogs where some dogs are lactose tolerant and others are lactose intolerant. I could not find any paper on the underlying genetics of lactose intolerance in dogs or other pets. It is not impossible our pets have also been under selection to be able to digest lactose as we humans could have given milk to them. It is also possible that pets do not actually produce any lactase at adult age but rather that some pets are just able to deal with having indigestible lactose in their guts! But then again, "Drink" and "digest thoroughly" are two different things.
Tits and robins in 20th century England
A funny and famous case is the case of blue tits and robins in the 20th century, in England. At that time, in England, the milkman was bringing the milk at home in the morning and would leave glass bottles with a simple aluminum cap in front of people's home. At some point, blue tits and robins learnt that by pecking through the aluminum they can get access to the milk. See this (non-peer-reviewed) article that tells the story.
Somewhat related
There are already a number of good posts on milk digestion in humans on Biology.SE. Consider having a look at:
The problem with this question is 1) wording and 2) access.
"Drink" and "digest thoroughly" are two different things. The latter does not prohibit the former. It is only lactose that is not tolerated; Calcium and other minerals, proteins, fats, etc. are also present in milk and are beneficial to the consumers if the lactose intolerance is not incapacitating.
One could argue that cats might but it isn't good for them to do.
I don't know about this. My barn cats drank milk all the time without diarrhea or other untoward consequences; so did my dogs. Furthermore, feral cats steal milk, as do sea birds!
While censusing, satellite tagging adult males and rototag tagging weaned pups of northern elephant seals (Mirounga angustirostris) at Playa Norte, Isla de Guadalupe (14 - 19 February, 2003), we opportunistically observed the stealing of elephant seal’s milk by feral cats (Felis catus) and western gulls (Larus occidentalis). Western gulls have been previously reported stealing milk from female northern elephant seals in several elephant seal colonies... and this behavior has also been observed in other seabirds such as the Pale-faced sheathbill (Chionis alba), feeding on southern elephant seal (M. leonina) milk (Favero 1996). However, this is the first report that focuses on feral cats stealing milk from elephant seals.
Regarding access, how would most adults go about getting milk? If offered, though, many adult animals will drink milk preferentially over water if offered at the same time. When I had dairy goats, excess milk was fed to adult dogs, adult cats, and many adult birds (e.g. chickens, guinea hens, peacocks, etc.) The chickens acted like it they were children and the milk was free candy! (I never had pigs, but my uncle and grandfather did, and fed excess milk to the adult pigs.)
As noted above, wild birds will also steal milk when the opportunity arises. Cows will sometimes suck their own teats.
Humans only appear to be isolated in drinking milk as adults by their ability to obtain milk through farming.
Just to point out an exception, adult cows sucking other cows' milk are not uncommon. At least, it's common enough to be a concern in dairy farms and for simple fixes to have been developed. The usual fix is just a piece of plastic pinned to the offending cow's nose that makes the suckled cow walk away.
I'm not sure of how much of this behaviour is feeding related, but concerns about direct impact on production make me suppose that inter-sucking cows are actually drinking a sizeable amount of milk - of course, there are also concerns about indirect impact on production by affecting the involved cows' health.
Here is a bit of a tangential example, because it isn't milk from cows. There are some varieties of ants (Primarily the Leaf-cutter ant, if memory serves) which will collect, breed, feed, protect and "milk" aphids for the honeydew they produce. It is generally classified as a mutually beneficial symbiotic relationship, though one could argue that it is closer to domestication. The adult ants will eat the honeydew secretion.
Source: https://en.wikipedia.org/wiki/Aphid#Ant_mutualism
The reason I bring this up is because one argument against raising cows in a "factory environment" is because humans are the only species that drinks milk after adolescence. While this was not hinted in the original question post, the wording made me think of this.
Pigs can. At least one milk, and milk derivates, producer around here used/uses their own pig farms to dispose of all byproducts of production, overdue products returned from stores, same day unused milk, etc. As they owned the pigs, that should not cause too many problems.
I also remember a photo of depression era USA where a farmer is giving milk to pigs. The caption said that the price was so low that they used it for animal feeding instead of selling for human comsumption. I can't find it now.
At some times and places, butter was the primary reason for dairies. After the cream was skimmed off, the remaining milk had a much lower sale value. In some cases, it wasn't worth the effort of trying to sell it, or no one wanted it, and so it was fed to the hogs.
Milk (or food prepared with milk) is used to feed various domesticated animals like pigs or cats, also when they are adult.
